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RHS Level 3: Plant taxonomy, structure, and function Q3

-3. Understand photosynthesis, respiration, water/solute relations in the plant and their relevance to horticulture.

3.1 Describe the process of photosynthesis.

Describe the structure of the chloroplast to include: stroma,thylakoids, grana.

Chloroplast

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Note: diagram is included for clarity, no mention of needing to know it.

A chloroplast is an organelle where photosynthesis takes place. The chloroplasts are filled with stroma, a colourless liquid, and with grana which are a stack of thylakoids. A thylakoid is a membrane bound compartment containing chlorophyll which absorbs sunlight for photosynthesis.  Photosynthesis begins in the grana and then continues in the stroma. Chloroplasts also contain starch grains and ribosomes.

Describe the process of photosynthesis: The interception of light by chlorophyll (wavelengths absorbed, PAR). The light dependent reaction (splitting of water to release oxygen and electrons, production of ATP and NADPH).

Photosynthesis

Photosynthetically Active Radiation (PAR)

Light wavelength is the distance between wave peaks that we perceive as colour. The range of wavelengths in photosynthesis is PAR, it is between 400 and 700nm (red+blue light). Chlorophyll absorbs this red and blue light, then each photon of light becomes excited which in turn excites an electron, which in turn excites another electron and so on, creating a chain of reactions known as the electron transport chain.

dependent and independent

Photosynthesis

Light dependent

  • In the thylakoid membranes,  light is absorbed by the chloroplasts.
  • Water (H2O) is broken down by electrons that are energized by the sunlight. The oxygen (O2) in the H2O is released as a waste product and the hydrogen (H) converts the NADP+ to NADPH.
  •  ADP (Adenosine Di-Phosphate) becomes ATP (Adenosine Tri-Phosphate) by the addition of P (phosphorus). ATP is an energy molecule that is used to create the sugars in the next stage.

The light independent reaction/dark reaction (carbon dioxide fixation, use of ATP, NADPH and electrons from the light reaction to produce sugar (glucose)).

Light independent (aka Calvin cycle)

  • This happens in the stroma and the energized electrons from the light dependent reactions provide the energy for the following processes.
  • The NADPH become NADP again. The ATP becomes ADP +phosphate (P) again.
  • Carboyhydrate (C6 H12 O6) is formed from: Hydrogen (H) originally came from water and then stored as NADPH, released during the light independent reaction. Carbon (C) comes from the carbon dioxide (CO2) , and oxygen (O) which comes from water and air.

The light dependent stage feeds the light independent stage feeds and vice versa.

Significance of enzymes in photosynthesis: catalysts, temperature dependency.

Carbon fixation is catalysed by the enzymes RuBiSCo, and ATPase.

Enzymes have an optimum temperature and the higher and closer to that temperature the plant gets, the faster the enzyme works. However, above that temperature the enzymes slow down again, as they are denatured and finally they stop working.

ATPase catalyses the reaction to change ATP to ADP plus a phosphate ion, this releases energy for other chemical reactions (see above for light dependent and independent reactions for where this happens).

RuBiSCo is responsible for fixing carbon (C) in the light independent stage so that it can be converted to complex sugars (C6 H12 O6)

Specify differences in photosynthetic efficiency in: C3, C4 and CAM plants in outline using NAMED plant examples.

NO DETAILS OF METABOLIC PATHWAYS REQUIRED.

C3 – occurs in  85% of all plants, for example rice or Helianthus annuus. It’s slightly inefficient in that 25% of carbon fixation is lost due to RuBiSCo which causes an oxidation reaction, this is known as photorespiration – whereby oxygen reacts with the carbon to form carbon dioxide which escapes as gas.

C4 – occurs in 3% of all plants, most of which are angiosperms. It has evolved in plants growing in areas with low light or water. For example Zea mays, Cyperus rotundus. It has one extra step before the calvin cycle that reduces photorespiration and loss of carbon fixation. It does this by moving the process to a special part of the leaves that contain little oxygen (which is needed to fix the carbon), leaves with this area are known as Kranz leaves.

CAM – occurs Clusia spp., Sedum spp., some bromeliads, Orchids, and cacti. CAM plants (crassulacean acid metabolism) are very efficient in their use of water, an adaptation needed by xerophytic plants. In CAM plants the stoma are closed during the day, so that water cannot escape, however this also means that gases cannot enter or leave. During the night the stoma open, so that CO2 can enter. CO2 is needed during the day, so it is converted to malate at night and stored until needed.

3.2 Explain how the rate of photosynthesis can be manipulated.

Describe how the following factors influence the rate of photosynthesis and explain how they can be manipulated: Light: light intensity (light compensation point, light saturation point); supplementary and replacement lighting; choice of lamp in relation to wavelength. Carbon dioxide: levels; ventilation; CO2 enrichment methods. Temperature: temperature range, heating, shading, ventilation, damping down. Water and mineral nutrients: the need for adequate levels of irrigation and essential elements for chlorophyll formation (nitrogen, magnesium and iron).

The main limiting factors in photosynthesis are light, temperature and CO2 concentration.

Light

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Light compensation points and light saturation point

Light compensation point  – the points on the light curve when the rate of photosynthesis matches the rate of respiration.

Light saturation point – the point at which increasing the light levels no longer increase the photosynthesis rate.

Light limits the light dependent phase of photosynthesis, and that in turn stops the light-independent phase. As light intensity increases the rate of photosynthesis will increase given that other factors are sufficient. However, there is a point of light intensity at which chlorophyll is damaged and photosynthesis stops.

Supplemental light is usually used during the winter, but if growers want to maximise crops then it can be used all year round.

More info here on lighting for the greenhouse.

Light can be supplemented with:

High Intensity Discharge Lamp (HID)s – either Metal Halide (MH) lamps or High-pressure sodium lamps (HPS).

All HIDs are good at lighting a large area without taking up much space. This is important, because lamps that take up a large area will block natural sunlight when the lamps aren’t switched on. HIDs generate a lot of heat, which is an advantage in the winter, but potentially a problem in summer.

MH – create white light, and this is better for some crops.

HPS – create yellow/orange light. They are more efficient and last 3x longer than MH

Fluorescents – cover a large area, more efficient than HIDs. The light wavelength is the least effective for photosynthesis. Inexpensive, widely available and don’t generate so much heat. However, the light is not as intense as HIDs and the units are large, so block out the sun.

LEDs – these have become more popular recently. They are long lasting and give off the least heat. They have a similar spectrum to sunlight, with the highest wavelength in red and blue light, ideal for photosynthesis. They take up small area and are efficient to run. The initial cost is greater, although worth it in the long run.

Carbon Dioxide

Good article where much of this info comes from

CO2 is in the air at the very low percentage of 0.04%, and increasing this percentage increases photosynthesis rate, until a maximum level of carbon fixation is reached. Above 1% and it becomes toxic to plants, causing necrosis and malformation. It can also cause humans to become dizzy.

The simplest form of increasing CO2 is by having vents open, so that as plants use up CO2 it is replenished from otuside. Disadvantages are that levels won’t get higher than 0.04% and if it is too cold or windy outside, vents should not be open. The opposite issue occurs with artificially created CO2 – that air escapes, and air with lower CO2 gets in from outside.

CO2 is heavier than air, so must come from above.

Natural gas, propane or kerosene all give off Co2, but also heat (which may be an advantage, but not automatically so). If fuels happen to be contaminated, then that can harm the plants. They also generate moisture. Both moisture and heat may lead to moulds growing on plants (botrytis)

Propane gas burners – CO2 generators, good for larger areas.

Paraffin heaters – for smaller areas.

Exhale bags filled with mycelium (fungus) give off CO2. These are a cheap, but not hugely effective method.

Liquid carbon dioxide – more expensive, but pure, doesn’t create heat or moisture and is more exact, with a timer and regulator. However, it requires special tank for storage. For smaller areas, simple canisters can be used.

Organic materials – eg manure, coir – give off CO2 while breaking down, this is a simpler way, requires no additional cost or machinery, but only lasts for a month, and can’t be regulated like a machine .

Temperature

The light dependent stage of photosynthesis is not affected by temperature, but the light independent stage is. The rate of photosynthesis is driven by enzymes (catalysts to the reaction) which have an optimum temperature. The higher the temperature, the better the catalysts work, until they reach optimum.

The optimum temperature for enzymes is 25-35°C, minimum is 5°C max is 45°C. However, different plants have other temperature needs, so a plant is not automatically happier in that temperature range.

Increasing heat

Electric – cheaper to install than gas and requires less maintenance, but daily costs are higher. Also easier to control effectively.

Gas – cheaper to run than electricity.

Paraffin – requires ventilation to release the gases given off, only useful for small scale and crops that require low temperatures.

Ground source heat pumps – are more environmentally sustainable, but expensive to install.

Hot water pipes –requires a complex water system and a lot of water. This is usually considered the most efficient for large areas since the heat spreads evenly throughout the greenhouse.

Biofuel  – sustainable plant based fuels (although their production is harmful to the environment). Require less maintenance, but costly.

Another aspect to be considered when heating, is insulation so that heat is not lost.

Reducing heat

Damping down – a quick, cheap way of cooling a room, plus increasing humidity (depending on the plants, that may or may not be useful).

Shading can be used to reduce heat, but will also reduce sunlight, although if there is a danger of scorch, that is a good thing.

Here’s an article on ventilation and shading

Water and Mineral Nutrients

Note: there is nothing specified as to whether this means outside or inside irrigation, and both are quite different. Light and heat are usually only supplemented inside, so maybe it’s assumed this is the same for irrigation. For this reason, I will talk about watering inside, but include a few articles on outside irrigation, so you can decide for yourself.

Irrigation is essential in a protected environment, and necessary for crop maintenance even outside during dry periods.

Forms of irrigation:  overhead, drip, manual and capillary matting.

Overhead – sprinkler systems, not very efficient (water goes everywhere), all plants are watered the same, and not usually used with feed. Not used with plants in pots, only those on inside/outside beds or large crop areas.

Drip – pipes lead directly to plants (from a main hose). Very efficient and can be used with feed. However it is tricky to install and once installed it’s even more tricky to move the plants around. All plants will be watered the same.

Manual – slow, using labour, however it means that each plant can be watered according to its needs. Often used with feed, and the feed can also be changed part way through for different requirements.

Capillary matting – polythene mats, with an absorbant layer on the bottom and perforated layer on top. The mats are kept permanently saturated. Expensive to set up, but plants are never overwatered because they soak up what they need. Can’t be used with feed.

Feed:

Nitrogen is a macro nutrient, common in most fertilisers. Levels of nitrogen are shown in the ratio N:P:K (N=nitrogen, P=phosphorus, K=potassium). For example a high nitrogen feed  would be 6:4:4. Magnesium and iron are micro nutrients and require more specialized fertilizers. Iron (Fe) is found in ferrous sulphate, usually a applied as a foliar feed. Magnesium (Mg) is found in epsom salts which will raise Mg without increasing pH, lime will increase Mg and alkalinity.

Article on how to add magnesium to soil

Article for outside irrigation

Article on soaker hose

3.3 Describe the processes of respiration. Structure of the mitochondrion to include: matrix, cristae. Describe aerobic respiration: Glycolysis (sugar (glucose) split to form pyruvate, production of ATP in the cytoplasm).

Aerobic Respiration

Aerobic respiration = glucose breaking down to CO2 +H2O + 38 ATP (energy)

Note: I’ve included more detail than the syllabus requires, that only asks for the glycolysis part and the structure of the mitchondrion. I’ve included the rest for completeness and it may be useful to be aware of it.

1. Glycolysis – 1 molecule of glucose (6C) splits to form 2 molecules of pyruvate (3C). ATP is created, and NAD+ is converted to NADH. This occurs in the cytoplasm.

2. Pyruvate oxidation – The pyruvate is converted to acetyl CoA in the mitchondria matrix, CO2 and NADH is created.

3. Citric acid cycle/ krebs cycle – The acetyl CoA goes through chemical processes which result in ATP, NADH, FADH2 being created and CO2 released. This occurs in the mitochndrial matrix.

4.Oxidative phosphorylation – More ATP is produced in the mitochondria, NADH and FADH2 return to being NAD+ and FAD. Water is formed.

Energy output up to 38 ATP for EACH glucose molecule.

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Mitochondrion – consists of two mebranes, an outer one that covers and contains the organelle, and an inner one that is folded over many times into cristae. The outer layer has pores to allow ions and small proteins through. The cristae is folded to create a large surface area. The matrix is a gel like fluid inside the cristae. The mitochondria also contain ribosomes (that syntehsize proteins), mitochondrial DNA and enzymes.

Anaerobic Respiration

Anaerobic respiration = glucose breaking down to ethanol (toxic) and CO2+ 2 ATP

This consists of the same first two stages, both glycolysis and pyruvate oxidation occur. However the next two stages do not happen in anaerobic respiration. Instead a process called fermenation occurs. In fermentation, zymase (an enzyme not used in aerobic respiration) is used to produce ethanol. Ethanol is toxic and will eventualy kill the plant. Anearobic respiration only produces two molecules of ATP, unlike aerobic respiration’s 38.

Anaerobic equation: C6H12O6 + zymase >> 2CO2+2C2H5OH + 2ATP released

Summary. Processes in  RED CAPITALS, products in blue lower case:

Glucose > GLYCOLOSIS > pyruvate > FERMENTATION > lactic acid + ethanol

Describe examples of horticultural situations leading to anaerobic respiration.

Anaerobic respiration is used intentionally when making alcohol, a natural by-product of anaerobic respiration in some plants. For example, barley is fermented to make beer.

Anaerobic respiration is also used by plants as a way of surviving in bad conditions, but it will eventually kill the plant. Some conditions that leads to this are:

  • Waterlogged or compacted soils in which oxygen can’t be accessed.
  • Fruits and vegetables continue to respire after picking. If packaged with too little oxygen, they will respire anaerobically, which causes spoiling.
  • Germinating seeds use anaerobic respiration because they don’t have the ability to take in oxygen yet.

3.4 Explain the movement of water and solutes through the plant.

Explain how water and solutes enter, move through, and leave the plant. To include: apoplast, symplast, endodermis, Casparian strip, transpirational pull, root pressure, capillary action, guttation.

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How water enters and moves through the root:

  • Water is drawn into the root hairs by osmosis
  • Water moves through the cortex through the symplast pathway or the apoplast pathway
  • The apoplast pathway ­– water moves through the space outside the plasma membrane. Plant materials can diffuse freely through it, except when blocked – eg by the casparian strip in the roots. CO2 needs to be made soluble in the apoplast before it can travel through the symplast.
  • The symplast pathway – water moves through the cytoplasm around the vacuoles of adjacent cells.
  • Most water travels by the apoplastic pathway, but is then stopped by the casparian strip.
  • Water is pushed across the root by root pressure.

How water moves up the stem:

  • Transpirational pull –  because water evaporates from the surface of the leaves, more water moves to the surface to replace it, and more follows on behind.
  • Capillary action – this is when water can move upwards through a narrow tube despite gravity. It occurs because adhesion of water to the walls of the capillary is stronger than the cohesion between the water molecules. However, the cohesion is what keeps the water flowing.

Endodermis –   this is a single layer of cells around the cortex. It controls water movement into the plant.

Guttation – Transpiration does not usually occur at night because stomata are closed. This leads to a build up of pressure – root pressure – and causes droplets of water to appear on leaf tips or margins in the early morning – guttation. This happens most often in tropical climates.

Describe the significance of the following terms in relation to water movement in the plant: diffusion, osmosis, mass flow, capillarity, adhesion and cohesion.

Diffusion – this is the method by which oxygen in solution in the soil moves into roots, from high concentration to low. Also it enable carbon dioxide and oxygen to move in and out of the leaves through stoma.

Osmosis – this enables water to move into roots from a less concentrated solution (ie where there is more water) to a higher concentrated one. Osmosis is also how water moves across the leaves to the surface where it evaporates.

Mass flow – this is the movement of disolved nutrients through the phloem or water through the xylem. Nutrients are moved from the source to the sink (ie from where the nutrients are stored to where they are needed.) This direction changes throughout the year because sugar-needs change.

Capillarity (capillary action) – how water moves from the roots to the xylem (description above).

Adhesion – water molecules adhere to celluose molecules in walls of xylem

Cohesion – sticking together of water molecules so they form a continuous stream from leaves to roots.

Explain how environmental factors affect the rate of transpiration: temperature, water availability, relative humidity, wind speed.

  • An increase in atmospheric temperature leads to an increased rate of evaporation, decreased relative humidity outside the leaf and increased water potential gradient. Therefore  transpiration increases. (ie when it’s hot, moisture content around the leaf is reduced, so more water is drawn up the plant to replace it).
  • A decrease in water available to the plant leads to water stress which leads to closure of stomata therefore transpiration decreases.
  • A decrease in humidity leads to drier surroundings, gives a steeper water potential gradient leads to an increase in transpiration.
  • In still air, water that has transpired from a plant forms a vapour around the leaf and water potential gradient from inside to outside is slightly less. Transpiration decreases. (ie humidity increases around the leaf so more water is not drawn up the plant to replace it).

Describe the uptake and distribution of mineral nutrients in the plant: nutrients from soil solution/foliar feed, distribution in the xylem and phloem, active uptake against concentration gradient into cells by membrane carriers.

carrier proteins

Membrane carriers transporting proteins against the concentration gradient

Xylem transports water and mineral salts from the roots up to other parts of the plant, while phloem transports sucrose and amino acids between the leaves and other parts of the plant. Because the minerals need to move against the concentration gradient (ie from a low concentration to a high concentration) when moving from the soil to the root hairs, the plant uses active transport. This where membrane carriers in the membrane bind to the mineral on the soil side and then release it on the other side of the membrane inside the root.

Describe transport of sucrose in the phloem: mass flow hypothesis, phloem loading and unloading.

translocation

Mass flow hypothesis is the most likely explanation for how sap (sugar and water) moves through the phloem, but nobody is certain this theory is correct. The movement can be in either direction, unlike water moving through the xylem, which only moves from the roots and up the stem.

The sap moves from sugar sources (where sugar is stored) to sugar sinks (where sugar is needed), although where the sinks and sources are changes depending on need and time of year.

Mass flow hypothesis, the stages:

The movement of sap is called translocation.

  1. Phloem loading entails a sieve tube element picking up the sugar solution by active transport. This requires energy from ATP and is controlled by companion cells that are connected to the sieve tube elements.
  2. The sugar increases the concentration in the sap causing water to move into the sieve tube element from the xylem by osmosis.
  3. This creates pressure that pushes sap down the phloem to the sink.
  4. Once the sink is reached, the sugar is unloaded.
  5. The sap now has a lower concentration, so water leaves the phloem by osmosis, lowering the turgor pressure and stopping the movement.
  6. That water is drawn into the xylem by transpirational pull.

Describe the concept of sources and sinks in relation to: plant organs; seasonal changes.

During the growing season (spring, summer) the roots are sugar sources and the growing points are sugar sinks, because the leaves need sugar to grow. During dormancy, the leaves become the sugar source and the roots become the sinks, as sugar is stored.

3.5 Explain the interrelationships between photosynthesis, respiration and transpiration and describe their control in horticultural situations.

Describe how photosynthesis, respiration and transpiration are interrelated when optimising plant yield and/or quality in the following horticultural situations: outdoor planting; intensive glasshouse production; post harvest storage (manipulation of temperature and water loss, controlled and modified atmospheres, climacteric/non climacteric behaviour).

Outdoor planting:  light and carbon dioxide can’t be controlled in outside planting (but are free) water can be added. Transpiration is increased by high winds, high temperature, low humidity and light levels. Wide spacing of plants lowers humidity and consequently increases transpiration, it can also increase access of sunlight to plants. To maximise photosynthesis, plants need positioning in the ideal sunlinght for those plants (some plants prefer shade).

Intensive glasshouse production: carbon dioxide is usually the limiting factor (ie what limits photosynthesis), water must be supplied and light can be increased.

It’s possible to increase CO2 during the day (especially in the case of C3 plants). Humidity can be increased to reduce transpiration (to avoid wilting), but excess humidity can cause botrytis or other fungi to grow. Spacing plants further apart increases photosynthesis, by allowing them access to more light. However, this also increases transpiration, because the water can evaporate more easily, so more water will be needed required.

Post harvest storage: the ideal conditions to prolong storage life is a low temperature and O2 levels, and high CO2, this is to reduce respiration (respiration increases when temps and oxygen levels are high). Fruits transpire after harvest, but without connection to the plant, they can no longer replace that water, so humidity must be kept high to reduce transpiration. However, if too high humidity will encourage the growth of botrytis (mould).

So increasing CO2, decreasing O2, keeping the temperature cool, with humidity not too high or low maintains a good quality of fruit.

Modified atmosphere means the fruit product is sealed, but not manipulated.

In a controlled atmosphere the atmosphere is constantly monitored and adapted.

The effects of ethylene/nitrogen levels and humidity on fruit storage – ethylene starts the fruit ripening process and as the fruit ripens, more is produced which speeds up the process.

Climacteric – some fruits and vegetable continue to ripen after harvest, these are climacteric. Climacteric fruits are often picked before fully ripe, so they can store for longer and then ethylene is controlled.

Non climacteric fruits don’t ripen once picked, so must be picked at the ideal stage to eat.

Article on fruit storage

Good PDF on water loss.

Good article on respiration in fruits

3.6 Explain how plants are adapted to different habitats.

Explain how plants are adapted for: low light levels (shade plants – to include differences in leaf surface area, chlorophyll density, palisade layer); anaerobic conditions (bog plants – to include aerenchyma, pneumatophores); reduced water supply (xerophytes – to include xeromorphic adaptations in Pinus). To include ONE NAMED plant example for EACH adaptation.

Low light levels

Sun and shade leaves can exist within the same plant.

IMGP2565

Codiaeum variegatum

Red/purple chlorophyll – to absorb different frequency of light, eg Codiaeum variegatum.

Broader, thinner leaves – to catch more sunlight, eg Monstera deliciosa.

Shade leaves – horizontal leaves; low saturation point; low compensation point; less rubsico produced, more chlorophyll; photosynthesis is limited.

 

IMG_8252

Pneumataphores

Anaerobic Conditions

Aerenchyma – spaces or air channels in leaves, stems and roots that allows exchange of gases between shoot and root eg in Phaseolus vulgaris.

Pneumataphores – special aerial  roots, either grow down from stem or up from typical roots, they move air from where the plant can access it to where it is needed. Surface is covered in lenticels eg Black mangrove, Avicennia germinans.

Adapted stomata – that can discharge excess water through guttation eg Zantedeschia aethiopica

Carnivorous plants  trap insects for nitrogen which are unavailable in waterlogged soil. eg Drosera spp.

Prop roots for stability eg Ficus elastica.

Other adaptations:

  • Excreting of excess salt eg Spartina.
  • Capable of growing in acidic conditions eg Sphagnum.

 

Xerophytes

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Hairy leaves to reduce air flow and increase humidity

 

Hairy stems – to reduce air flow eg Opuntia erinacea

Spines instead of leaves – to reduce surface area and therefore transpiration. eg Opuntia chlorotica

Increased thickness of cuticle – a thick, waxy surface to the leaf eg Lophophora williamsii

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Leaves reduced to spines and thick stems for water storage

Water is stored in bulbs and fleshy stems – eg Saxifraga tridactylites

Other adaptations without examples: Fewer stomata, sunken stomata. Dormant during drought conditions

Xeromorphic adaptations in Pinus

  • Lateral root system with sinker roots – greater surface area to soak up water
  • Tightly assembled needle structure – slows air movement and evaporation
  • Needles covered with waxy cuticle to reduce transpiration

 

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RHS Level 3: Plant taxonomy, structure, and function Q2

I have taken the Level 3 myself, but I’m not a teacher, so if you notice any problems with the information, then please let me know in the comments below.

Question 1 here

2. Understand the structure and function of plant tissues and organs in the life of the plant.

2.1 Identify a range of plant tissues and describe their structure and function.

Identify and describe the structure and function of plant tissues, to include:

Simple tissues:

Parenchyma

These are thin-walled, living cells, unspecialised and therefore can be adaptable to different functions such as photosynthesis and food storage. They are found throughout the plants in stems, roots and leaves.

Collenchyma

These are living cells with thickened cell walls. They are used for support and regeneration, they are found in shoots and leaves.

 Sclerenchyma (fibres and sclereids)

This is support tissue, sclerenchyma cells contain lignin as well as cellulose. They are dead once mature. There are two types:

Sclereids are toughened gritty bodies, found in nut shells, peach stones and Camellia leaves. They can also be found in the phloem, xylem and cortex of the stem.

Fibres are elongated cells that interlock to provide support for the plant. They are in the stems, roots and leaves.

Epidermis

The epidermis is made of live cells with thin cell walls. It may have a cuticle and is to protect the plant and prevent water loss. It is found on the entire surface of the plant, except where the epidermis is replaced by the periderm, a corky layer that performs the same function in older plants.

Meristem (cambium)

This is located at the growing points, and consists of actively dividing cells to increase the size of the plant – either primary or secondary thickening. Located at growing points. Its cells are thin walled and living.

 

Complex tissues:

Xylem (vessels, tracheids, parenchyma, sclerenchyma fibres)

Vessels are made up of dead cells containing lignin, that have perforated ends. They carry out the main part of water transportation but are in angiosperms only.

Tracheids are also made of dead cells and don’t have perforated ends, but overlap instead. They are narrower than vessels. They are found in all vascular plants and also transport water.

Parenchyma (see above for more) in the xylem are involved in the storage of carbohydrates and oils.

Sclerenchyma fibres (see above for more) are dead cells for support and structure.

Phloem (sieve tube elements, companion cells, parenchyma, sclerenchyma fibres)

Sieve tube elements are dead cells. They are long tubes with sieve plates at either end. Perforated. They transport sugar around the plant to where it is needed.

Companion cells are smaller, live, and contain nuclei for controlling the sieve tube elements.

Parenchyma acts as packing for the other types of cells, surrounding them and helping with transport.

Sclerenchyma fibres provide structural support for the plant.

 

Secondary tissues:

Periderm (outer bark) is the corky outer layer of a plant stem formed in secondary thickening or as a response to injury or infection.

Phellem (cork) is a tissue formed on the outer side of phellogen. It is composed of dead cells and is used for protection.

Phellogen (cork cambium) is the meristematic cell layer that creates the periderm. Cells that grow inwards from there are termed phelloderm, and cells that develop outwards are termed phellem or cork.

Phelloderm (secondary cortex) is the layer of tissue, often very thin, produced on the inside of the cork cambium in woody plants. It forms a secondary cortex.

Secondary phloem (inner bark) is a type of phloem that forms from the vascular cambium during the secondary growth. The secondary growth is responsible for the growth in girth in plants, especially trees.

Vascular cambium is the meristematic tissue in between the xylem and phloem that creates new xylem and phloem cells for secondary growth in the stems and roots.

Secondary xylem is created during secondary growth and is for an increase in width of the stem, rather than height.

Radial parenchyma (ray) is for the transport of water and goes across woody stems.

Annual rings are concentric circles found in the trunk of a tree. They show the amount of wood produced during one growing season. The rings are caused by a change in density of cells throughout the year (see below).

 

Describe the process of secondary thickening in the stem of a woody perennial (e.g. Tilia), from primary tissues to two years old.

cross section young stem

Secondary thickening in young stem

cross section mature stem

Secondary thickening in a stem

cross sectioon root

Primary thickening in a root

Secondary thickening (aka secondary growth) is whereby a plant’s stems or roots increase in width, whereas in primary thickening, a plant increases in length. Most seed plants (ie not ferns or mosses which have spores not seeds) have secondary thickening, but notable exceptions are monocots (eg orchids, irises, grasses). There are a few monocots that have a different type of secondary thickening, not described here (eg Palms).

Secondary thickening occurs in two lateral meristems: vascular cambium and cork cambium, and is similar in both stems and roots. Vascular cambium (comprising of fascicular cambium and inter-fascicular cambium) is only found in herbaceous perennials, and is located within the primary phloem and primary xylem in the vascular bundle. The cells of the vascular cambium divide to create new phloem and xylem cells, known as secondary xylem and secondary phloem.  It produces xylem on the inside and phloem on the outside.

In a woody stem (such as the Tilia example) the secondary xylem contains lignin which forms the wood in the stem. In woody plants there is also the cork cambium which is the outermost lateral meristem. It produces cork cells which create a waterproof covering. It also creates a layer of cells called the phelloderm, which grows inwards from the cork cambium.

Each year a layer of xylem and phloem are added during the growing season. The interior xylem die off as the plant gets bigger, they then fill with resin and supply structural support only. This is known as the heart wood. The still living xylem layer that transports water is known as the sapwood. The exterior layers of phloem are crushed against the cork cambium, caused them to break down. This means the plants contains increasing amounts of old xylem, but little older phloem.

The secondary thickening cells grown earlier in the season, in spring, do not have such thick cell walls, leading to less dense wood. Later in the year, the wood is denser, this leads to the annual rings.

2.2 Identify and describe types of inflorescence.

Note: an inflorescence is not the same thing as a flower. A flower has a single carpel.

Note: the diagrams are for clarity, it isn’t stated that these are needed, only necessary to describe them.

Monopodial/ Indeterminate – not having all the axes terminating in a flower bud and so potentially of indefinite length.

Sympodial/determinate – the terminal bud forms a flower and so ceases to grow.

Identify and describe types of inflorescence, to include:

raceme

Raceme (Digitalis) – a tall, thin inflorescence where the flowers are attached to the peduncle by smaller pedicels. Monopodial/ Indeterminate.

spike

Spike (Acanthus) – a tall thin inflorescence where the flowers are attached directly to stem without pedicels. Monopodial/ Indeterminate.

umbel

Umbel (Allium) – flowers are attached from a single point on a stem with pedicels of equal length, creating a dome shape. Monopodial/ Indeterminate

corymb

Corymb (Sambucus) – flowers are attached to a stem with pedicels of different lengths creating a flat inflorescent head. Monopodial/ Indeterminate.

cyme

Cyme (Myosotis) – terminal bud dies and growth is from the lateral bud. The first flower to open is at the top or middle. Sympodial/determinate.

panicle

Panicle (Syringa) – a number of racemes connected to central peduncle. Monopodial/ Indeterminate

capitulum

Capitulum (Helianthus) – many small flowers grouped together as if making one single flower. Monopodial/ Indeterminate

verticillaster

Verticillaster (Phlomis) – a ring of flowers, then a small length of bare stem, then another ring of flowers. Sympodial/determinate.

2.3 Describe plant adaptation for pollination.

Describe how the flowers/inflorescences of named plants are adapted or pollination by different named agents, in relation to flower structure/shape, position, colour, scent, provision of food, flowering time, mimicry.

(to include:

Wind – eg Poa annua. The pollen is small and inconspicuous, but numerous so that it can carry on the wind and allows for the large amount of pollen that is wasted. The stigma are long and feathery, hanging outside the flower to catch the pollen. With no insects involved, there is no need for food, scent, bright colours or mimicry. Usually green.

Bee – eg Aster. These flowers provide nectar that is difficult to access to ensure the bee picks up the pollen. The flowers occur in spring through to autumn when bees are active, and during the day. The flowers are usually large, solitary and upright. The colour is usually blue, violet or yellow and often has landing guides in ultra violet. If a bee is drawn to a red flower, there is usually yellow at its centre. They have a strong sense of smell, so flowers are usually fragrant. Flowers have been known to mimic bees, eg bee orchid.

Moth – eg Ipomea alba. These usually flower at night when moths are active, providing scent either evening or early morning. The flowers are large, tubular and white, the scent is strong and sweet. I haven’t found evidence of mimicry.

Butterfly – eg Buddleja. Flowers have a cluster of tubular flowers, in the case of Buddleja, they flower late summer when there is less competition. The flowers have a landing platform, are held up high and are usually red or purple. They have no scent and don’t use mimicry.

Fly – eg Dranunculus vulgaris. These don’t provide any food. Their structure consists of a single spadix, central. These flowers are brown, red or orange and mimic faecal matter or rotting meat in both smell and appearance. Flowering time is usually summer and into autumn.

Bird – eg Penstamon barbatus, Columnea. These flowers are large, tubular and positioned beneath leaves (which may have red patches to guide the birds.) They provide a dilute nectar, but have no scent or mimicry. They flower in the summer.

Draw and label diagrams to show the structure of grass and legume flowers and relate to mode of pollination.

lolium

Grass Flower

legume flower

State the meaning of cross pollination and self pollination. Explain the benefits of EACH using plant examples.

Cross pollination – eg Ilex aquifolium. The transfer of pollen from the anther of one flower to the stigma of a flower of a different plant in the same species. This form of pollination results in more variety and chance to adapt to changing landscape/predators etc.

Self pollination  – eg Senecio vulgaris. The transfer of pollen grains from the anther to the stigma of the same flower, or to the stigma on a different flower, but on the same plant. With this type of pollination the plant does not need to expend energy attracting pollinators and is more likely to make seed.

State the means by which cross pollination is favoured:

Self incompatibility means that hormones stop self-fertilisation, by not allowing a pollen tube to grow from the pollen. Eg Trifolium repens.

Flowering time – when a plant is monoecious, containing both male and female flowers on the same plant, they usually don’t open at the same time.

Heterostyly – 2 or 3 morphological types of flowers exist in the population so it’s less likely that self-pollination is possible.

Protandry is where the anthers mature in a flower first, so that both female and male parts aren’t functional at the same time. Eg Lamium album.

Protogyny is where the stigmas mature first , so that both female and male parts aren’t functional at the same time. Eg Hyacinthoides non-scripta.

Dioecious plants have the male and female sexual organs on completely different plants (ie there are male plants and female plants) so self fertilisation is impossible.) eg Skimmia japonica.

2.4 Describe fertilisation and the structure of fruits.

Describe the process of fertilisation, to include: pollen grain, pollen tube, two male gametes, ovary, ovule, micropyle, ovum/egg cell/ female gamete, endosperm nucleus, zygote, double fertilisation.

Note: diagrams are for clarity, it isn’t stated in the syllabus that they are needed.

Description

 

 

When the pollen cell lands on the stigma, a pollen tube starts to grow, it contains two nuclei: the tube nucleus (also known as vegetative cell) and the generative nucleus (aka generative cell). The tube nucleus causes the pollen tube to grow down the style to the ovary. The generative nucleus divides to form two gametes.

When the tube nucleus reaches the ovule, the first male gamete fuses with the female gamete to form the zygote. The second male gamete fuses with the polar nuclei (two specialised nuclei within the ovule, aka the central cell). This fusion produces the endosperm which forms a food store. This is called double fertilization.

State the advantages and limitations of fertilisation resulting from cross pollination and self pollination.

Cross pollination advantages: results in more variety and chance to adapt to changing landscape/predators etc. Creates hybrid vigour and healthy plants. The seeds are more often viable.

Cross pollination limitations: requires other plants of the same species flowering at the same time. As a crop, this means more than one plant must be planted. It requires pollination by an agent (eg an insect or the wind) which can be limited by weather or pesticides. It needs to produce a lot more pollen to ensure pollination.

Self pollination advantages: more likely to result in seed being produced. Less pollen is needed because it doesn’t have so far to travel, so there is less waste. The plant will remain stable so if it is adapted to the environment it will continue to be so. If it forms a genetic defect, that will continue to the next generation.

Self pollination limitations: it won’t adapt to environment in time. The plant will weaken over time.

Describe the relevance of cross/self pollination to horticulture, to include: top fruit production (apple), vegetables (maize, cucumber), the use of cross/self pollination in the production of F1 hybrids.

The Cucumber is monoecious. If there are not many insects (eg lack of bees due to pesticides or poor weather) the female flowers need to be pollinated by hand or fruit will not form. This will take a great deal of time and inconvenience.

Maize is fine to be cross pollinated, and this is generally a good thing. Unless it is a GM crop, then it must not cross pollinate with neighbouring fields. A buffer zone is needed, alternatively have only GM crops and non GM cultivars that flower at different times.

State the advantages of F1 hybrids.

  • They are uniform, every plant is the same, with the exact same genetics and phenotype. So not only the appearance, but also the yield, health and disease resistance are uniform.
  • The offspring of the plants always different (a benefit to those who own the copyright of the plant, since they cannot be easily reproduced. A disadvantage to those who grow the plants)
  • They have ‘hybrid vigour’, which means they are strong plants, with larger flowers and better health.

 NO DETAILS OF THE GENETIC BASIS OF F1 HYBRIDS REQUIRED.

Describe the development and structure of a true fruit: pericarp exocarp/epicarp, mesocarp, endocarp).

Following on from the double fertilization shown above. After double fertilization,  one or more ovules becomes seeds, the zygote becomes the embryo of the seed, and the endosperm mother cell becomes the endosperm – nutrition for the embryo. As the development to seeds occurs, the ovary ripens and the ovary wall, the pericarp, will become either fleshy (eg in drupes) or hard (eg nuts). Usually the pericarp splits into three layers, the epicarp (outer layer aka exocarp) mesocarp (middle layer) and endocarp (inner layer). The epicarp usually forms the rind of a fruit, the mesocarp usually forms the edible flesh and the endocarp is the layer closest to the seed. The nature and toughness of each layer varies depending on how the seed is to be dispersed – ie whether the plant wants the fruit to be eaten, and by what or if it wants the seed to be carried on the wind and so on.

The rest of the flower (eg petals, sepals) either fuses with the ovary and becomes part of the fruit (known as an accessory fruit) or falls off.

Recognise and describe the following fruit categories and their fruit examples:

Note: diagrams for clarity etc etc

Dry dehiscent:

legumecapsulefollicle

Legume – multiple seeds in a line, twists and expels

Capsule – pores at top or open to release seeds

Follicle – splits only along one side

Dry indehiscent:

 

 

Nut – hard fruit containing a single seed,

Achene– single seed that nearly fills thin pericarp

Succulent/fleshy:

 

 

Drupe – 1 carpel, hard endocarp,

Berry – 2 or more carpels, many seeds

Name ONE plant example for EACH fruit example.

  • Legume – pea, lentil
  • Capsule – Nigella, orchid
  • Follicle – Consolida, poppy
  • Nut – walnut, hazelnut
  • Achene – sunflower seed, sycamore seed
  • Drupe – apricot, cherry
  • Berry – banana, tomato

Describe what is meant by a false fruit.

A false fruit is formed from other parts of the plant as well as the ovary, especially the receptacle, such as the strawberry or fig.

Draw and label a diagram of a pome from a named plant.

pome

 

Latitudinal Diversity: my theory

IMG_0539

Growing up in England I was fond of woods, but when I went to live in a cloud forest (which is a tropical rainforest in the mountains) in central America, three things stood out as massively different to the forests I knew in England.

  1. There was a huge variety of plants. In the UK, most woods have the same few trees and plants repeated – oaks, birches, sycamores and ash for the trees; bracken and brambles at ground level. In the cloud forest, almost no tree or ground level plant was repeated, to put it technically, it had a greater species diversity.
  2. The ecosystem was far more interactive than I was used to. Trees were laden with epiphytes, and there was barely a leaf without fungus, insects or a virus. It seemed that almost every plant surface had something else growing on it. In the UK, there are only occasional galls and nests, the odd bit of moss, and epiphytic plants are rare.
  3. The variety in colour, shape and habit of the cloud forest plants was huge, at every turn I uncovered new leaf shapes and colours, whereas in the UK there are mostly green leaves growing in a few different patterns. (Note that I am talking about native woods in the UK here, we import many different garden plants from other countries and those have a greater variety of shapes and colours)

2413

The differences were so immense and fascinating, I started researching as soon as I was able to.

What I discovered is that there is higher diversity of species and greater number of species (species richness) the closer to the equator you get. This is known as the latitudinal diversity gradient.

Science hasn’t quite explained it yet, but there are theories, I also have my own which I want to share. I’ve not come across anyone suggesting the same explanation, although the chances someone has, somewhere.

IMG_2401

An aroid flower in Ecuador

The Facts

  • The greatest number of species for the major taxa – flowering plants, ferns, mammals, birds, reptiles, fresh water fish, amphibians, insects and snails – are in the tropics.
  • Species diversity and richness increase as you travel towards the equator.

For example: The 1950s doc ‘Evolution in the Tropics’ by Dobzhansky stated that Greenland had 56 species of birds, New York 195, Guatemala 469, Panama 1100 and Colombia 1395.

  • While tropical moist forests have the greatest diversity, even tropical savannahs and grasslands are more diverse than similar landscapes in temperate areas. This is especially important, because it suggests that the difference is not just due to terrain, but also latitude.
  • Recent research suggests that there are more fungi species in the tropics too. There isn’t enough known about diversity of bacteria species across the globe.
IMG_1454

A pink-leaved climber growing within a green-leaved plant.

The Theories

There are a number of theories. They include factors such as the Ice Age, which affected the poles greatly and the tropics less so; the size of the tropics compared to other areas; and the higher levels of predation so that the fight to survive drives evolution. All of the theories are contested, a few can explain part of the difference, but not all. Some are circular, eg. there is greater species diversity, because there are more competitors for food sources.

For more detail try A Neotropical Companion by Kricher (where much of my info comes from) or Wikipedia which has a number of other theories too.

IMG_3408

My Theory – it’s all about the small things

One notable difference about the equator is that there is little change in light and temperature. Where as in the UK the nights are very long in the winter and short in the summer, in Central America it gets dark at 5pm all year round. Temperatures are also more stable; slightly closer to the poles, the more temperatures can fluctuate from minus degrees in the winter to scorching heat in the summer. In the tropics, it’s pretty much hot all year round – or in high up cloud forests it’s consistently warm.

This lack of change makes some difference to larger animals and plants since they don’t need to go into dormancy they can grow and reproduce all year round. But their life cycles are still fairly slow, reproducing once a year or every few years. However, this difference is far more significant when it comes to very small organisms because their lifecycles are so much shorter, and they are more affected by changes in temperature and light. Those quick lifecycles mean they can mutate, adapt and evolve at far greater rates too.

So I believe that is why there is greater species richness, abundance and diversity of small organisms at the equator, but the difference is not so pronounced in larger organisms, so what else is a factor?

I believe that it is the species diversity of smaller organisms that directly causes the diversity in larger organisms through parasitism and symbiosis. Parasitism drives evolution and symbiosis aids survival.

IMG_0734

First, some terminology

Parasitism

Parasites are usually insects, fungi, small plants or bacteria and are harmful to larger plants and animals, taking what they need without concern for the host. Organisms often evolve to protect themselves from threat. Parasites are a threat. The more threats, and the more varied the threats, the more animals and plants need to evolve to fight them. This is a common, but as yet unproved theory.

For example: If a plant has a mutation of hairy leaves that deter insects, then in an environment with many insects, that mutation is more likely to lead to the survival of that plant and the proliferation of the hairy-leaf gene.

Symbiosis

Symbiosis between insects and fungi?

Symbiosis

Symbiotic relationships tend to drive specialisation and help organisms to survive. Because the rainforest is so crowded, there is a constant battle for nutrients, space and light, so forming an alliance is beneficial. Through the generations, that alliance tends to become tighter and more exclusive. Symbiosis can be seen between many animals, plants, fungi and bacteria.

For example:  Ants forming a protective army inside an acacia tree and fighting off any animal that comes to eat it. Or aroid flowers that are only pollinated by one type of fly, so they evolve to give off a scent that attracts that specific fly (often rotting meat). In a crowded rainforest, if all plants targeted all insects, then many plants would get missed and never pollinated. Forming a symbiotic relationship is like putting an address on a letter, instead of flinging up in the air and hoping someone reads it.

The Small Things

Fungi

Fungus in Ecuador

The ideal conditions for fungi to grow are warm moist ones. In the UK fungi live in the ground unseen, all year round. Then in autumn they produce fruiting bodies – ie the mushrooms that enable them to reproduce – that’s because the soil has warmed over the summer and there’s plenty of rain. In the tropics, the soil never cools, and humidity is constant, this means the reproductive phase can also continue all year round.

 

 

 

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Why fungi are important to larger organisms – fungi can be both beneficial and harmful to plants. Some fungi form a symbiotic relationship with them, growing on their roots and enabling them to take in nutrients that the plants would struggle to access on their own – these are known as mycorrhizal fungi. This harmoniuos relationship can take centuries to form. This is one reason why it’s so difficult to regrow plants on an area that has been de-forested, because the mycorrhizal fungi are no longer there, and the plants can’t access nutrients without them.

Fungi can also be parasitic and break down healthy wood. That’s what fungus does essentially, breaks stuff down, it’s a decomposer – that’s good when it’s breaking down dead matter to release the nutrients, but bad when it breaks down living material.

Bacteria

IMG_1502

Either bacteria or insect galls

Like fungus, the ideal conditions for bacteria to grow is warmth and moisture, they are also sensitive to light changes. So, the equator, and especially the rainforest at the equator, has perfect conditions.

Why is bacteria important to larger organisms – like fungi, bacteria can be both good and bad for plants. Some bacteria work in a similar way to fungi, attaching to roots and breaking down nutrients (specifically nitrogen) into a form the plants can absorb. And, like fungi, bacteria can be harmful, causing diseases.

Insects

 

 

 

Insects also like warmth and wet. We know in the UK if there is a warm summer followed by a lot of rain, then the insects will increase. In the tropics, those are the constant conditions. Even in drier areas, the consistency of temperature is enough to maintain insect populations.

Why insects are important to larger organisms – insects can also be a blessing or a burden to plants. Leaf cutter ants will ravage a tree, defoliating it, but as described above, ants can protect trees too. The photo above shows a number of insect galls on plants, where parasitic insects alter how a plant grows to create their habitats.

 

 

 

 

 

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Conclusion

To put it simply…

According to my theory,

  • Smaller organisms thrive in stable environments where light and temperatures are fairly constant all year round.
  • The resulting high numbers and quick life cycle leads to greater opportunities for them to mutate and evolve.
  • Smaller organisms affect the number and diversity of larger organisms through parasitism and symbiosis.
  • Parasitism drives species richness, by forcing larger organisms to evolve to survive. Symbiosis aids survival and promotes specialisation.
  • So the numbers and diversity of larger organisms increases.

 

1

Mosses and lichen on a branch

 

RHS Level 3: Plant taxonomy, structure, and function

  1. Understand the Plant Kingdom and the taxonomic hierarchy.

1.1 Describe the major groups of the Plant Kingdom.

List the main groups within bryophytes, pteridophytes, gymnosperms and angiosperms.

This is quite an archaic way of grouping plants. The kingdom Plantae is usually divided into 10 divisions, listed below, with the groups in the syllabus in bold. Gymnosperms consists of Pinophyta, Cycadophyta and Ginkgophyta. Angiosperms = Magnoliophyta:

  • Anthocerotophyta – hornworts
  • Marchantiophyta – liverworts
  • Bryophyta – mosses
  • Lycopodiophyta – club and spike-mosses
  • Pteridophyta – ferns and horsetails
  • Gnetophyta – 3 extant genera of woody plants
  • Cycadophyta – cycads
  • Ginkgophyta – Ginkgo
  • Pinophyta/Coniferophyta – conifers
  • Magnoliophyta – flowering plants
divisions-for-rhs

Plant Characteristics

Describe and compare the structural and reproductive characteristics of: mosses, ferns, conifers and flowering plants in relation to their adaptation to terrestrial life.

DETAILS OF ALTERNATION OF GENERATIONS AND HAPLOID/DIPLOID STRUCTURES ARE NOT REQUIRED.

I’ve written about these four groups previously, the information about structural and reproductive characteristics is in the first two paragraphs of each blog

  1. Mosses
  2. Ferns
  3. Conifers
  4. Flowering Plants

Brief description of reproductive characteristics:

Bryophytes – have sporophyte and gametophyte stages. Gametophyte is dominant.

Pteridophytes – have sporophyte and gametophyte stages. Sporophyte is typical fern, gametophyte is small and rarely noticed.

Gymnosperm – have male and female cones. Male cones drop pollen which is carried by wind.

Angiosperm – have flowers that may be dioecious, monoecious or hermaphrodite. Usually wind or insect pollinated (but other methods of pollination exist).

1.2 Describe features of plant classification and nomenclature relevant to horticulture.

State the hierarchy of botanical units and explain how and when they are used.

To include: family, genus, species, subspecies, varietas, forma.

To include ONE NAMED plant example for EACH of the above terms showing how it is written.

Family

Have the ending -aceae (many family names were recently changed to conform to this). Plant families are usually named after the biggest or most well known genus in that family. eg Euphorbiaceae, the family that the genus Euphorbia is in.

Genus

Genus is a subdivision of family. The genus of a plant is used as the first part of its binomial name, and is always capitalised. It should be written in italics (or underlined). eg Euphorbia.

Species

Species is a subdivision of genus. The species of a plant is used as the second part of its binomial name and is never capitalised. It should be written in italics (or underlined). eg characias  (as in Euphorbia characias.)

Subspecies

Recommended abbreviation is subsp. but ssp. is sometimes used. Subspecies are written in small italics, but the word subsp. is not. A subdivision of species. Plants within different subspecies but within the same species are capable of interbreeding, but don’t due to geographical separation. eg Euphorbia characias subsp. wulfenii.

Varietas

A subdivision of species, similar to subspecies (and the two terms often overlap) however, different varieties within a species may geographically overlap, unlike subspecies. Recommended abbreviation is var. Varieties are written in italics, but var. is not. eg Malva alcea var. fastigiata.

Forma

If a plant shows uncharacteristic appearance of its species (such as habit or colour) then it can be known as a different form. These differences are usually due to environmental reasons and won’t be passed to the next generation. Recommended abbreviation is f. The form is written in italics, but f. is not. eg Vinca minor f. alba.

Explain the meaning and use of the terms: cultivar, Group, trade designation (selling name), Plant Breeders’ Rights, interspecific, intergeneric and graft hybrids, naming authority.

To include ONE NAMED plant example for EACH of the above terms, showing how it is written.

Cultivar: This is short for ‘cultivated variety’ and refers to plants that have been bred for their characteristics. The names are often chosen as a selling point, for example using somebody’s name, making them a good present for people of the same name. eg Clematis ‘Willy’ (note the cultivar name is capitalized, in single quotes and not italicized. Because of the complexity of cross breeding across species, the species of a cultivar is only sometimes used.)

Group: If several cultivars are similar, they can be grouped together to make customer selection easier. eg Lilium Darkest Red Group (note the group is capitalized, not italicized, and not in quotes.)

Trade designation: Cultivar names cannot be legally protected. If a plant breeder wishes to keep sole legal rights to a plant, then he/she uses a trade name. This a commercial synonym that is legally protected. eg Rosa FASCINATION = Rosa ‘Poulmax’. (note: the writing method for ‘Fascination’ changes, sometimes it is in quotes, like a cultivar; other times it is in square brackets. The correct notation is all in capitals, not italicized, not in quotes, often in a different font.)

Plant Breeders’ Rights: Breeders using a Trade designation have Plant Breeders’ Rights which are recognised internationally. If you own the rights to a cultivar, it cannot be bred by anyone else without your permission. If somebody buys one specimen of your cultivar, you still have exclusive rights to all propagation material of that plant: seeds, cuttings etc.

Interspecific, intergeneric and graft hybrids: Unlike with animals, plants can be bred across species and genera. Plants of different genera can, in some cases, be grafted together, occasionally this will lead to a mixing of cells where the scion and the rootstock meet, this is not a true hybrid. It is also known as a graft chimaera.

examples

  • Interspecific hybrid –  Mahonia × media (bred from Mahonia lomariifolia and Mahonia japonica, note the ‘x’ in the middle and new specific epithet.)
  • Intergeneric hybrid× Cupressocyparis leylandii (bred from Cupressus macrocarpa and Chamaecyparis nootkatensis, note the ‘x’ at the beginning and the genus which is a combination of the parents’).
  • Graft hybrid – +Laburnocytisus ‘Adamii’, (a graft hybrid between Laburnum and Cytisus, note the ‘+’ at the start and genus which is a combination of the parents’.) This graft contains flowers of Laburnum and Cytisus (ie both yellow and purple) but also flowers that are a pinky colour, a mix of the two.

Naming authority: The International Cultivation Registration Authority is a naming authority, responsible for seeing that cultivar names are not duplicated. The naming authority can also be the person or organisation who first name a species. (thanks to sam c for this)

State the significance of the ICN (The International Code of Nomenclature for algae, fungi and plants) formerly ICBN (International Code of Botanical Nomenclature) and the ICNCP (International Code for Nomenclature for Cultivated Plants) in the naming of plants.

The ICN (The International Code of Nomenclature for algae, fungi and plants) A code that governs plant discoveries in the world – ensuring that plants aren’t given different names by different discoverers, or that already named plants aren’t given new names without reason.

International Code of Nomenclature website Contains complex set of rules to standardise naming and classification eg changing all plant families to end in -aceae, Compositae > Asteraceae.

ICNCP (International Code for Nomenclature for Cultivated Plants) – a code that governs the naming of newly created cultivars.

Cultivated Plant Code

(For more information about the latest ICNCP, there’s an interesting article on Gardening Wizards, here)

Explain the reasons for name changes: reclassification (scientific research, new discovery), changes in nomenclature (rule of priority), incorrect identification.

To include TWO NAMED plant examples for EACH.

Reclassification (scientific research, new discovery)

  1. With advances in DNA technology, African Acasias were found to not be related to Australian Acasias. Australian Acasias have kept their name, while African have become Vachellia or Senegalia.
  2. Coleus became Solenostemon, but was then found to be part of the Plectranthus genus. Plectranthus scutellarioides used to be Coleus blumei.

Rule of priority

This is where a plant is discovered to have been named previously, and its old name is found on record. When an existing name is discovered, the plant should revert to this name, but occasionally, if the new name is far more familiar it will be kept.

  1. Platanus ×acerifolia was the name of the London Plane, but this name was recorded in 1805 and it was discovered later that an earlier name of Platanus ×hispanica had been recorded in 1770. Therefore Platanus ×hispanica became the official name.
  2. Festuca subgenus Schedonorus was moved to the genus Lolium and its name became Lolium subgenus Schedonorus.

Incorrect identification

Sometimes a name change is due to a simple mistake, when one plant becomes mixed up with another.

  1. Archontophoenix cunninghamiana was for a long time incorrectly sold as Seaforthia elegans.
  2. Syzygium australe was often sold as Syzygium paniculatum

 Explain how plant names can indicate: plant origin, habitat, commemoration, colour, growth habit, leaf form.

To include TWO NAMED plant examples for EACH. 

It is often the plant species that indicates origin, colour etc, but not always (see below). The Latin will only refer to one characteristic (when Latin plant names were first used, botanists tried to include every characteristic, leading to ridiculously long names, then Linnaeus reduced it to two).

Plant origin: Mahonia japonica (Japan), Arum italicum (Italy)

Habitat: Clematis alpina (alpine plants), Pinus sylvestris (wood or forest)

Commemoration: Photinia fraseri (John Fraser1750-1811 nurseryman), Weigela (Christian Weigel 1749-1831 German botanist)

Growth: Briza maxima (large or largest), Vinca minor (smaller)

Habit: Cotoneaster horizontalis (growing horizontally), Phlomis fruticose (shrubby)

Leaf form: Acer palmatum (palmate leaves), Ilex aquifolium (pointed leaves)

Plant Families: Araceae (aka Aroids or Arums)

Zantedeschia Inflorescence

Zantedeschia Inflorescence

A Few Basic Facts

  • Aroids are monocots in the family Araceae (aka arum family), in the order Alismatales. Most other families in this order contain tropical or aquatic plants, eg Hydrocharis and Saggitaria.
  • Araceae has 104-107 genera. The largest genus is Anthurium with over 700 species.
  • Location: Latin American tropical regions have the greatest diversity of aroids, however, they can also be found in Asia and Europe. Australia has only one endemic species – Gymnostachys.
  • Habitat: Aroids can be aquatic (water), epiphytic (air) and terrestrial (ground). Most are tropical, but there are also arid and cold loving aroids.
  • Distinctive features: All have an inflorescence (a structure containing a group of smaller flowers) which consists of a spadix (always) and a spathe (sometimes).
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Aroid Flowers

  • Aroids can be hermaphrodite (each flower is both male and female), monoecious (male and female flowers on the same spadix) or dioecious (male and female flowers on completely different plants).
  • This family contains one of the largest flowers (Amorphophallus titanium, the titan arum) and the smallest (Wolffia, duckweed).
  • Some aroid leaf and inflorescence shapes:
Aroid Leaf Shapes

Aroid Leaf Shapes

Leaves

Aroid Leaves

Fruits

Aroid Fruits

Adaptations

Like many tropical families, aroids have evolved a number of adaptations to stay healthy and propagate. Some examples of adaptation:

  • The spathe protects the flowers and in some cases is used to trap insects for pollination. It is not a petal, but a modified leaf. Many spathes turn green and photosynthesize after flowering has finished.
  • Aroids have different types of roots adapted to their purpose. They have different adventitious roots  for climbing, attaching to rocks or taking in water.
  • Many tropical species have shiny leaves to deter the mosses and lichens that grow in abundance in the rainforest.
  • Smell is used by many species to attract pollinators. The smell of rotting meat, fungi and excrement is used for flies and beetles. Fragrant scents are used to attract bees.
  • In many species the spadix actually heats up and can reach 25°C, even in near freezing conditions. This increases the release of smells to attract pollinators. The heat also makes visiting insects more active.
  • Aroids that want to attract flies and beetles often have a warty, hairy, twisted appearance, with dark colours. This is to mimic the effect of dead animals, fungi or excrement.
  • In some species, leaves may change shape from juvenility to adulthood – changing from variegated to unvariegated, pale red to green, or altering the number of lobes of the leaf. Colour change may deter animals from feasting on the fresh young leaves by making them look less leaf-like.
  • Most species in Araceae have tubers or rhizomes, this means a damaged plant has the food storage and ability to grow new shoots from many points beneath the ground. Some aroids have other means of vegetatively propagating themselves, such as bubils and offsets.
  • A number of aroids are poisonous, some are edible. Aroids have evolved poisons in some species as protection. Those that are edible did not evolve to be eaten by us, rather we have evolved to be able to eat certain plants.

Reproduction

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Male and Female Flowers

Because many aroids are monoecious there is a danger of self-pollination. While self-pollination is easy (a guaranteed fertilisation), it leads to less genetic variety and less ability to adapt to changes in the environment. Aroids are particularly variable plants, in one small area of the Thai Peninsula 22 distinct varieties of the plant Aglaonaemia nitidum f. curtisii were found. However, in order to achieve this variation, the plant needs to cross pollinate reliably. It does this by being protogynous, meaning the female flowers on an inflorescence ripen first and then later male flowers produce pollen.

The Generic Process for Monoecious Aroids

A beetle, fly or bee (hopefully covered in pollen) is attracted by the scent given off by the heated spadix. The insect flies around inside the spathe, lands on the slippery surface and falls into the gap between the spadix and spathe. At this point only the female flowers are mature, and the  insect, made more active by heat from the spadix, moves about bumping into the flowers and depositing the pollen. Now, the insect has fulfilled the first part of its function, the aroid would like it to pick up pollen from the male flowers. However, the male flowers will not ripen for a day or so yet, so the insect needs to be held hostage. The slippery spathe ensures that the insect can’t escape, it is given sustenance in the form of nectar. Once the male flowers are ready and producing pollen, the slippery surface of the spathe breaks down, allowing the insect to escape. As it flies away it bumps into the male flowers, picking up more pollen to take to the next plant of the same species that it comes to.

Two Specific Examples of Monoecious Reproduction

Philodendron auminatissimum: Sometimes the pollinating insect can outstay its welcome, perhaps damaging flowers or laying eggs. This Philodendron has overcome the problem by shrinking the spathe after the male flowers have become active. This means that the beetle must leave or become crushed.

Arum nigrum: This arum doesn’t trap visiting flies, it merely confuses them. The hood of the spathe hangs over the spadix, obscuring the  sunlight, and there are translucent marking in the base of the spathe. When a visiting fly tries to escape, it heads for the light, but this just guides it deeper into the spathe. This leads to panicked and more active movement, ensuring pollination.

Arum nigrum

Arum nigrum

Reproduction in Other Aroids

In dioecious aroids the female flowers are found on a different plant to the male flowers, so a genetic mix is guaranteed. Not many aroids are dioecious, but a few species of Arisaema are.

A few aroids are even paradioecious and change gender to suit circumstances.

Hermaphrodite aroids are similar to monoecious ones, the male and female parts on each flower mature at different times so self pollination cannot occur.

Habitat

Arid

For the most part, arid aroids have not evolved the typical shrunken leaves and thickened cuticle of other desert plants. Instead they tend to grow under trees and bushes and at the base of rocks where a damp, shady microclimate allows them to survive. They have unusually lush foliage for arid plants. This would make them a target for being eaten, but they have dealt with this by producing harsh toxins and needles of calcium oxalate that pierce and poison the throats of animals. Animals know to stay well clear of aroids.

Some Examples

Dead Horse Arum

Heliocodicerous muscivorus

Heliocodicerous muscivorus: This is called the dead horse arum. It has an inflorescence 35cm long and wide. It grows in the shelter of rocks on a few islands on the Mediterranean. It is pollinated by either flies or beetles and grows where sea birds have their colonies at nesting time. Sea birds live in a mess of rotting fish and eggs, dead chicks and excrement, which attracts the flies/beetles. The arum must then compete with the smell of these, in order to attract those same insects for pollination. It mimics the dead not only in smell, but also by looking like the corpse of part of a horse, complete with tail. Visiting insects find themselves falling into where the ‘tail’ is and becoming trapped by the slippery walls. Many insects lay their eggs inside, although any maggots that hatch will likely starve to death. The insects are held for two to three days and are fed by nectar.

Note: It’s worth looking at photos of the dead horse arum, my painting doesn’t really do it justice.

Sauromatum venosum: This is the called the voodoo lily because it has the ability to flower without soil or water, using only the energy stored as starch in the corm. It smells rotten.

Stylochaeton lancifolius: This aroid has flowers and fruits half buried in the ground. I have been unable to find information about why this is. My suspicions are:

  1.  It is pollinated by animals that are close to the ground. This can be seen in Aspidistra flowers, pollinated by slugs and snails. The flowers grow on the ground, under the leaves.
  2. Being submerged provides a little protection, even if eaten or stepped on, the Stylochaeton still has half a flower remaining.
  3. The fruits are eaten by something small. Having eaten the fruit, the seeds can be dispersed in the faeces.
Stylochaeton lancifolia

Stylochaeton lancifolia

Tropical

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Rainforests are dense, shady, and teeming with aggressive life. Animals, plants, fungi and bacteria are locked in a constant arms race. Consequently aroids have developed strong poisons, shiny leaves and the ability to climb to cope with some of these problems. In the tropics, latitudinal diversity (a wider variety of organisms that occurs close to the equator) means that it may be many miles through dense forest between plants of the same species. For this reason, aroids use very strong, and often unpleasant, smells to attract the right kind of insect.

A tropical rainforest has distinct layers and aroids grow in each of these. There are terrestrial aroids growing in the ground and epiphytic ones that climb into the canopy.

Climbers and epiphytes have only aerial, adventitious roots. There are two types: those that are sensitive to light and make for dark crevices where they can grip, and those that are sensitive to gravity and hang down from the plant in order to soak up rain and humidity.

Terrestrial Examples

Deiffenbachia grows in the Americas, while Aglaonema is native to Asia, they are both highly variable, but virtually indistinguishable from one another. This is an example of convergent evolution. Both contain toxins as a defence; Deiffenbachia is commonly known as dumb cane, because the if eaten, it causes the throat to swell, so that speech is impossible.

Ag 2

Aglaonema and Deiffenbachia – both highly variable, but in similar ways

 

Amorphophallus: This is a genus of tropical and subtropical aroids, native to Asia, Africa and Australasia. They attract flies and beetles by giving off the smell of rotting meat. Unusually, Amorphophallus species only put out one leaf or one inflorescence at a time, one a year. The single leaf is highly divided.

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Some Amorphophallus inflorescences

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Single, highly divided leaf of Amorphophallus

Some species in this genus also have white patches on the stem, these are to mimic lichen growing on trees and serve to protect them from stampeding elephants. When tramping through the jungle elephants have learnt to avoid trees, which are usually covered in lichen. Amorphophallus would be very easily damaged by an elephant, so by looking a bit more like a tree they can fool the elephant into avoiding them.

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Lichen mimicking stem

Epiphytic Examples

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Monstera

Monstera: These are one of the few plants to have holes in their leaves. Recent research shows that leaves with holes benefit in shady areas because the light coming through the trees is often dappled. By having holes in their leaves, Monstera cover a larger area with the same amount of leaf (so the same amount of energy used to make it) as a smaller leaf without holes. This allows the plant to take advantage of any sunlight that gets through the canopy.

Anthurium punctatum: This is an aroid from Ecuador. It has formed a symbiotic relationship with ants. It has nectaries away from the flowers because it is not trying to attract pollinators, but protectors. The ants set up home in the Anthurium and guard it from animals and insects that may eat it. However, in this Anthurium the ants are particularly aggressive and keep away pollinators also. The ants also secrete an antibiotic substance called myrmiacin, which is antibiotic and protects the ants from moulds and bacteria that might cause disease. However, this substance prevents pollen tube formation needed for the plant to be fertilised. These two barriers to pollination mean that the species can only propagate itself vegetatively.

Philodendron: This is a diverse genus. Plants can be epiphytic, hemiephytic or (occasionally) terrestrial. Hemiepiphytic means that the plant spends part of its life-cycle as an epiphyte (in the air). It may start off on the ground and then wind its way up a tree, then let its original roots die back. Or it may start as a seedling in the branches of a tree and a root will trail its way to the ground.

Philodendron bipinnatifidum

Philodendron bipinnatifidum

Temperate Woodland

Arisarum proboscideum

Arisarum proboscideum

Arisarum proboscideum: aka the mouse plant. This is a woodland aroid, native to Spain and Italy. It has flowers like little mice. The ‘tails’ of these give off a mushroomy odor, that attract fungus gnats for pollination. The flowers have a spongy white appendage inside the spadix that looks like a mushroom to complete the deception. Fungus gnats often lay their eggs in the flowers, although the maggots won’t live to adulthood.

Aquatic

As I have blogged before, plants never evolved much in water. This means that all aquatic plants have evolved on land and then evolved again to cope with life in water. Some problems faced are – damage to flowers and leaves due to water currents, lack of access to pollinators, water blocking out light, lack of oxygen (leading to rotting roots), and the heaviness of water (800 times as dense as air) putting pressure on foliage.

Some solutions to problems:

  • Aerenchyma:  these are gas filled cavities that improve buoyancy and oxygenation.
  • Fish shaped foliage: these offer less resistance to water currents, so less damage occurs.
  • Larger surface area in relation to volume: ie filmy leaves. This increases photosynthesis  eg Cryptocoryne
  • Roots: These are not needed to transport water, since it can be taken in by all parts of the plant. However, roots are used to anchor the plant and stopped it being carried away by currents. eg Jasarum steyermarkii
  • Reproduction: Many aquatic aroids find it easier to spread vegetatively rather than by flowering, in order to avoid flowering problems.

An Example

Pistia stratiotes 2.JPG

Pistia stratiotes: This is the only floating aquatic aroid, growing in swampy deltas in India and West Africa. It is adapted to staying still in fast moving currents, and has found the balance between sinking and blowing away.  The inner tissues have aerenchyma and the outer surfaces are ridged, velvety and with dense covering of hairs. This makes it unable to sink, and water repellent. Feathery roots act as an anchor. It has tiny flowers in a protective hairy spathe.

Pistias form a dense mat on the surface of the water, and can create mats of 15m wide. This makes Pistia something of a weed, causing problems to the ecosystem because the water underneath is deprived of light.

However, Pistia is not only harmful, some ecological benefits:

  • The darkness caused by the Pistia mats has led to the evolution of blind elephantnose fish, which live beneath the mats. They hunt by electricity and have well developed brains and learning abilities.
  • Birds and animals often make the floating island their home.
  • Pistia can purify stagnant water.

 

Note: outdoor photos are mostly taken in Ecuador and indoor photos mostly from Wisley Gardens.

 

Fasciation

Normal Flower and Fasciated Flower

Normal Flower and Fasciated Flower

Fasciation in plants is a bizarre mutation in the meristem (growing point) leading to flattened flower stems and distorted flowers, fruits and roots. It can also lead to a ring of small flowers surrounding the main flower, this is known as ‘hen and chicks’ and can be seen in some of the Veronicastrum pictures below. The meristem is where cells actively divide in order to grow or create new flowers and leaves, a disturbance to this process can lead to the cell division intensifying and occurring in a haphazard manner, leading to distortion. Essentially the growing point ceases to be a point and instead forms a cockscomb. For many plants this is most commonly noticed with flowers, which then go on to form distorted fruits, but with cacti and ferns it is often seen in the leaves.

Causes

Genetic

In some plants, such as the soybean (Glycine max), fasciation is caused by a single recessive gene. This means that fasciation will only occur if both parents of a plant have that gene and pass it on.

Multiple distorted flowers Veronicastrum 'Fascination'

Multiple distorted flowers Veronicastrum ‘Fascination’

Physiological

Normal and Fasciated Spathyphyllum

Normal and Fasciated Spathyphyllum

In plants without the gene, fasciation is caused by disturbance to the meristem at the time of growth. This disturbance can be caused by

  • Mites or insects feeding on the shoot
  • Fungal, bacterial and viral diseases
  • A sudden change in temperature – eg going from low to high or high to low (especially in Hyacinthus)
  • Zinc deficiency or nitrogen excess
  • Drought followed by heavy watering

Frequently Fasciated Plants

The following plants have exhibited fasciation: soybean, many cacti, ferns, Euphorbia, Prunus, Salix, cannabis, Aloe, Acer, Forsythia, Delphinium, Digitalis, Taraxicum and Syringa.

Artificially Induced Fasciation

In some cases fasciation is seen as a desirable characteristic, it can lead to increased yield in crops due to the enlarged heads, or provide a talking point in ornamental displays. Examples are the maize, Celosia cristata and Asplenium cristata (note the species name ‘cristata’ – cristate is another word for fasciation). To this end, the above conditions can be induced or one of the following methods used:

  • Manipulating the photoperiod (exposure to light)
  • Using susceptible cultivars (see below)
  • Using radiation – gamma rays or ionizing x-rays.
  • Chemical application – growth regulators or polyploidzing agents
  • A cutting or scion taken from a fasciated plant will create a new fasciated plant

Veronicastrum ‘Fascination’ is a cultivar grown for its tendency to fasciate.

Fasciated stem of Veronicastrum 'Fascination'

Fasciated stem of Veronicastrum ‘Fascination’

Veroncastrum 'Fascination'

Veroncastrum ‘Fascination’

Fasciation in Cacti and Other Succulents

Many cacti and succulents are subject to fasciation, although the word more commonly used to describe this state is cristate. More than fifty cacti genera have shown cristation, as well as the succulent families Crassulaceae, Asclepiadaceae and Euphorbiaceae. Some cacti have ‘Cristata’ in the name. Fasciated cacti form ribbon like weaves, or have many divisions. Cristation is often cultivated in cacti, with cuttings used to perpetuate the cristate cacti. It is thought that some cacti species have a genetic propensity to cristation and somatic mutation (genetic alteration caused by environmental factors as described above) leads to the physical changes. Seeds from fasciated stems in cacti often lead to fasciated seedlings, although this is not necessarily true of other plants, Digitalis, when fasciated, does not produce fasciated seedlings.

Mammillaria elongata cristate

Mammillaria elongata cristate

Some more cacti showing signs of cristation

Fasciated Mammillaria compressa

Fasciated Mammillaria compressa

Normal and fasciated Mammilaria

Normal and fasciated Mammilaria

Euphorbia

Euphorbia

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 Fasciation in Ferns

Several ferns are especially cultivated to be cristate, such as Dryopteris affinis ‘Cristata’ or Asplenium cristata

Asplenium cristata

Asplenium cristata

 Additional information and pictures:

Plant Divisions: Flowering Plants

Leaf Variety in Magnoliophyta

Leaf Variety in Magnoliophyta

Plants in the Magnoliophyta Division may also be called Angiosperms or flowering plants, they include grasses, palms, oak trees, orchids and daisies. Magnoliophyta is the only division that contains plants with true flowers and fruits, and all plants in this division use those flowers and fruits to reproduce. It is not known exactly when flowers first appeared, but definitely by 125mya and probably as far back as 160mya.

Flowers have proved to be an extremely successful adaptation, and despite its recent appearance, Magnoliophyta is by far the largest and most diverse plant division with over 250,000 different species and 500 families. (For comparisons to other divisions and their sizes see here)

Leaf Variety in Magnoliophyta

Leaf Variety in Magnoliophyta

Flowers

In Magnoliophyta, flowers replaced the cones of more primitive plants, as a means of reproduction. Some flowers are brightly coloured, have a scent or produce nectar in order to entice animals to pollinate them, but others use wind or water and, having no need to draw attention, are barely noticeable.

Flower Variety in Magnoliophyta

Flower Variety in Magnoliophyta

Flower Variety in Magnoliophyta

Flower Variety in Magnoliophyta

Fruit and what that really means…

All plants in this Division produce fruits of some kind, even though what they produce may not be easily recognised as fruit. The botanical definition of a fruit is a matured ovary (the ovary is the female part of the flower that contains the ovules which become the seeds once fertilised), this includes peppers, tomatoes, aubergines, nuts, peas, wheat grains, but not apples or rhubarb. There is another meaning for the word fruit, which is culinary and refers to a sweet part of a plant that is eaten, this is the more familiar term and includes rhubarb and apples, but not tomatoes and nuts, etc. ‘Vegetable’ is only a culinary term, referring to parts of a plant used in savoury cooking, it may refer to any part of the plant: leaves (lettuce) flower buds (broccoli), stems (celery) or roots (carrots) and has no botanical equivalent.

Classification

Being such a large and interesting division means that the classification of Magnoliophyta has received more attention and undergone more changes than any other division.

How Many Flowering Plants Are There?

It was believed for some time that there were over 400,000 flowering plants, but it turns out that many species of plant (not known as yet how many) have actually been named twice or even three or four times. The binominal naming system (using two Latin names, eg Helianthus annuus) was designed to make plant naming international and straightforward, but with people all over the world discovering and naming plants and no comprehensive way of cross referencing them, we have ended up with a lot of confusion. Now, partly due to the international power of the internet, serious attempts are being made to work out how many actual species there are and to remove duplications. The Plant List is a collaboration between a number of botanical gardens around the world and has an impressive online collection of these names.

DNA Alters The Family Tree – Cronquist to APG III

Before DNA testing was possible (or DNA was known about) plants were collected into families, classes and orders according to detailed studies of how they looked.

Over the past few hundred years there have been many different classification systems, but one of the most commonly used and straightforward was the Cronquist System, devised in 1968. This System grouped plants into families, with the families grouped into orders, orders then grouped into sub classes and sub classes grouped into two classes: monocotyledons and dicotyledons. However, with genetic testing, it has been found that many of these groupings were wrong. A new system, called APG (Angiosperm Phylogeny Group), was introduced in 1998, but has subsequently been updated twice since then and will no doubt change in the future.

Frustratingly, what was once a very neat and straightforward system of classification has become an unwieldy, confused and messy system, because nature is never neat. The new system, called APG III, does not use classes and subclasses, instead it groups orders within clades, nested within other clades, nested within other clades; with some families not fitting into any clade at all.

The following diagrams are an attempt to show the changes in a simple manner, using images of plants to represent different orders and showing how those orders have altered their connection to others. It is clear that some assumptions were completely wrong, for example some dicots are more closely related to monocots than other dicots; the buttercup is not kindred with the water lily; cacti are more connected to Heuchera than originally thought and oak trees are closer to Euphorbia than London planes.

Cronquist system

Cronquist system

APG III System

APG III System

Key to Magnoliophyta plants

Key to Magnoliophyta plants

Note: I was unable to take photos of a tulip tree or Rhododendron in flower, so used photos I got online from here: Rhododendron and tulip tree

 

It was also fairly tricky to find all the necessary information about where plants appear in the Cronquist system, if anyone spots any faults, please contact me at the email to the right. Most of my information came from Wikipedia, and from here

To enlarge the key click the thumbnail

Anthurium and Ctenanthe - two flowering plants

Anthurium and Ctenanthe – two flowering plants

Plant Divisions: Gnetophyta

Ephedra cutleri

Ephedra cutleri

Gnetophyta is a plant division containing only 3 genera and approximately 80 species. It isn’t known when plants in this division first evolved, but somewhere between 140 and 250mya. Although gnetophytes are gymnosperms, with no true flowers or fruits, they have some features in common with flowering plants:

  • Vessel elements in the vascular system not seen in other gymnosperms
  • Both Welwitschia and some Gnetum species are pollinated by insects
  • Flower like structures on male cones of Welwitschia
  • Nectar – produced on the tip of the cones rather than in a flower

All gnetophytes are evergreen and woody, and may be trees, vines or in the case of Welwitschia, difficult to classify. These plants have not been studied much and it is tricky trying to find out information about them. For example, although they are mostly considered dioecius (male and female cones on separate plants) all three genera sometimes produce bisexual cones, containing both stamen and ovules, but it isn’t really understood why, or if these cones can then reproduce.

Gnetophyta Family Tree

Gnetophyta Family Tree

Gnetophyta Family Tree

Gnetum

Gnetum gnemon

Gnetum gnemon

There are 30-35 species of Gnetum, including two trees, many vines, and shrubs.

One tree, Gnetum gnemon, reaches 15-20m tall, and does not have fruits, but a fruit-like juicy covering for the seeds, which, like fruit, are edible to birds and aid in the spreading of seed.

Many Gnetum have seeds and leaves that are also edible to humans. Leaves of Gnetum have network of veins, something seen in dicotyledonous flowering plants, but no earlier evolved plants. All are dioecious. Gnetum are thought to be the first plants to be insect pollinated, by now extinct scorpion flies.

Welwitschia

Welwitschia

Welwitschia

Drops of nectar on female cones - Barry Rice/CalPhotos/EOL

Drops of nectar on female cones – Barry Rice/CalPhotos/EOL*

There is only one species of Welwitschia and it only grows in the deserts of Namibia and Angola. Despite sometimes growing 10m wide (although more commonly 4m wide), Welwitschia has just two strap like leaves that grow continuously. The longest recorded leaves were 37m long, but most leaves break up in the harsh desert environment and become tatty and brown at the ends. Unlike Gnetum, the veins are parallel, as seen in monocotyledonous flowering plants as well as some ferns and cycads.  Welwitschia probably live 1000-2000 years, although this is difficult to know for sure. The female cones produce drops of nectar to entice insects to pollinate them. They have a single tap root grows deep into the sandy desert soil in search of water.

* Photo from The Encyclopedia of Earth with some more technical details about Welwitschia

19th July 2013

19th July 2013

31st August 2013

31st August 2013

I recently bought some Welwitschia seeds to see how they would grow. I planted them in a pipe to give space for the deep tap roots, 2:1 sand to compost. Within a week, three had germinated. Two died a few weeks later, I believe because I didn’t take into account that the single root only takes water from deep in the soil, so watering from above was pointless. I spray with fungicide every week or so. As can be seen from the pictures, Welwitschia has two cotyledons that start out orange and turn green.

Ephedra

Ephedra chilensis

Ephedra chilensis

There are about 50 species of Ephedra. They have slender stems with needle like leaves and small, sometimes brightly coloured, cones. They grow in dry areas in the Northern hemisphere, such as North Africa, Europe and North America. Ephedra looks very much like a gigantic version of psilotum (see previous blog about ferns) and can grow up to 3m. Some are monoecious.

The Evolution of Attracting Insects

While researching the previous blog about Ginkgophyta I learnt about terpenoids. Terpenoids are chemicals produced by both primitive plants (eg mosses and ferns) and flowering plants, the last group of plants to evolve. However, the function of terpenoids has altered as the plants have evolved. Terpenoids attract certain beneficial insects that feed on other insects that are harmful to the plant and this is an advantage to all plants, however, in later plants, Cycadophyta, Gnetophyta and Magnoliophyta, the insects attracted are also used to pollinate the plants and it was presumably because of the existence of terpenoids that such a partnership of plants and insects was able to form. Insect pollination is a far more efficient means of transporting pollen than wind, because an insect seeks out another plant, often a specific insect becomes an exclusive visitor to a specific plant. In the case of Welwitschia, growing in the desert, there may be many kilometres between plants, an awful lot of pollen would need to be produced in the hope of it being carried on the wind. Using insects to transport the pollen is akin to getting the postman to post a letter through the letterbox of the person you want to reach, instead of throwing  a thousand leaflets down the road they live in, in the hope they pick one up.

Plant Divisions: Ginkgophyta

Ginkgo biloba

Ginkgo biloba

Ginkgophyta is a plant division of non-flowering trees originating over 250 million years ago, in which all plants except for one, Ginkgo biloba, have become extinct. Ginkgo bilobas are large, deciduous trees with unusual looking cones and distinctive leaves, they can live for up to a thousand years. A few hundred million years ago whole forests existed around the world filled with different species of Ginkgos, but now the one remaining species is native only to China.

Ginkgophyta Family Tree

Ginkgophyta Family Tree

Ginkgophyta Family Tree

Leaves

Ginkgo biloba leaf

Ginkgo biloba leaf

Ginkgo leaves are bi-lobed, tough and more resistant to decay than other leaves. Some leaves are borne on long stems and turn yellow, die back in winter, then reappear in spring, while others are on shorter stems that may survive the winter.

Trunk and Vascular System

The bark of Ginkgos is fissured and the trunks may reach to 4m in diameter.

The vascular system of Ginkgos, and also conifers, are different to that of flowering plants. While flowering plants have a series of tube-like cells to conduct water, Ginkgos have connecting cells with tiny perforations, these are valves that close when water is in short supply so that turgidity is preserved.

Reproduction and Survival

Ginkgo biloba

Ginkgo biloba with male cones

Cone on female Ginkgo

Cone on female Ginkgo

Ovules

Ovules

Ginkgos are dioecious. The male cones grow from the shoot tip in clusters and release pollen. The female ovules (cones) appear in twos on the end of a stalk and do not look much like the cones of conifers. Each ovule has a drop of fluid, the pollination drop, that traps pollen to enable fertilisation.

Ginkgo fruit

Ginkgo pseudofruit

Ginkgo sperm cells are motile, swimming to the ovule using thousands of hairs. This is something that occurs in cycads too (see previous blog) and in ferns, but not conifers or flowering plants, so is  a throwback to a more primitive form of reproduction. Once fertilized the ovule grows into something resembling a fruit containing the seed.

Ginkgo seedling

Ginkgo seedling

Ginkgo seeds contain two cotyledons (seed leaves), but these never expand or emerge, instead they remain embedded in the seed providing nutrition for the seedling. The first leaves to appear above ground are true leaves with the distinctive Ginkgo shape, this is called hypogeal germination.

Ginkgos have a few clever ways of surviving and reproducing:

Like cycads, Ginkgos have been known to change sex, so that the male trees start producing ‘fruits’ and seeds. This is an effective way of propagating when there are no females around.

Ginkgos have a tendency to put out suckers from the ground that point upwards, but older trees sometimes also have odd downward growths, called Chichi, hanging from a single branch like stalactites. When these growths hit the ground they can start growing new roots and eventually form into a new tree, this is seems to be a form of reproduction for when the main tree is coming to the end of its life.

Chichi on Ginkgo

Chichi on Ginkgo

The brilliant photo above was taken by Rebecca Sweet and posted on Gossip in the Garden

If Ginkgos are hacked right back to the bare trunk they can regrow, either growing from the damaged stem or by putting out new shoots from the ground.

Ginkgos are also very resistant to pests, diseases, fires and pollution.

Medicinal Properties

Ginkgo biloba

Ginkgo biloba

Ginkgo biloba contains Flavonoids and Terpenoids which are naturally occurring chemical groups found in plants.

Flavonoids

Use for the plant: pigmentation, assisting in nitrogen fixation and cellular function

Use for humans: thought to have anti-allergic, anti-inflammatory, anti-microbial, anti-cancer and anti-diarrheal properties although this is not fully proved.

Terpenoids

Use for the plant: provide pigmentation and smell. They are thought to act as a deterrent to herbivorous insects and an attractant to insects that may eat herbivorous insects. They also are found in flowering plants and are used to attract pollinators. They may have antioxidant benefits for plants.

Use for humans: they have been used in traditional medicines for many years, although their effectiveness is not proved, they may have antibacterial properties and they may also have antioxidant benefits.

(note: I have been unable to ascertain exactly what Terpenoids and Flavonoids do in Ginkgo biloba specifically, so this information refers to their function in plants in general.)

Why do plants have medicinal properties?

We have enemies in common: plants have evolved chemicals that fight some of the same insects, fungi and bacteria that also plague humans.

Poisons can also be cures: mammals are often problematic for plants and so they have evolved ways to fight them off, but these ways may also, in small amounts, be cures. For example, Digitalis affects heart rate and is fatal in large amounts, but in small amounts can regulate heart rate.

While researching this question I have come across a common belief that plants evolved medicines in order to benefit humans, that by cultivating plants we made it beneficial for them to produce certain chemicals. However since plants first evolved 400 million years ago and evolved those chemical defenses millions of years ago, yet Homo sapiens only evolved a few hundred thousand years ago and only started cultivating plants 12,000 years ago, this isn’t really likely.

Further information about Ginkgos:

A very good website here, with clear pictures and video ( although the video is unfortunately difficult to hear):

http://kwanten.home.xs4all.nl/ovule.htm

Ginkgo biloba

Plant Divisions: Cycads

Cycas

Cycas

Cycadophyta is a plant division that contains only trees, the cycads, which are palm-like gymnosperms. They first appear in the fossil record 280mya and haven’t changed much since then, although a fair few genera became extinct 200mya.

Cycads have long, narrow leaves, with either pinnate or bipinnate leaflets forming in a whorl at the top of a trunk or growing on slender stems from the ground. They vary greatly in size, with some reaching 18m high while others are only 30cm. Cycad leaves often unfurl as they grow and as lower leaves die their bases remain attached to the stem to form an armour-like casing. Like all gymnosperms, cycads do not produce flowers or fruits, instead they reproduce by cones.

Cycads appear in a geographical band that stretches from the Tropic of Cancer to the Tropic of Capricorn. They are mostly found in tropical and subtropical regions in both the northern and southern hemispheres, for example Central and South America. They can live up to a thousand years, but tend to grow very slowly.

Leaves

Leaves of Zamia furfuracea

Leaves of Zamia furfuracea

All cycads are evergreens, with stiff long leaves. Many are pinnate (opposite leaflets along a central stem) or bipinnate (each pinnate leaflet divided into further leaflets). There is a little variety in the leaves of cycads, adult leaves can be blue-grey or green and they may be twisted or spiky. The veins in cycad leaves run in parallel lines from the leaf stem. Young leaves may be copper-coloured and in some genera they unfurl like fern leaves. Many cycads have stipules, these are small outgrowths at the base of the main leaf and can be useful in identification of plants. In general stipules can have different forms such as glands, hairs, miniature leaves, spines or scales, but on cycads they take the form of stunted leaf-like structures. In Bowenia and Stangeria the stipules are larger and fleshy, and their function is to protect the leaves as they grow.

Unfurling leaves of Cycas rumphii

Unfurling leaves of Cycas rumphii

Trunk and roots

Trunk of Encephalartos ferox

Trunk of Encephalartos ferox

Cycad stems are often thick, but are not true wood, instead they are fibrous and contain a lot of starch. Many cycads, for example Bowenia and Stangeria, have subterranean stems, these are carrot-like. Cycads also have specialised roots, termed collaroid roots, that form into coral like structures. Coralloid roots contain blue green algae that form a symbiotic relationship with the plant, fixing nitrogen for it.

Subterranean Stem of Cycas

Subterranean Stem of Cycas

Reproduction

Cones of Dioon and Ceratozamia

Cones of Dioon and Ceratozamia

It has recently been confirmed that cycads are pollinated by insects, which is rare for gymnosperms, but a distinct advantage for a plant that grows in tropical forests where there is little wind.

Cycads are dioecious which means that there are separate male and female plants, however cycads are one of the few plant groups that can change their gender, seemingly in response to stress such as physical damage or extreme cold. Plants are not as gender specific as mammals, (most are hermaphrodites) and although x and y chromosomes (the ones that differentiate gender) have been found in a few plants, the significance of these chromosomes in plants is still not fully understood.

Seeds with Red Sarcotesta in  Female Zamia loddigesii Cone

Seeds with Red Sarcotesta in Female Zamia loddigesii Cone

When cycads are not changing gender, the process of reproduction is very similar to that of conifers, the male cones release pollen, the female cones receive the pollen and form seeds. However, being dioecious, the male and female cones do not appear on the same tree as they do with conifers. On many cycad genera, the seeds that form have brightly coloured seed coats called sarcotestae (sarcotesta singular). This is slightly fleshy and edible to birds and animals which aids in the spreading of seed and is a forerunner to fruit. (This can be seen in the picture to the right, with the red seeds forming beneath the brown scales, pushing them out as the seeds enlarge) Pomegranates and Ginkgos also have sarcotestae.

Male Cones of Ceratozamia,, Chigua and Cycas

Male Cones of Ceratozamia, Chigua and Cycas

What is the difference between Cycads, Tree Ferns and Palms?

Cycads, tree ferns and palms can all easily get confused, all may have a central trunk, usually without side branches, and then a whorl of leaves at the top, consisting of a central rachis (the middle part of the leaf) with leaflets either side. All grow mainly in the tropics, although tree ferns can grow in more temperate areas too. However, the three types of trees are not closely related at all, each is in a different division, this can be seen most clearly in their very different ways of reproducing.

Palm Tree, Cycad, Tree Fern

Palm Tree, Cycad, Tree Fern

Cycads – Cycadophyta Division – reproduce by pollen, cones and seeds.

Tree ferns – Pteridophyta Division – reproduce by spores and have separate sporophyte and gametophyte generations. Leaves are divided into distinctive fern leaf shapes. Trunks tend to have leaf bases still attached, same as with cycads, and their trunks are also not true trunks, however tree fern trunks are made of modified roots.

Palms – Magnoliophyta Division – reproduce with flowers and fruits. Palms are more varied than cycads and may have palmate or fan leaves, or a trunk with spikes or smooth bark.

Cycadophyta Family Tree

Cycadophyta Family tree

Cycadophyta Family tree

Cycadophyta is a small division with only three families and eleven genera. A number of genera are now extinct, for example Beania and Crossozamia.

Zamiaceae

Zamiaceae

Zamiaceae – Zamia pseudomonticola, Dioon purpusii, Encephalartos horridus

This is the family with the most genera – Chigua, Zamia, Ceratozamia, Macrozamia, Lepidozamia, Dioon, Encephalartos, Microcycas – and consequently the widest variety of shapes. The sarcotesta in this family are red, yellow or brown.

Zamia pygmaea is the smallest cycad and, at 30cm, is in fact the smallest gymnosperm.

Some species of Macrozamia have leaves so fine they look almost like pine needles or grass, whereas Zamia have wider leaves. Encephalartos has some species with distinctive spiky leaf shapes (see photo).

Ceratozamia with Female Cone

Ceratozamia with Female Cone

 Cycadaceae

Cycadaceae - Cycas seemannii

Cycadaceae – Cycas seemannii

Mature Female Cycas cone with seeds forming

Mature Female Cycas cone with seeds forming

Cycadaceae contains only one genus Cycas. Although Cycas has many species, there is little variation in appearance, with all species having a stout trunk and a whorl of leaves at the crown. Cycas have notably different female cones to other cycads. The scales of the cones are open with seeds forming in between the scales, rather than underneath as in other cycads. The young leaves of Cycas are coiled (see near top for photo of unfurling leaves).

Stangeraceae

Stangeraceae - Stangeria eriopus

Stangeraceae – Stangeria eriopus

Stangeriaceae contains two genera, Bowenia and Stangeria. In this family, young leaves are coiled or folded. Stipules are present and the sarcotesta is purple. Most plants in this family have wider leaves than other cycads, although Stangeria paradoxa has leaflets as finely divided as a fern. Both genera have slender stems, with the trunk underground.

Bowenia spectabilis

Bowenia spectabilis