Tag Archives: theory

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:


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.


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.


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


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


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


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


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


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


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.


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.




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.


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:


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




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.




Latitudinal Diversity: my theory


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)


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.


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.

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.


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.


First, some terminology


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 between insects and fungi?


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


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.



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 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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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.



Mosses and lichen on a branch