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.

Secondary thickening in young stem

Secondary thickening in a stem

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

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:

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.