RHS Level 2 study, & gardening know how
RHS Level 2 study, & gardening know how
Unit 2 1: Plant Science II RHS Level 2 (Old Syllabus) R2101

Plant adaptations: when plant structures have an unusual function.

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This blog is most relevant to R2101 (old syllabus) and some of it is relevant to Unit 2 of the new syllabus Topic 1 (Plant science 2)

Introduction

Plant adaptations are when a plant organ does a different job compared to its usual function. For example, the main job of a plant stem is to hold the leaves aloft for photosynthesis, but they can also adapt to twine around structures, turning the plant into a climber.

For each of the main plant organs you are required to know the function of that organ (what is usually does) and some named examples of adaptations. Adaptations are when the plant structure is modified to do something different compared to its usual function. Adaptations have evolved over 1000s of years so that plants can adapt and survive in different habitats.

We will look at each plant organ in turn, list the function(s) and then look at the adaptation examples in detail. We will focus on the named examples outlined in the RHS level 2 syllabus (old syllabus) but there are a few other examples listed just for interest.

The three organs we are focusing on are stem, root and leaves.

Stem

Primary functions of the stem

  • Holding leaves in the best position for photosynthesis.
  • Holding flowers in the best position for pollination.
  • Transport of water, nutrients and sugars.
  • Holding fruits in the best position for seed dispersal

Stem Adaptations

1. Protection

i)Thorns

Named Example: Crataegus monogyna (hawthorn)

The whole of the stem is adapted to form a sharp hard spike which deters predators. A thorn is very strong and cannot be snapped off easily when you push your finger against it. It arises from the whole body of the stem.

Other example of thorns for interest: Pyracantha coccinea (Firethorn).

Crateagus monogyna Thorns which deter predators and protect the plant. (A stem adaptation)

ii) Prickles

Named Example: Rosa rugosa and most rose species.

Technically the ‘spikes’ on rose stems are called prickles rather than thorns. Prickles can be ‘pushed off’ easily by pressing your finger against them. They are different to thorns in that they are made from the stem epidermis (the outer layer) and so are not as strong as thorns which are an adaptation of the whole stem. Prickles will protect from predators and will also help the rose to climb if it is a rambling or climbing rose, as they hook onto supports.

Other examples of prickles for interest: Rubus fruticosus (blackberry)

Rosa rugosa prickles which deter predators and protect the plant. (An adaptation of the outer part of the stem)

2. Storage / Perennation

Perennation is when a plant can survive between growing seasons when conditions are unfavourable with stored food reserves. For stems there are a number of ways that stems are adapted to assist with perennation.

i) Corms

Named Example Crocus sativus

A corm is a solid swollen underground food storage stem which perennates below ground. Corms are solid stem tissue which can be seen when they are cut in half. They also tend to be flatter in shape compared to bulbs. (see later) The dormant period can be during Winter or Summer depending on the plant species.

Other named examples of corms for interest: Crocosmia × crocosmiiflora (monbretia), Gladiolus species

This is a Monbretia corm. On the left is the outside. On the right the corm is cut in half and you can see the solid tissue inside which is stem. The food store in corms is mostly stem tissue.

ii) Stem tuber

Named Example: Begonia x tuberhybrida

Begonia tubers are large swollen underground food storage stems with buds. They are tender and need to be stored in a frost free place over Winter.

When you are talking about tubers don’t get confused. There are stem tubers and root tubers.(see later)

This begonia x tuberhydrida is not in very good condition. I fished it out of an outside container and you can see the areas of grey around the edge where it is has been too wet and is starting to rot. However you can see the solid tissue in the bulk of the tuber which is stem tissue.

iii) Rhizomes

Named Example: Iris germanica.

A rhizome is an underground stem. Iris germanica is unusual in that it is not completely below ground as it sits half in the soil and half above the soil. This rhizome is very large and swollen.

Other examples of perennating rhizome for interest: Ginger (Zingiber officinale)

This is Iris germanica with a thick food storage stem showing buds, and roots growing from the base.

3. Climbing

Twining stems

Named example: Wisteria passiflora

Movement of shoot tips allows the stem to twine around a structure when it touches it.

Other examples of twining stems for climbing for interest: Phaseolus coccineus (runner bean), Lonicera periclymenum (climbing honeysuckle), Humulus lupulus (hop)

This is a climbing bean (rather than a Wisteria passiflora which is the named example you have to learn.

4. Natural Vegetative Reproduction

These are structures which help the plant to spread and reproduce. This is not perennation, this adaptation makes MORE of the plant.

i) Stolon / Runner

Named Example: Fragaria x ananassa (Strawberry)

A creeping above-ground stem which originates from the base of the plant. The stolon then forms adventitious roots at the nodes where it touches the ground.

Other examples for interest: Ranunculus repens (Creeping buttercup), Chlorophytum comosum (spider plant)

Strawberry stolons can be left to root in the ground naturally, or can be placed in pots where they can develop a good root system and are then severed from the parent plant. This is a type of natural vegetatative reproduction.

ii) Rhizome

Named Example: Festuca rubra (A lawn grass)

A creeping below ground stem which spreads out from the parent plant and forms roots at nodes and sends up shoots.

Other named examples for interest: Many are weeds which helps them to survive control. E.g. Elymus repens (couch grass)

iii) Stem Tuber

Named Example: Solanum tuberosum (Potato)

A potato tuber will turn into lots of potatoes if planted. You can remember that a potato tuber is an adapted stem because it will go green if exposed to the light and the eyes you see on it are where buds will emerge.

I planted 4 potatoes in a container and harvested all these which are an example of natural vegatative reproduction. Potatoes are stem tubers.

Roots

Primary Functions of the root

  1. Anchorage
  2. Water uptake
  3. Mineral uptake

Roots are continually branching and exploring the soil for nutrients and water.

Root Adaptations

1. Food Storage / perennation

i) Thick Tap Roots

Named Example: Daucus carota

Thick tap roots store large amounts of starch in cells in the tap root. This storage of food allows perennation which is when the plant is carried through from one season to the next. Carrot is in fact a biennial and the root starts to wither as the food reserves are used up for growth and flowering in the second year. Gardeners will harvest the tap root before this process starts.

Daucus carota tap root – an example of perennation/food storage, a root adaptation

ii) Root Tubers.

Named Example: Dahlia ‘Bishop of Landaff’

Some plants have swollen root tubers which store food. Dahlia tubers should be lifted and protected from frost and then planted again in late Spring after growth has been started off under protection.

Dahlia root tubers – a root adaptation for perennation

2. Facilitate Climbing

Adventitious roots

Named Example : Hedera helix

Adventitious roots allow the plant to climb vertically if planted against a vertical surface. Adventitious roots grow from the stem and anchor the plant to the wall.

Other example for interest: Hydrangea anomala subsp. petiolaris (climbing hydrangea)

Adventitious roots on Hedera helix facilitate climbing

3. Support

Prop roots

Named Example Zea mays (Sweetcorn)

Prop roots are roots that grow above the soil level in order to stabilise the plant. These roots arise from the stem and so are adventitious roots (meaning roots arising from an unusual place) Prop roots often develop in plants that are tall and likely to be blown over by the wind.

Other named example for interest: Impatiens glandulifera (himalayan balsam), palm trees. There are also buttress roots found on mature trees but these are not covered in the syllabus.

Prop roots of Zea mays. These support the plant and provide special anchorage.
Thick prop roots seen on Impatiens glandulifera (the weed, Himalayan balsam) which enables this tall invasive plant to grow on steep river banks.
Prop roots at the base of a palm tree.

Leaves

Primary function of leaves

Photosynthesis.

Leaf Adaptations

1. Protection

Spines
Named examples: Berberis thunbergii (deciduous Berberis), Berberis darwinii (Evergreen Berberis)

When it is the leaf which is adapted to deter predators, the spikey parts are called spines. In some plants it is the edge of the leaf (the leaf margin) which is spiney and in other plants it is the whole leaf which forms a spine. Berberis shows both variations.

Other examples for interest: Ilex aquifolium (Holly) has spines at leaf margin.

Berberis thunbergii. The whole leaf is a spine and serves to protect the plant from predators.
Berberis darwinii. The leaf margin is spiney to protect the plant from predators.

2. Food storage / perennation

Perennation is when a plant can survive between growing seasons when conditions are unfavourable. There is a period of reduced growth between seasons.

Bulbs

Named Examples : Narcissus ‘February Gold’ and Lilium regale

Bulbs are made from swollen modified leaves (scales) If you cut an onion in half you can see the layers of swollen leaves.

There are 2 main types of bulbs. Bulbs covered with a papery tunic (Tunicate, eg Narcissus species) and scaly bulbs (No papery tunic, eg. Lilium species)

Both bulb types are leaf adaptations for storage and perennation

Other examples for interest: Tulipa species, Galanthus nivalis (Snowdrops), Muscari armeniacum (Grape hyacinth)

Hyacinth tunicate Bulb. A leaf adaptation for perennation and food storage which allows the bulb to survive from season to season. Tuynicate bulbs have a papery tunic which protects the inner fleshy leaves. The named example in the syllabus for a tunicate bulb is Narcissus.
Lilium Bulb which does not have a papery tunic – the bulb scales (leaves) are more exposed which is why, after purchasing, you should not allow lily bulbs to dry out.

3. Water Storage.

Leaf cells store water(succulent leaves)

Named Example: Hylotelphium telphium (syn. Sedum spectabile)

Leaves are swollen and store water. Often these types of plants are classified as succulents and thrive in dryer conditions. There are many other varieties of Sedum that are succulent which are often used on green roofs.

For more sedums see: https://worldofsucculents.com/?genera=sedum

Hylotelphium telphium (syn. Sedum spectabile)

4. Facilitate Climbing

i) Tendrils

Named Example: Lathyrus odoratus (sweetpea)

Modified upper leaflets that become very thin and long, forming tendrils. They respond to contact and coil tightly around an object whereby plants gain stability and can climb.

Lathyrus odoratus (Sweetpea) leaves are compound. Each of the upper leaflets forms a tendril which coils around structures enabling it to climb

ii) Twining petioles

Named Example: Clematis montana

Modified leaf petioles that twist around other stems or wire/trellis for anchorage so they can climb. The petioles are long and loop around thin structures for anchorage.

This picture taken in February shows the old clematis petioles from last year with new leaf buds beginning to emerge. Although the old leaves have fallen from the petioles you can see how the long petioles twist and coil in order to cling onto something and facilitate climbing.

5. Attracting pollinators

Bracts

Named Example: Hydrangea arborescens ‘Annabelle’

Bracts are modified coloured leaves which look like flower petals, in fact they can be mistaken for petals. Species with bracts generally have very small / insignificant flowers. The colourful bracts attract the pollinator to the small flower in the centre.

Other examples for interest: Euphorbia pulcherimma (Poinsettia) has large red bracts, Euphorbia characias has lime green bracts surrounding a tiny flower. Eringium have a ‘ruff’ of decorative spikey bracts.

Close up of hydrangea bracts with tiny flower in the centre.
Mop head hydrangea ‘flowers’ which are mostly made up of bracts (adapted leaves for attracting pollinators)

So what about flowers and fruits?

There is a lot of information to learn about flowers and fruits. However, both flowers and fruits do one main job and are not greatly adapted to perform different functions compared to stems, roots and leaves.

Functions of Flowers:

  • Aid pollination by attracting pollinating insects
  • Give rise to seeds
  • Give rise to fruits

Different flowers have various characteristics depending on whether they are wind or insect pollinated so although they can look very different, they are still doing their main function – pollination, giving rise to seed and fruits. Hence there are no special adaptations to describe for flowers, just different characteristics.

Functions of a Fruits:

  • To aid in protection of the seeds
  • To aid in dispersal
  • May impose dormancy on the seed

The same can be said for fruits, as for flowers. There is a diverse range of fruit types ranging from tomatoes to nuts to poppy capsules. The different designs relate to dispersal methods. Nuts are gathered and buried for winter stores by squirrels whereas a poppy capsule sways in the breeze and seeds exit via small pores. There is a lot to learn about the characteristics of fruits and seeds and methods of dispersal but again they are not adaptations as they are all doing the same job: The fruits are designed to protect the seeds held within as well as spreading the seeds as far away from the parent plant as possible to avoid competition and increase their chance of survival.

Flowers and fruits have very diverse characteristics, but their function remains constant. Flowers are designed to ensure pollination and fruits are designed to protect and disperse seed. It is important to study the characteristics of wind and insect pollinated flowers and the different methods of fruit and seed dispersal, but this is more about diversity of characteristics rather than doing a different job. Hence there are no specific adaptations covered in this blog.

Confusing bits clarified

Rhizomes are mentioned in 2 sections under stem adapatations. For the food storage/perennation example, the named example is a big fat rhizome (Iris germanica) so in this case the adaptation is more about perennation rather than spreading. Rhizomes are also mentioned as a means of natural vegetative reproduction (Festuca rubra, a lawn grass) In this case the rhizomes are thin and long so the main adapation is spreading. HOWEVER there are some rhizomes like hedge bindweed which are spreading and quite fleshy in nature so could fit under the perennation or vegetative reproduction heading.

Climbing – Be careful to remember that for Hedera helix (Ivy) it is the roots that are adapted, not the stem.

Rhizomes and stolons

Rhizomes spread sideways underground and stolons spread sideways above ground.

Bulbs and corms look very similar from the outside but when you cut them in half, bulbs show the fleshy layers of leaves and corms have a hard solid centre which is stem. You just have to learn which is which.

Finally here are summary tables of everything covered above.

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