Sample units & exercises

From Unit 2:

PLUMBING AND STRUCTURAL ENGINEERING

Plants don’t need a skeleton because each cell is surrounded by a rigid wall. In herbaceous (non-woody) plants the principal cell wall material is cellulose, but herbaceous plants are actually supported mainly by water. The vacuole in the centre of each cell is full of water or, rather, an aqueous solution. As long as the vacuole is full, the cell contents push against the cellulose wall making the cell hard like a blown-up balloon. In a drought, the cells lose water and shrink so the ‘balloons’ become soft and the whole plant will flop, or wilt.

Plants are non-motile, i.e. the whole plant is unable to move, but a limited range of movement of some cells or organs is possible. Examples are the opening and closing of stomata, leaf rolling or folding at night or in response to drought, the snapping shut of leaves of the insectivorous Venus Fly-trap and the sudden folding of the leaves of the Sensitive Plant. All of these movements are activated by the transfer of water between cells causing some to shrink and others to swell, usually with a hinge effect.

Where are the water pumps?
The answer is that there are none, despite the fact that huge volumes of water are moved from the soil to treetops, and that water movement is responsible for the opening and closing of stomata and the other movements described above.

Two physical processes are responsible for the bulk of the water movement in plants:

1. Evaporation from the leaves, caused by the heat of the sun
2. Osmosis: the movement of water molecules through a partially permeable membrane from a less concentrated solution to a more concentrated one.

Water and sugary sap are moved around the plant through veins called vascular bundles. These contain two different channels: the xylem (zy-lem), which transports water from the roots upwards, and the phloem (flow-em), which transports sap from the leaves down to the roots and up to the apical growing region and the flowers. Xylem cells have to be strong enough to avoid collapsing under the great tension that sometimes develops within them. Their walls are strengthened with lignin, and may become very thick. Lignin has amazing properties of strength combined with slight flexibility and durability.

Because it contains lignin, vascular tissue has the second function of providing strength to a plant. In the stem of a herbaceous plant the vascular bundles are arranged around the periphery. This is the best mechanical arrangement to resist bending or breaking in the wind. At the air-soil boundary, where shoot becomes root, the vascular bundles converge towards the centre. This is the best arrangement for resisting the vertical pull that happens when a herbivore (or gardener) tugs on the plant from above.

From Unit 6:

TAXI SERVICE

Many plants simply drop their seeds and rely on animals’ feet to transport them. Pineappleweed prefers motor transport. Its seeds are picked up in the treads of car tyres and it can be found in practically every gateway, lay-by and car park. It originated in Asia and N America but is reckoned to be one of the fastest spreading species in the 20th century and can now be found throughout Europe.

Birds are excellent seed-carriers and almost all of our hedgerow trees, shrubs and climbers are adapted to them, producing succulent fruit. Most berries are red, this being the most attractive colour to birds, although some are blue or black. The bird digests the fruit but the hard seeds pass through its gut and are deposited under another hedge. Unripe berries are green, hard and bitter but the ripening of the seeds is a signal for the colour change accompanied by softening and sweetening of the fruit.

Yew is unusual because it is not a flowering plant but technically a conifer although it doesn’t bear cones. Its ‘fruits’ are called arils. It is a poisonous plant with toxic alkaloids in all parts of the tree except the red flesh around the aril. The dark seed in the middle is poisonous. If a child swallowed the arils it would be fine provided that it didn’t scrunch the seeds.

Another way of hitch-hiking is to produce hooked or spiky seeds (burs) that will stick to the coats of passing animals. Many plants do this, one of the commonest being Cleavers / Goosegrass / Sticky Willy. It is so common around gardens and hedges that humans and their dogs probably carry it more than wild animals, and it was apparently the inspiration for Velcro.

Pirri-pirri-bur is an alien from Australia and New Zealand that arrived attached to imported sheeps’ wool. This might hold the record for long distance travel.

PRACTICAL EXERCISES

From set 1:

BEAN SHOOT GROWTH

(Unit 3, p.1)
You need: 2 flower pots with soil or compost
A few broad beans
Method:
Soak the beans for 24 hrs. Moisten the compost, making sure that it is slightly damp but still friable. Plant 2 or 3 beans in each pot, 1-2 cm deep. Enclose the pots in plastic bags and, if the weather is cool, put them in a warm place until you can see shoots appearing. (If nothing happens check that the compost is not too dry or, conversely, that it is not too wet and they have rotted.) When germination is visible select one in each pot and remove the others. This is day 1. Remove the plastic bags and place one pot in the light, the other in a dark cupboard. Water the compost as required. The ‘dark’ pot should only be removed from its cupboard briefly for observation and watering.

You should see the stages of germination listed below. Record the day on which each stage is reached as day 1, day 2 etc – although you can add the date if it helps you to keep track.

• Shoot appeared
• Shoot in a hook shape
• Green colour
• Shoot becomes straight
• Leaves spread sideways
• Shoot grows towards window (in the ‘light’ pot)

Record the height of both seedlings on two or three occasions, measuring both on the same day, maybe after 3 days, 1 week and 2 weeks. (You can change this according to the growth rate of your own plants – ambient temperature will affect this.)

Interpretation:
Light is crucial to plant growth. A seedling must reach the light and start to photosynthesize before its food store is exhausted, or it will die. This experiment is designed to show several of a seedling’s responses to light. Have you been able to conclude any of the following?

• The shoot initially forms a hook which protects the apical bud.
• Light induces the hook to straighten
• Light induces chlorophyll to be formed
• Light induces the leaves to spread
• Light inhibits (reduces) growth in length of the shoot.
• If the light is coming from one side the plant grows towards it

Note that darkness is the absence of light. It is not possible to measure the amount of darkness but only the amount of light.

ENTER YOUR OBSERVATIONS AND CONCLUSIONS ON THE ANSWER SHEET

From set 2:

MOSSES

(Unit 7 part I, pp 4-6)

Mosses are most conspicuous in winter when most of the flowering plants are dormant. This is also the time when many of them form their capsules. Walking through a wood you will see their green cushions on the otherwise bare ground, on logs and climbing up tree trunks. But there are plenty of them in town also, on walls, rooves and other masonry and amongst the grass on lawns. Have you ever looked at them closely? If this is the first time you are bound to be impressed, but you do need to use a lens.

The task is to find 3 different mosses:

1. growing on soil, maybe amongst grass or other vegetation
2. growing on the bark of a living tree, dead log or tree stump
3. growing on a hard substrate – rock, stone, brick, tiles etc.

If the plant has dried up, put it in a plastic bag or box, spray with water, seal it and leave it for a while. Mosses will revive.

Examine each one carefully, using a handlens. Since their habitats are different you will have found three different species. Record the following features, giving measurements in mm:

• Are capsules present?
• Plant height (excluding capsules)
• Are stems upright or prostrate or arched?
• Do main stems branch often/rarely/never?
• Approx. length of longest leaves.
• Are leaves wrinkled or smooth?
• Are leaf edges smooth or minutely toothed?
• Do leaves end in a long white spike?
• Are leaves ± flat or strongly curved?
• If curved, do they curve inwards or outwards (giving a snake-like or spiky appearance)?
• How long are the capsules (excluding stalk)?
• Do the capsules have hairy caps?

There are so many species of moss that your tutor won’t give you an identification unless it appears to be one of a small number of extremely common species. The object of this exercise is for you to look closely at these fascinating little plants and to observe some of the characters that are used in identification. As you will realise, a magnification greater than x10 is necessary for accurate moss identification and capsules may be required.

ENTER YOUR OBSERVATIONS ON THE ANSWER SHEET