These are movements made by parts of plants in response top non-directional stimuli such as changes in temperature or light intensity. They may be growth movements or turgor movements.
If, in the petals of a young flower, the lower surface grows faster than the upper surface, the petals will curl upwards and inwards, closing over the reproductive organs. If the upper surface grows faster, the petals open outwards exposing the stamens and gynaecium. Certain flowers, e.g. Nicotiana, pollinated by night-flying insects, close during the day and open at night. Flowers of morning glory (Ipomea), Commelina and Hibiscus open during the day and are pollinated by birds or insects active in the light. The advantage of opening when pollinating insects are active is obvious but the benefit of closing at other times is not known. Closure during the day may reduce evaporation by transpiration and at night may prevent excessive cooling and prevent dew from forming on the stamens and carpels The nastic movements of leaves are usually due to changes in pressure in a group of specialized cells, called the pulvinus at the base of the leaf stalk. The stimulus may be a change in light intensity, temperature, wind velocity, touch or combinations of these. There may also be a rhythm of opening and closing independent of external stimuli. When the cells in the pulvinus of a leafor leaflet are fully turgid the leaf is held out from the stem with the leaflets expanded, so receiving maximum light. When the appropriate stimuli are received, water is withdrawn from the cells of the pulvinus, the cells lose their turgor and the leaf stalk droops. The pulvini of the leaflets behave in a similar way so that the leaflets fold over each other, reducing the exposed surface. The advantage of these movements, sometimes called "sleep movements 1s not clear but they may reduce transpiration at a time when photosynthesis (p. 46) cannot proceed or they may protect the leaf from damage in heavy rain or strong winds. The leaves of Acacia, Caesalpinia, Cassia and Delonix, for example, exhibit such "sleep movements" in response to reduction in light intensity and perhaps temperature at night time. When light intensity increases, the pulvinus cells take in water, increase their turgor and the leaf is lifted and expanded once again. The turgor movement in the leaves of Mimosa pudica is well known.
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ECOSYSTEMS, ENERGY FLOW IN AN ECOSYSTEM.
Dormancy
Most seeds when shed contain only about 10 per cent of water by weight. In this dry condition all the chemical processes of living are very slow and little food is used. The sects may remain alive in this state for long periods without germinating, though still able to do so when conditions are favourable. The seed is said to be dormant. If properly stored, wheat can still be germinated after about fifteen years, but the para rubber seed loses the power of germination after only a few days. The percentage of seeds which will germinate usually decreases with the length of time they are kept dormant.
In many tropical plants there is no dormancy period and in the mangrove, for example, germination of seeds occurs while they are still on the parent plant.
CONTROLLED EXPERIMENTS
A controlled experiment is one in which the experimenter controls the conditions. By this method he can be sure of the way in which these conditions influence the animal or plant.
If, out of 50 seeds shed by a wild flower, only 20 germinate and only 10 of these produce mature flowers, it is possible only to guess at the factors that might have been responsible. These could include dead seeds, attack by fungus or bacteria, un- favourable conditions of light, temperature, moisture or air.
All these conditions are beyond the control of an observer because he is unable to influence them in the plant's natural environment and he does not know the variations in them that have already taken place during the development of the plants.
To find out the importance of a particular condition to the normal development and existence of an animal or plant, the usual experimental practice is to exclude or vary this particular condition, keeping all others constant, and observe the effect on the plant or animal. Most experiments involve placing the plant or animal in an unusual situation, in boxes, jars or cages, when it can be argued that the peculiar experimental conditions are responsible for the observed results. For this reason, it is necessary to set up two almost identical experiments, the one with normal conditions and the other with the single eliminated or varied factor, any difference can then be attributed to the latter. The first experiment is called the control.
The control also enables the researcher to be sure that the results" of his experiment would not have occurred quite irrespective of the experimental conditions.