We have already seen how stimuli cause a taxis to occur in the case of simple plant organisms. Flowering plants, being fixed in, usually respond to a stimulus by a change usually of a type, called a tropism, of some particular part, so that its activities as a whole are controlled to some extent at least. Before dealing with these tropisms we will examine the case of opening and closing of the stomata, since these are the main openings by which carbon dioxide for enters a , photosynthesis being the fundamental initial process occurring in such plants.
Moreover, the rates of transpiration, of respiration in darkness, as well as of photosynthesis during daylight are dependent upon the condition of the stomata and lenticels. The plant cannot alter the size of the lenticels, but it is able to change the shape of the guard cells so that the stomata are variable in size. The factors controlling the size of the stomata are light, theof the surrounding air and the water supply from the . The stomata are generally much more abundant in or confined to the lower epidermis, and the guard cells are the only cells in that layer possessing chloroplasts. In darkness starch grains are found in the chloroplasts, but in light the starch is rapidly converted to sugar, causing the cell sap of the guard cells to acquire a higher osmotic pressure. More water is, therefore, drawn from the neighbouring epidermal cells into the vacuoles of the guard cells, which become more turgid in consequence. Their outer thinner walls stretch to a greater extent than those adjacent to the pores, causing the guard cells to curve apart and to widen the pores. This enables photosynthesis throughout the leaf to be speeded up, as more carbon dioxide can now enter the stomata. Thus the rate of photosynthesis is partly dependent upon the plant ’s response to the presence of light. When light fails, sugar in the cell sap is reconverted to starch in the chloroplasts, the guard cells lose water to the epidermal cells and straighten with resulting narrowing of the pores.
An artificial stoma can be made by taking two strips of celery leaf stalk or dandelion flower stalk, leaving them until they curl slightly and then binding them by rubber bands with the original outer surfaces in contact. Each strip represents a guard cell. On immersion in tap water they bend more, I.e. the pore opens as water goes into each strip. If placed in a salt solution the pore shuts, as water nowthe strips by osmosis.
THE GENERAL TROPISMS OF FLOWERING PLANTS
The response ofto the stimulus of sunlight is termed heliotropism. The aerial parts of plants are particularly sensitive to light, generally bending their or leaf stalks so that the are arranged in a fashion characteristic of each species of plant—generally with plants bearing wide leaves the arrangement is a mosaic, I.e. the leaf blades fit in together so that they do not overlap and so shade each other in order to obtain the greatest quantity of sunlight—and the blades are arranged normally to the sun ’s rays so that the intensity of light received is greatest. Some plants do not thrive in the most intense light. These either are most active in spring, their aerial parts dying off with the approach of summer, or they live in shady situations, or, if living in the open in summer, arrange their leaves diatropically to the stimulus of light—such a plant is the common iris, the spear-shaped leaves of which are arranged fanwise with their edges north and south, catching the morning and evening light and exposing the least available area during the middle of the day.
Proof that the stems and leaves respond to sunlight is easily obtained by the following experiment:—
Experiment 64—70 show the Effect of Light on Seedlings
Suitableare those of mustard. The in A develop in complete darkness, those in B are subjected to one-sided illumination, while those in C are equally illuminated on both sides.
Later it is found that thein A have long straggling stems with.sickly yellow undeveloped leaves. Those in B have stems bending towards the light with their leaves at right angles to the incident rays of light, but those in C have erect stems with spreading leaves.
It is evident that light has a tonic effect on the growth of stems and leaves. In its absence growth is acoelerated in stems, I.e. light retards growth, but the development of leaves and of chlorophyll is prevented. Such light-starved plants are said to be etiolated.
We can in some cases find experimentally the regions which are sensitive to light as distinct from those which respond.
Experiment 64a—To show that the Tips of Stems are Light-sensitive Organs
Young oat-grass seedlings can be shown to be sensitive at their tips by setting up three: A, normal seedlings ; B, seedlings capped with silver foil ; C, seedlings having the tips cut off previously with a sharp scalpel.
It is best to mark on each pot that side which is to face the light before setting up, and then to put each pot into a box lit from the side so that the mark li is facing in the direction of light. Suitable caps are easily made by rolling the foil around match sticks, care being taken not to make them too heavy. Only the seedlings in A curve towards the light.
In general, stems are negatively geotropic and roots positively geotropic—both responses useful to the plant, as stems reach light and air, whilst roots reach water and make a firm anchorage.
Experiment 65—To shozo Negative Geotropism of Stems
A pot containing broad bean seedlings is placed on its side and left overnight in a dark cupboard. The response near the growing tip is obvious, the tip curving upwards.
Experiment 66—To shozo Positive Geotropism of Roots
Another pot of broad bean seedlings is inverted or placed on its side. After two or three days hose out the soil gently from around the roots in order to expose them without damage or distortion. The mainwill have curved downwards and the lateral roots also will have curved correspondingly.
Experiment 67—To show that the Root Tip is the Sensitive Region
Three sets of bean seedlings are pinned to squares of cork which float in water, so that the main roots are in the water :—
Bean seedlings with their main roots horizontal.
Bean seedlings with thetips cut off and the main root horizontal.
As, but with the root tip stuck on again with a thickish layer of thick vaseline.
Only in docs a downward curvature result. This is not due to the weight ol ’ the root, for a curvature does not occur in either or.
The Klinostat. —This is an instrument used to demonstrate both helio- tropism and geotropism. Its action is to nullify the effects of unilateral illumination and of gravity on the direction of growth of main stems and roots. It consists of a cork disc to which germinating, e.g. beans, can be pinned. This. disc is rotated slowly by clock- work at about one revolution every fifteen minutes. The disc can be made to rotate in a vertical plane, , or swung round into a horizontal plane.
Experiment 68—To nullify the Effect of Unilateral Illumination
The disc is placed in the horizontaland a bean with its vertical is pinned on to the disc. The clockwork is started and the apparatus is placed on a window-ledge. No curvature results since each side of the stem is illuminated equally in turn. The clockwork is then stopped and curvature results.
Experiment 69—To nullify the Effect of Gravity
The disc is placed in the vertical position. A beanis pinned on to the disc with the main root horizontal. It is packed in damp moss and the celluloid cover is placed over it. The clockwork is started, and so long as the rotation is maintained no curvature results since each side of the root comes in turn nearest to the ground. On stopping the rotation a curvature soon appears.
Experiment 70—To show the Positive Hydrotropism of Roots
Broad bean seeds are planted at one end of a long narrow box. Once the seedlings are established, water the whole box thoroughly and then proceed to water it only at the end farthest from the seedlings. If the box has a sloping glass side the response of the roots can be readily seen when they reach this side. Failing this, the soil can be hosed away from the roots, the plants being left as found in the box for demonstration purposes. The roots will be seen to have grown towards the damper soil.
Experiment 70b—An Alternative to Experiment
Wet fibre is placed in a coarse-meshed sieve, cress seeds are scattered over it and then more fibre spread on top. This is kept well watered, most easily done by placing a flat sheet of filter-paper on the fibre and pouring water gently on it. The sieve is placed in a vessel of water covered by a bell jar and left until the young roots show clearly below the mesh of the sieve. Their appearance is then noted. The water in the vessel is now replaced by calcium chloride, making the atmosphere inside the bell jar very dry. The fibre is kept wet as before, so that the roots of young seedlings can only obtain water by bending back into the fibre. Some of them shrivel and die, but many respond. In this experiment the roots have to respond cither to the stimulus of water or of the force due to gravity, the former proving to be the more effective stimulus.
Manyflowers, e.g. common sunflower, dandelion, daisy, crocus and tulip, shut up with the approach of darkness, whilst others open, generally due to unequal growth of the cells at opposite surfaces of the petals. The cause of unequal growth is not definitely known, but the purpose of the movement is. Those which shut keep moisture from the parts within, whilst those which open are pollinated by nocturnal moths.
The compound leaves of some plants, e.g. mimosa, wood sorrel and clover, fold up their leaflets in a definite order in dim light so that the surface exposed is very much reduced, thereby reducing the amount of dew deposited, the amount of water lost by transpiration and consequently the amount of heat lost prior to transpiration. In the case of clover this is emphasized by the fact that the upper surface possesses more stomata than the lower surface. The movements are brought about by changes in turgidity of a set of cells called a pulvinus at the base of each leaflet.
It is well exhibited by all those plants which twine themselves about a support, e.g. hop, honeysuckle and runner bean, or which possess tendrils by which they cling to a support, since they lack the necessary rigidity for their own support. Various parts of particular plants function as tendrils; they include the bladeless leaves of cucumber, the terminal leaflets of pea and those of white bryony, the nature of which is doubtful. The ’sensitive plant ’ is particularly sensitive to touch. Leaves near the affected leaf also subsequently show a response. In this case the response is due to the passing of aproduced by the stimulation in the sap streaming along the stem to other parts.
All tropisms are growth responses. This may be illustrated, for example, in geotropism, as follows :—
A well-grown primary root produced by a germinating beanis marked with equidistant lines. The seed is then pinned to a cork with the root held horizontally and left in a damp atmos- phere. The region of curvature will be seen to occur in the growing region. If the root tip is removed, no curvature occurs. A similar result is obtained by treating a stem of a germinating bean in the same manner.
The elongation of the growing regions of stems and roots is due to the effect of growthon the cells of those regions. These substances are produced by the cells at the tips of the stems and roots and diffuse to the growing regions where they induce growth. In the case of gcotropism and phototropism, it has been shown that the effect of unequal stimulation on different sides of the stems or roots by gravity or light is to cause a redistribution of the auxins so that one side receives more than the others. A corresponding alteration in the rate of growth on the different sides occurs producing a curvature.
The difficulty experienced by some plants in obtaining certain substances in which their environment is deficient has been overcome in at least two ways. Leguminosa?, e.g. gorse, living in a soil lacking nitrates, house nitrogen-fixing bacteria by means of which they obtain their supplies of protein. Other plants are active in catching small animals, digesting them and absorbing the digested parts. Many show a marked response to the stimulus of touch and of particular chemical substances. Sundews curl their tentacles over insects and pieces of meat, secrete digestive juices and curl back the tentacles when absorption is complete so that the wind will blow the light remains away. The butterwort curls its greasy leaf-edges over its prey, uncurling them when the ’meal ’is over. Tropical insectivorous plants have specially sensitive areas, whilst the response is the shutting of a trap, e.g. the hinged leaf in Venus ’s fly-trap. The bladderwort catches small ’water-fleas ’in its bladders and digests them.
Protective movements other than those already dealt with include the movements of the flower stalks of the bluebell and ivy-leavtd toadflax, prior to fruit dispersal, and of the new leaves of horse-chestnut. Bluebellhang downwards, thereby preventing the inner parts from getting moistened by rain. Once has occurred each flower stalk bends upwards so that the fruit will have the open ends upwards, dispersal being effected by censer action. The flowrers of ivy-leaved toad-flax grow towards the sun, but their stalks bend away from light once the fruits begin to form, so that the seeds are deposited in the crevices of the wall where the plant commonly grows.