WHAT DOES A PLANT DO WHEN IT GROWS?

Up to this point we have considered some of the processes which go on in plants, and for convenience they have been considered separately. Some attempt must now be made to take the plant as a whole. One of the most striking features of plants is the fact that they grow. This usually means that they increase in size, protecting their real living substance inside walls of cellulose and other substances probably composed largely of carbohydrate waste from the various processes going on in the plant. As most plants are unable to move about, and therefore have to obtain their supplies from a restricted field, it is hardly surprising that the actively growing parts of the plant lie either at the tips of the roots and branches, or disposed in a layer as close to the air as possible, just within the protective coating which covers the surface and does much to check excessive loss of water.

Mere increase in size and weight does not constitute growth; a piece of dead dry wood will absorb water and increase both in weight and size, but it returns on drying to its former

state. On the other hand, when a plant grows, it undergoes a permanent change, and this change cannot be reversed. Once a seed has germinated, it cannot be put back into the resting condition; cells within it have divided, and chemical changes have gone on; changes of this sort accompany all true growth. For a short time roots may actually decrease in diameter as they grow, but this is a somewhat unusual phenomenon.

It will be remembered that the unit of plant construction, and the unit of plant activity, is the cell. The growth of the cell starts with the division of its nucleus into two nuclei and the organisation of a cell around each of the two. In general, cells which are able to divide are distinguished by the possession of rich and dense cytoplasm, and by the small development of their vacuoles or sap-containing spaces; it is only when the cell is approaching maturity that a large single central vacuole becomes prominent. Some cells undergo very considerable changes in size and form before they are fully mature, and, once mature, they have generally lost the power to divide any more, and become incorporated in some part of the plant which is fully organised.

FINDING OUT HOW FAST A PLANT GROWS

THERE are various methods of measuring the rate at which a plant grows. The elongation of stems and of roots can be directly measured, and it has been shown that the rate at which a whole plant is growing can be very satisfactorily determined by periodical measurements of the total area of the leaves on the plant. In stems and roots the whole of the organ does not elongate, but growth is localised in well-defined regions a little behind the tips, and, as the whole organ increases in length, the portion that is actually elongating keeps at an approximately constant distance behind the tip. In roots the elongating region lies between the area covered by root hairs and the tip; it is obvious that root hairs could not make and keep contact with soil particles if they were attached to a portion of the root which was increasing in length. In stems, the elongating region corresponds approximately to the portion bearing partly developed leaves.

In the whole plant, so long as it is not preparing to flower, •measurements of leaf area show that the rate of growth increases in the same way as money increases when it is put out at compound interest, that is, that the rate of growth is

proportional to the amount of plant material already present. When, however, the plant is preparing to fiowrer, this relation ceases to hold, since the reproductive processes divert the energy and material of the plant to uses other than ordinary growth in size.

INFLUENCES WHICH CHANGE THE BEHAVIOUR OF PLANTS ALTHOUGH plants are mostly sedentary organisms, anchored to the soil by their roots, yet many of them are capable of limited movements, which, in the higher plants, usually depend on changes in the direction of growth. It is well known for example that if plants are set in a window and left undisturbed, their branches tend to grow towards the light, and, it may be shown in some plants, that if their roots are exposed to one-sided illumination, they tend to grow away from the light. Similarly, if plants with erect stems and downwardly pointing roots are set on their sides, one soon finds that the ends of the young stems begin to grow upwards, and the ends of the young roots downwards. These changes in direction occur in the parts of the organs that are still elongating, and are caused by one side of the organ growing faster than the other. Old organs which have got beyond the stage when they can elongate are generally unable to adjust themselves in this way.

When plants show a change in behaviour which can be related to some change in the conditions surrounding them, they are said to react to a stimulus. For instance, if a plant is placed on its side its position is changed in relation to gravity; if a plant which has been growing in a well-lit situation is put into a place where light comes to it from one side only, the direction of the light falling on the plant is altered. These changes provide the stimuli, but it by no means follows that the stimulus provides the energy used up in the subsequent reaction by the plant. Indeed, the stimuli which influence the behaviour of plants have been compared with the trigger of a gun : the trigger starts the explosion, but the violence of the discharge depends on the amount of explosive present; a stimulus calls forth a change in the behaviour of the plant, but the nature and extent of the change depends on the plant itself.

The chief stimuli which provoke response in plants are connected with light and with gravity; contact with solid objects is an important stimulus to climbing plants, and

variations in the humidity of the soil may provoke response in roots. If a plant is placed on its side it does not at once begin to curve; it has to be left for half an hour or so before the change in the direction of gravity begins to affect it. Consequently, if we support a plant in a horizontal position on a vertical turntable which rotates slowly and thus neutralises the one-sided effect we find that the stem and root continue to grow straight on; similarly, if a plant is rotated on itself slowly in a vertical position in light falling from one side only there is no response. Such experiments indicate clearly that gravity—or light—have some part in the behaviour of the plant when it is displaced and left so for some time.

Measurements show that a stem elongates faster in darkness than in light. When, therefore, we place a stem so that light falls on it on one side only, it is not surprising that the tip of the stem is brought towards the light, since the badly-lit side elongates faster than the well-lit side, and so must turn the tip to the light. In a horizontal stem the lower side elongates faster than the upper side, and so the tip is re-erected. The advantage of this to the plant is obvious, but it must not be concluded that the movement is consciously purposeful; of that we have no evidence whatever.

Movements of these kinds can only occur when the conditions surrounding the plant are favourable to growth, and of these conditions, a suitable temperature is the most potent; response is always slow in cold weather. It has already been pointed out that the plant has to be exposed to the changed conditions for a definite time if response is to follow. If, for example, an erect stem is placed in one-sided light for 20-40 minutes, and then, while still vertical, is rotated about its own long axis in complete darkness, it nevertheless proceeds to develop the characteristic curvatures, indicating clearly that response does not depend on the continued presence of the appropriate stimulus.

There is reason to think that stimuli are perceived by the tips of growing organs, and that in some way these tips then transmit something to the regions of elongation beneath them, where the reaction becomes apparent. If the tips are cut off from roots, and the roots are then placed on their sides, very little curvature occurs, though the region of elongation remains active. After some two days, however, curvatures may begin to appear, and it may now be shown

that the end of the stump has regained some measure of sensitiveness.

If, however, roots are placed on their side for a time, and the tips are then cut off and replaced in position with a little gelatine between them and the stumps, curvature follows in an almost normal way; presumably something diffuses through the gelatine from the tips and the usual growth changes follow.

Many and diverse experiments of this sort suggest strongly that the responses of plants are connected with the production of some special substance inside the plant, and that this affects growth. We know little as yet of these substances, and of the manner in which they affect the behaviour of the plant, but it seems probable that they are akin to certain substances which are already known to have noteworthy physiological effects, and to substances which are believed to cause cancer in animals.

A PLANT THAT COLLAPSES AT A TOUCH

IN addition to these localised movements, many plants show movements of a more general kind. Of such plants, the best known is probably the Sensitive Plant. This is a small plant distantly related to peas and beans. It has feathery-leaves which collapse suddenly if the plant is shaken or exposed to the vapour of ammonia or other potent fluids. If the small leaflets at the end of a part of the leaf are gently touched they close together, and then successive pairs farther and farther away from the end do so in turn; this looks as though a stimulus is transmitted along the leaf. With a rougher touch the stimulus travels more quickly, and the whole leaf may collapse. In warm weather the leaf regains its stiffness in about half an hour. Although the exact mechanism remains obscure, it seems probable that the collapse of the leaves is due to the temporary loss of water from the cells, as a result of stimulation.

The opening and closing of flowers is usually due to growth at the bases of the floral leaves. The flowers of tulips and crocuses, for example, open in warmth and close in the cold. When the flowers open there is sudden growth at the base of the floral leaves, and as they increase in length they necessarily separate; when they close the growth occurs at the base of the other sides of the leaves, causing them to shut together.

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