THE NATURALIST.

Otago Witness, Issue 3671, 22 July 1924, Page 66

THE NATURALIST.

MOVEMENTS OF PLANTS. By the llon. G. M. Thomson, M.L.C. On asking a schoolboy, “What do you think is the chief difference between animals and plants?” he answered that animals can move, but plants cannot. That is a very common idea, but a quite erroneous one. We have only to consider what takes place in certain carnivorous plants, especially sundews and Venus’s fly-trap, when catching their prey, namely, that some plants cannot only move, but that they move, as it were, sentiently. One statement can be made which may seem very improbable to many people at first sight, but which is absolutely true. It is that at some stage or period in its life, every plant—whether its development be high or low —must move either actively or passively. It must either be furnished with motile organs or it must be carried. This is a necessity of its existence. In the -lowest or most primitive plants the cells are frequently furnished with cilia, or fine strands of naked protoplasm, by means of which they swim actively in the water in which they live. As we consider the higher stages of development, we find that the spores must be carried by wind or by water. In the highest plants there is a resting stage in the development of each, which we call the seed, and this must be moved away by some agency from the parent plant in order to find room to live.

But that is not what I mean by movements of plants. In a great number of the higher plants, probably in all, there are active movements in the plant itself caused by some outside agency, but originating in the internal organisation. The most potent of these agencies is the action of light, and this manifests itself in many ways and in many directions. An enormous literature has grown up oil this subject during the last eighty years; a literature which received its greatest stimulus since the publication in 1865 of a paper by Charles Darwin on “The movements and habits of climbing plants,” which appeared in the journal of the Binnean Society. I can here only indicate some of the most conspicuous and familiar facts, proof of some of which can be ascertained for themselves by any careful observer. In the first place all plants bearing green leaves, indeed all plants which contain chlorophyll in their external tissues, mast have access to light; this is an essential in their existence. If a mass of seedlings are allowed to grow on any cleared piece of ground, those which arc strong enough to overtop their neighbours and secure the maximum of sunlight, will in time put most of their competitors out of existence. If you plant a potato in open ground where it is subjected to the fullest action of sunlight, it will produce stout stems, perhaps a foot or more in height. But if at the same time you plant another potato tuber in the midst of dense bush, it will develop long, weak, straggling stems in its efforts to get its foliage up towards the sunshine.

Many plants with long erect stems, though apparently exhibiting no power of movement, will be found to possess some power of response to the direct action of light. If two or three straight sticks are thrust vertically into the ground round and close to the growing stem or shoot, and the position of the latter be noted early in the day before the sun is up, it will be found that it does not grow perfectly straight up, but sways slightly, so that its apex usually describes a curve, or portion of an elliptic movement during the hours of sunshine. Some do this to such an extent that they become what are called “twining” plants. To say that twining plants revolve round a support in .response to the influence of light would he quite misleading. But that the original swaying or revolving movement which ultimately developed into the twining habit arose in response to the active influence of light is quite a probable hypothesis. It was long ago suggested that the twisting of plant stems was due to the outer layers of tissue in the stem continuing to grow after the inner layers had ceased to grow, or were slowing down in their growth. In twining plants every part of the revolving slioot lias its own separate and independent movement. If you tic the lower half or more of the long revolving shoot of a French bean, a eonvolvulus. or a hop-plant to a straight upright stick, you will find that the upper free part continues to revolve, and every internode—that is, the portion of stem between the points of attachment of the leaves —moves independently. Darwin wrote: —-“If a coloured streak be painted along the convex surface, the streak will after a time be found to be running laterally along one side of the bow, then along the concave side, then laterally on the opposite side, and, lastly, again on the originally convex surface. This clearly proves that during revolving movement the internodes become bowed in every direction. The movement is, in fact, a continuous self-bowing of the whole shoot, successively directed to all points of the compass.” All these movements are certainly due to growth, but the growth itself Is dependent on the influence of sunlight. The rate of revolution depends on the vigour of the growing plant, and the conditions prevailing at the time, but a French bean or scarlet runner will—when these are favourable—-make a complete revolution round a support in two hours. These revolutions are not made with the sun in all cases, ns we might expect. The hop plant and the honeysuckle move with the sun, but climbing asparagus, wistaria, French bean, convolvulus, and our rar« climbing fern Jygodium, all move

against the sun. The black bindweed — Polygonum convul vulus—which is a very common weed in many gardens, grows as a twiner during the middle of the summer, but plants growing vigorously in tlie autumn do not seem to twine at all.

A more specialised kind of movement is shown by such climbers as the native ■species of clematis. In these the petioles of the leaves and of the leaflets are irritable, especially when the leaves are very young, and tend to move in the direction from whence the irritation comes. Irritation in most highly organised structures, whether plant or animal, is nearly always accompanied by increased cell growth, as we know, for example, when a boot's continuous pressure on part of the foot develops a corn. But in the plants referred to, the first effect of irritation is movement. If the stalk of a young leaf of clematis is gently “rubbed with a thin twig a few times on any side, it will in the course of a few hours bend to that side.” If this happens to a plant growing iii the open, say among scrub or on the edge of the bush, the young leaf-stalk continues to move or bend to the side which is rubbed. So that if the wind sways the young leaves against the twigs of a tree or bush, the stalk continues to bend until it encircles the object which it was rubbed against. If you closely examine any clematis growing on the edge of the forest, you will find that it is the leaf-stalks or petioles which have taken hold of the supporting plant. Wherever this holding of the support has taken place, the tissues of the clematis have greatly increased in size, so that they are there thicker and stronger than in other parts. The common garden nasturtium or Indian cress—Tropmolum majus—shows the same characteristic to a slight extent, but some other species of the same genus have much more sensitive leaf-petioles. A climber which is very commonly grown in Dunedin gardens is Solatium jasminoides, and this species is a true leaf-climber. Though a strong-growing plant, the leaf movements are slow com-p-'rpd to many others, but they are very effective. “The flexible petiole of a half or a quarter-grown leaf which has clasped an object for three or four days increases much in thickness, and after several weeks becomes so wonderfully hard and rigid that it can hardly be removed from its support.” Still another example of leaf-climbing is found in tlio common fumitory—Fumaria officinalis or allied species—which is such an abundant weed in this country. The leaves and the petioles, when young, appear to be continually moving in search of supports to which they can cling. Of course, it is impossible to see these movements of twining plants and leaf-climbers, because they are so slow, but they can be quite easily observed by marking the position of the parts at any particular time, and then recording the change of position at later periods. One object gained by these climbing plants is the faculty of exposing tlieir loaves and other green tissues to the action of light, and of producing their flowers in conspicuous positions, where they can be seen, and visited by insects. By using other plants as supports they are able to gain these ends by the expenditure of the minimum amount of energy, and the production of the minimum amount of merely supporting material, such as goes to form the strong rigid stems of the plants they grow upon. Economy of labour and production is a law of nature —all unnecessary organs and tissues tend to be reduced or suppressed, or to disappear, and this is

seen in these twiners and leaf-climbers which have thin stems only large enough to contain the conducting tissue they require, but useless for support. There are climbing plants which appear to have very little power of free movement. and three common examples may be mentioned here. The bush lawyer— Bobus australis—climbs by hooks on the midribs of its leaves and leaf-petioles. These are originally trichorr.es or liair-lilce structures of the epidermis, which become thickened and much curved. The young shoots and leaves of this plant certainly look as if they were in search of suitable supports to which they might cling, but no one seems to have detected any definite movements. The plant is so abundant, however, that it should not be difficult to observe and test them. An extremely common weed in hedges and on waste ground is known as cleavers, or Galium aparinc. The stems are strongly ridged, as are the petioles and the midribs of the leaves, and these ridges .are very hard anti sharp, and are hooked at very frequent intervals. This weed also does not show any free power of movement, or at least such has not been observed. Lastly, a certain number of plants, of which ivy is the most familiar example, climb by means of specially developed roots. These show the true root character of always growing away from the light, and seeking to penetrate dark crevices, whether in banks, walls, or tree trunks. These also exhibit no free power of movement. But in a great number of plants, organs specially designed to act as climbing organs are developed, which possess highly sensitive powers of movement but these must be left for another paper.