in this case the organ grows away from the light. These movements are spoken of as heliotropic and apheliotropic curvatures. The purpose of the movements bears out the contention that the plant is trying to adjust itself to its environment. The stem, by pointing directly to the light source, secures the best illumination possible for all of its leaves, the latter being distributed symmetrically around it. The root is made to press its way into the darker cracks and crannies of the soil, so bringing its root-hairs into better contact with the particles round which the hygroscopic water hangs. Leaves respond in another way to the same influence, placing themselves across the path of the beam of light.
Similar sensitivenesses can be demonstrated in other cases. When a root comes in contact at its tip with some hard body, such as might impede its progress, a curvature of the growing part is set up, which takes the young tip away from the stone, or what-not, with which it is in contact. When a sensitive tendril comes into contact with a foreign body, its growth becomes so modified that it twines round it. Many instances might be given of appreciation of and response to other changes in the environment by the growing parts of plants; among them we may mention the opening and closing of flowers during the days of their expansion. One somewhat similar phenomenon, differing in a few respects, marks the relation of the plant to the attraction of gravity. Observation of germinating seedlings makes it clear that somehow they have a perception of direction. The young roots grow vertically downwards, the young stems vertically upwards. Any attempt to interfere with these directions, by placing the seedlings in abnormal positions, is frustrated by the seedlings themselves, which change their direction of growth by bringing about curvatures of the different parts of their axes, so that the root soon grows vertically downward again and the stem in the opposite direction. Other and older plants give evidence of the same perception, though they do not respond all in the same way. Speaking generally, stems grow upwards and roots downwards. But some stems grow parallel to the surface of the soil, while the branches both of stems and roots tend to grow at a definite angle to the main axis from which they come. These movements are spoken of as different kinds of geotropic curvatures. This power of perception and response is not by any means confined to the growing organs, though in these it is especially striking, and plays a very evident part in the disposition of the growing organs in advantageous positions. It can, however, be seen in adult organs, though instances are less numerous.
When the pinnate leaf of a Mimosa pudica, the so-called sensitive plant, is pinched or struck, the leaf droops rapidly and the leaflets become approximated together, so that their upper surfaces are in contact. The extent to which the disturbance spreads depends on the violence of the stimulation—it may be confined to a few leaflets or it may extend to all the leaves of the plant.
The leaves and leaflets of many plants, e.g. the telegraph plant, Desmodium gyrans, behave in a similar way under the stimulus of approaching darkness.
A peculiar sensitiveness is manifested by the leaves of the so-called insectivorous plants. In the case of Dionaea muscipula we find a two-lobed lamina, the two lobes being connected by a midrib, which can play the part of a kind of hinge. Six sensitive hairs spring from the upper surface of the lobes, three from each; when one of these is touched the two lobes rapidly close, bringing their upper surfaces into contact and imprisoning anything which for the moment is between them. The mechanism is applied to the capture of insects alighting on the leaf.
Drosera, another of this insectivorous group, has leaves which are furnished with long glandular tentacles. When these are excited by the settling of an insect on the leaf they slowly bend over and imprison the intruder, which is detained there meanwhile by a sticky excretion poured out by the glands.
In both these cases the stimulation is followed, not only by movement, but by the secretion of an acid liquid containing a digestive juice, by virtue of which the insect is digested after being killed.
The purposeful character of all these movements or changes of position indicates that they are of nervous origin. We have in them evidence of two factors, a perception of some features of the environment and following this, after a longer or shorter interval, a response calculated to secure some advantage to the responding organ. We find on further investigation that these two conditions are traceable to different parts of the organs concerned. The perception of the changes, or, in other words the reception of the stimulus, is associated for example, with the tips of roots and the apices of stems. The first recognition of a specially receptive part was made by Charles Darwin, who identified the perception of stimulation with the tip of the young growing root. Amputation of this part involved the cessation of the response, even when the conditions normally causing the stimulation were maintained. Francis Darwin later demonstrated that the tips of the plumules of grasses were sensitive parts. The responding part is situated some little distance farther back, being in fact the region where growth is active. This bending part has been proved to be insensitive to the stimuli. There is consequently a transmission of the stimulus from the sensitive organ to a kind of motor mechanism situated some little way off. We find thus three factors of a nervous mechanism present, a receptive, a conducting, and a responding part. The differentiation of the plant's substance so indicated is, however, physiological only; there is no histological difference between the cells of these regions that can be associated with the several properties they possess. Even the root tip, which shows a certain differentiation into root cap and root apex, cannot be said to be a definite sense organ in the same way as the sense organs of an animal. The root is continually growing and so the sensitive part is continually changing its composition, cells being formed, growing and becoming permanent tissue. The cells of the tip at any given moment may be sensitive, but in a few days the power of receiving the stimulus has passed to other and younger cells which then constitute the tip. The power of appreciating the environment is therefore to be associated with the protoplasm only at a particular stage of its development and is transitory in its character.
What the nature of the stimulation is we are not able to say. The protoplasm is sensitive to particular influences, perhaps of vibration, or of contact or of chemical action. We can imagine though perhaps only vaguely, the way in which light, temperature, moisture, contact, &c., can affect it. The perception of direction or the influence of gravity presents greater difficulty, as we have no clear idea of the form which the force of gravity takes. Recently some investigations by Haberlandt, Noll, Darwin and others have suggested an explanation which has much to recommend it. The sensitive cells must clearly be influenced in some way by weight—not the weight of external organs but of some weight within them. This may possibly be the cell sap in their interior, which must exercise a slightly different hydrostatic pressure on the basal and the lateral walls of the cells. Or more probably it may be the weight of definite particulate structures in their vacuoles. Many experiments point to certain small grains of starch which are capable of displacement as the position of the cell is altered. Such small granules have been observed in the sensitive cells, and there is an evident correlation between these and the power of receiving the geotropic stimulus. It has been shown that if the organ containing them is shaken for some time, so that the contact between them and the protoplasm of the cells is emphasized, the stimulus becomes more efficient in producing movement. This reduces the stimulus to one of contact, which is in harmony with the observations made upon roots similarly stimulated from the exterior. The stimulating particles, whether starch grains in all cases, or other particles as well, have been termed statoliths.
We have spoken of the absence of structural differentiation in the sense organs. There is a similar difficulty in tracing the paths by which the impulses are transmitted to the growing and curving regions. The conduction of such stimulation to parts removed some distance from the sense organ suggests paths of transmission comparable to those which transmit nervous
