Page:EB1911 - Volume 21.djvu/783

Rh the framework of the fabric of the cell, and the construction of a continuously increasing skeleton; part is used in maintaining the normal temperature of the plant, part in constructing various substances which are met with in the interior, which serve various purposes in the working of the vital mechanism. A great part again is utilized in that increase of the body of the plant which we call growth.

Growth, as usually spoken of, includes two essentially different processes. The first of these, which may be regarded as growth proper, is the manufacture of additional quantities of living substance. The second, which is usually included in the term, is the increase of such accessories of living substance as are necessary for its well-being. These include cell walls and the various stored products found in growing cells. There is clearly a difference between these two categories. The formation of living substance is a process of building up from simple or relatively simple materials; the construction of its cellulose framework and supporting substance is done by the living substance after its own formation is completed, and is attended by a partial decomposition of such living substance.

Growth is always going on in plants while they are alive. Even the oldest trees put out continually new leaves and twigs. It does not, of course, follow that increase of bulk is always conspicuous; in such trees death is present side by side with life, and the one often counterbalances the other. As, however, we can easily see that the constructive processes are much greater than those which lead to the disappearance of material from the plant-body, there is generally to be seen a conspicuous increase in the substance of the plant. This is, in nearly all cases, attended by a permanent change in form. This is not perhaps so evident in the case of axial organs as it is in that of leaves and their modifications, but even in them it can be detected to a certain extent.

In the lowliest plants growth may be co-extensive with the plant-body; In all plants of any considerable size, however, it is localized in particular regions, and in them it is associated with the formation of new protoplasts or cells. These regions have been called growing points. In such stems and roots as increase in thickness there are other growing regions, which consist of cylindrical sheaths known as cambium layers or phellogens. By the multiplication of the protoplasts in these merismatic areas the substance of the plant is increased. In other words, as these growing regions consist of cells, the growth of the entire organ or plant will depend upon the behaviour of the cells or protoplasts of which the merismatic tissues are composed.

The growth of such a cell will be found to depend mainly upon five conditions: (1) There must be a supply of nutritive or plastic materials, at the expense of which the increase of its living substance can take place and which supply the needed potential energy. (2) There must be a supply of water to such an extent as to set up a certain hydrostatic pressure in the cell, for only turgid cells can grow. (3) The supply of water must be associated with the formation of osmotic substances in the cell, or it cannot be made to enter it. (4) The cell must have a certain temperature, for the activity of a protoplast is only possible within certain limits, which differ in the case of different plants. (5) There must be a supply of oxygen to the growing cell, for the protoplast is dependent upon this gas for the performance of its vital functions, and particularly for the liberation of the energy which is demanded in the constructive processes. This is evident from the consideration that the growth of the cells is attended by the growth in surface of the cell wall, and as the latter is a secretion from the protoplasm, such a decomposition cannot readily take place unless oxygen is admitted to it.

When these conditions are present, the course of the growth of a cell appears to be the following: The young cell, immediately it is cut off from its fellow, absorbs water, in consequence of the presence in it of osmotically active substances. With the water it takes in the various nutritive substances which the former contains in solution. There is set up at once a certain hydrostatic pressure, due to the turgidity which ensues upon such absorption, and the extensible cell wall stretches, at first in all directions. The growth or increase of the protoplasm at the expense of the nutritive matter for a time keeps pace with the increased size of the cell, but by and by it becomes vacuolated as more and more water is attracted into the interior. Eventually the protoplasm usually forms only a lining to the cell wall, and a large vacuole filled with cell sap occupies the centre. The growth of the protoplasm, though considerable, is therefore not commensurate with the increase in the size of the cell. The stretching of the cell wall by the hydrostatic pressure is fixed by a secretion of new particles and their deposition upon the original wall, which as it becomes slightly thicker is capable of still greater extension, much in the same way as a thick band of india-rubber is capable of undergoing greater stretching than a thin one. The increase in surface of the cell wall is thus due—firstly to the stretching caused by turgidity, and secondly to the formation and deposition of new substance upon the old. When the limit of extensibility is reached the cell wall increases in thickness from the continuation of the latter of the two processes.

The rate of growth of a cell varies gradually throughout its course; it begins slowly, increases to a maximum, and then becomes slower till it stops. The time during which these regular changes in the rate can be observed is generally spoken of as the grand period of growth.

If we consider the behaviour of a growing organ such as a root, we find that, like a cell, it shows a grand period of growth. Just behind its apex the cells are found to be all in process of active division. Growth is small, and consists mainly in an increase of the quantity of protoplasm, for the cells divide again as soon as they have reached a certain size. As new cells are continually formed in the merismatic mass those which are farthest from the apex gradually cease to divide and a different process of growth takes place in them, which is associated more particularly with the formation of the vacuoles, consequent upon the establishment of considerable hydrostatic pressure in them, thus causing the bulk of the cells to be greatly enlarged. Here it is that the actual extension in length of the root takes place, and the cells reach the maximum point of the grand period. They then gradually lose the power of growth, the oldest ones or those farthest from the apex parting with it first, and they pass gradually over into the condition of the permanent tissues.

The same order of events may be ascertained to take place in the stem; but in this region it is complicated by the occurrence of nodes and internodes, growth in length being confined to the latter, many of which may be growing simultaneously. The region of growth in the stem is, as a rule, much longer than that of the root. The growth of the leaf is at first apical, but this is not very prolonged, and the subsequent enlargement is due to an intercalary growing region near the base.

The turgidity in the cells of a growing member is not uniform, but shows a fairly rhythmical variation in its different parts. If the member is one which shows a difference of structure on two sides, such as a leaf, the two sides frequently show a difference of degree of turgidity, and consequently of rate of growth. If we consider a leaf of the common fern we find that in its young condition it is closely rolled up, the upper or ventral surface being quite concealed. As it gets older it gradually unfolds and expands into the adult form. This is due to the fact that while young the turgidity and consequent growth are greater in the dorsal side of the leaf, so that it becomes rolled up. As it develops the maximum turgidity and growth change to its upper side, and so it becomes unfolded or expanded. These two conditions are generally described under the names of hyponasty and epinasty respectively.

Cylindrical organs may exhibit similar phenomena. One side of a stem may be more turgid than the opposite one, and the maximum turgidity, with its consequent growth, may alternate between two opposite sides. The growing apex of such a stem will alternately incline, first to one side and then to the other, exhibiting a kind of nodding movement in the two directions. More frequently the region of maximum turgidity passes gradually round the growing zone. The apex in this case will describe a circle, or rather a spiral, as it is elongating all the time, pointing to all points of the compass in succession. This continuous change of position has been called circumnutation, and is held to be universal in all growing cylindrical organs. The passage of the maximum turgidity round the stem may vary in rapidity in different places, causing the circle to be replaced by an ellipse. The bending to two sides alternately, described above, often called simple nutation, may be regarded as only an extreme instance of the latter.

Nervous System of Plants.—So far we have considered the plant almost exclusively as an individual organism, carrying out its own vital processes, and unaffected by its surroundings except in so far as these supply it with the materials for its well-being. When we consider, however, the great variability in those surroundings and the consequent changes a plant must encounter, it appears obvious that interaction and adjustment between the plant and its environment must be constant and well balanced. That such adjustment shall take place postulates on the part of the plant a kind of perception or appreciation of the changing conditions which affect it.

Careful examination soon shows an observer that such perceptions exist, and that they are followed by certain purposeful changes in the plant, sometimes mechanical, sometimes chemical, the object being evidently to secure some advantage for the plant, to ward off some danger, or to extricate it from some difficulty. We may speak, indeed, of the plant as possessed of a rudimentary nervous system, by the aid of which necessary adjustments are brought about. The most constantly occurring changes that beset a plant are connected with illumination, temperature, moisture, and contact with foreign bodies. Setting aside other susceptibilities, we have evidence that most plants are sensitive to all these.

If a growing stem receives stronger illumination on one side than another, its apex slowly turns from the vertical in the direction of the light source, continuing its change of position until it is in a direct line with the incident rays. If a root is similarly illuminated, a similar change of direction of growth follows, but