Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/709

] we collectively terra Fuwji may, and probably do, include descendants of the original stock which existed before plants possessed chlorophyll at all. No doubt also Fungi com prise plants which are destitute of the chlorophyll possessed by their near allies, in consequence of the degeneration due to a parasitic mode of life. Amongst Flowering Plants we cannot doubt that this has been the case with Cuscuta, Orobanche, Lathrcea, and many others. But besides plants which are actually parasitic, there are other degraded allies of green plants, which are content to work up again the imperfectly broken down products of decay. Such plants are termed Saprophytes; many examples of them exist amongst the Orchidacece, such as Neottia, Epipogium, and Corallorrhiza. They live upon the products of the decom position of vegetable matter, and have more or less com pletely lost the characteristic green tint of chlorophyll, which would be useless to them if they possessed it. But perhaps the most curious case of the occasional disposition of even green plants to seize upon nutritive matter in an available state of chemical aggregation, is that which is met with in the numerous examples now known of insectivorous plants. In these latter cases we certainly find morphological adaptation of considerable complexity for purposes of nutrition. But in the vegetable kingdom generally this is certainly the exception rather than the rnle. In the animal kingdom it is very different. Amongst plants, however, adaptations of structure which have reference to reproduction assume far greater importance, and these have to a large extent to be relied upon for taxonomic purposes. Even in the highest plants the physiological division of labour is very small compared with the extent to which it exists amongst animals. From plants of the simplest structure up to the most complicated, the plan of nutrition retains the same broad features. There are few physiolo gical facts of real importance to be observed iu the highest terms of the series which may not be equally well studied in the lower. Amongst such of the lower plants as are aquatic in their mode of life the protoplasm of individual cells is often broken up into fragments, very minute in size, which are set free in the surrounding fluid, and being furnished with cilia or motile filamentous prolongations of their protoplasm, rapidly disperse themselves over a consider able area. Such locomotive organisms are usually called zoospores. After a time each is invested with a cell-wall composed of cellulose; this differs entirely in composi tion from protoplasm, especially in containing no nitrogen. The production of the cell-wall is not therefore to be regarded as a modification of any part of the proto plasm, but as a segregation of particles of cellulose which were intermixed with it ; such a segregation goes on repeatedly, wherever life exists in plant tissues. Starch, which is identical in ultimate composition with cellulose, we know to be fabricated from inorganic materials in the chlorophyll-granules (which are specialized portions of protoplasm) under the influence of light. Cellulose is derived from the starch so manufactured, and is dispersed in a state probably of molecular subdivision throughout the protoplasm. The cellulose wall is not apparently essential to the con ception of a vegetable cell, but it is, perhaps, not going too far to say that its existence has conditioned almost all the histological and morphological peculiarities of plant con struction. The cell, as already pointed out, although bounded with what is relatively a tough and even rigid cell-wall, is by no means debarred from further nutrition and growth. If destitute of chlorophyll, it may take in nutrient matter, which only requires some moderate elabor ation to suit it for incorporation with the protoplasm. If, on the other hand, chlorophyll be present, it will do a good deal of preliminary work in preparing the substances which then, as before, the protoplasm will further appro priate and work upon. In either case the protoplasm of the cell will grow, and as the processes which have been described are generally accompanied by the imbibition of fluid, the cell-wall is subjected in consequence to a con siderable tension. The cell-wall, under these circumstances, grows also, and the experiments of Traube seem to show that, given the conditions under which it is known to take place, this growth is almost entirely a physical process. Carried beyond a certain point, tension must result in rapture ; but just short of this there appears to be a limit at which the intercalation of new molecules of cellulose is permitted, and so the surface of the cell-wall is enlarged. In (Edo- goniv.m there is a peculiar arrangement in which fracture actually does take place repeatedly. A circular cleft is formed, which is repaired within by the apposition of an annular splice. To the growth of a cell so conceived there would seem to be no limit, and in the Siphophycece, of which Vaucheria is a well-known type, there is apparently none. The vege tative portion of these organisms, however complicated, is always formed by the extension of a single cell ; the pro toplasm is continuous throughout every part, and except when zoospores are formed is never segmented. This, however, is a rare arrangement. Generally speak ing, there comes a time when the protoplasm, by a phase of contractility, divides itself into two masses, and between these a partition of cellulose is formed in the same way as the coat of the naked zoospores already alluded to. Each cell so formed possesses all the capacity for nutri tion and growth which the whole possessed. It divides therefore in its turn, and in this way we get the first in dication of an aggregate of cells. In the lower plants the cell is complete in itself ; in the higher, its independence is more or less merged in that of the others with which it is associated. This aggregation seems to begin in a purely mechanical way. In Pleurococcus, for example, cell division repeated a few times may produce aggregation of, at any rate, four cells. If it were not that the adhesion of these cells seems afterwards to fail, there would be no reason why the mere process of cell division should not produce larger aggregates. But the cell-wall common to two adjacent cells splits through its middle lamina, and the two neighbouring cells part company. In Hydrodictyon we have a remarkable example of the formation of an aggregate synthetically, owing to the action of some cause which is quite imperfectly under stood, but which is probably purely physical. An enor mous number of zoospores are formed from the contents of a parent cell, and these, after tumultuously moving within its cavity, come to rest, and at the same time arrange themselves in the well-known net-like fashion which is characteristic of the full-grown plant. The mechanical persistence of aggregates of cells formed by normal cell division is obviously the step which led to the evolution of such organisms as Volvox and Ulva, since these are merely aggregates of simple types, such as Ghlamydococcus and Pleurococcus. At first the independence of the individual aggregated cells would be little impaired. In a Spirogyra or Oscilla- toria, for example, the number of cells present in a fila ment is probably a matter which does not affect the cells themselves individually, and which conversely they have no power of influencing. The constituent cells might go on dividing, and so form filaments of unlimited length, but which occasionally would be liable to be broken up by arbitrary accident. In Cladopkora. however, the cells of a filament cease after a time to divide transversely, and 