Page:History of botany (Sachs; Garnsey).djvu/344

two spots, says Schleiden, in the plant, where the formation of new organisation may be most easily and most certainly observed, the embryo-sac and the end of the pollen-tube, in the latter of which, according to his theory of fertilisation, the first cells of the embryo are supposed to be formed, but where in fact no cells are formed. At both spots small granules soon arise in the gum-mucilage, which, before homogeneous, now be- comes turbid, and then single larger and more sharply defined granules, the nucleoli, appear. Soon after, the cytoblasts are seen as granular coagulations from the granular mass; they grow considerably in this free condition, but as soon as they have reached their full size, a delicate transparent vesicle is formed upon them; this is the young cell, which at first presents the appearance of a very flat segment of a sphere, whose plane side is formed by the cytoblast, the convex by the young cell (the cell-membrane), which rests upon the cytoblast as a watch-glass on a watch. Gradually the vesicle becomes larger and of firmer consistence, and now the whole of the wall, except where the cytoblast forms part of it, consists of a jelly. By-and-bye the cell grows beyond the edge of the cytoblast and rapidly becomes so large that the latter appears only as a small body inclosed in one of the side walls. The shape of the cell becomes more regular with advancing growth and under the pressure of adjoining cells, and often passes into that of a rhombododecahedron, which Kieser for reasons drawn from the nature-philosophy assumed to be the fundamental form. It is only after the resorption of the cytoblast that the formation of secondary deposits on the inner surface of the cell-wall commences, though some exceptional cases are adduced. Schleiden thinks (p. 148) that he may assume that the process here described is the general law of formation of vegetative cell-tissue in Phanerogams. He adds particularly that the cytoblast can never lie free inside the cell, but is always enclosed in a duplication of the cell-wall, and he thinks that it is an absolute law that every cell, except perhaps in