The theory of cells, as given in the "Phytogenesis," may be briefly stated as follows: There are two points in a plant well adapted for a ready and safe observation of the production of a new organization; these are the embryo-sac and end of the pollen-tube (according to his fertilization theory, the first cells of the embryo should form there, while in reality this is not the case). At both points, the formation of nuclei causes turbidity in the homogeneous gum-solution these increase in size, and soon cytoblasts (a granular coagulation) appear. In the free state the cytoblasts increase rapidly until they attain a certain size, when they are surmounted by a fine diaphanous bubble; this is the young cell, at first a segment of a sphere, its plane side formed by cytoblasts, and its convex side by young cells (the cell-epidermis) similar to a watch-crystal on a watch. Gradually the bubble expands, becomes more consistent, and the wall is composed of cytoblasts and of a gelatinous substance. The cell grows, overlaps the cytoblasts, and then increases so rapidly that the cytoblast appears as a small nucleus inclosed in a duplicature of the cell-wall. As the growth progresses, the mutual pressure, exerted by the cells upon one another, causes a certain regularity of form, frequently that of the rhombendodecahedron. Only after the resorption of the cytoblasts the formation of secondary deposits begins on the inner surface of the cell-wall. Scbleiden assumes the process thus described to be the general law of formation for the vegetable cell-tissues in the phanerogams. This theory was conceived while Schwann was still engaged with his theory of the origin and propagation of animal cells. Schwann has, in fact, acknowledged in his "Microscopic Researches" that Scbleiden communicated his observations on the subject to him before publishing them, and thus gave him the light that showed him the way to his own results. So it has come to pass that, by means of the joint labors of these two men, the cell has been recognized as the peculiar element in both kingdoms of organic nature, and all the processes of vegetable and animal life have been located in its little laboratory.
Schleiden's theory has been proved to be a premature generalization, based upon incomplete and inaccurate observations, and has been refuted by Nägeli; but, incorrect and of little consequence as it was in itself, it has also proved to be the grain of ferment which has worked a transformation and revivification of biological science.
Schleiden's theory of fructification was announced just at the time (1839) when those who denied sexuality in plants had seemed to carry the day, and all botanists had agreed, to use the language of M. Herrera, in attributing the production of the embryo to the ovule, while allowing to the pollen only a simple action of fertilization. "All at once a botanist, already celebrated, proclaimed that he had seen the embryo forming in the grain of pollen and penetrating the ovule with the pollenical tube. This unexpected animalculist was Schleiden. His
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