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PHYTOGENESIS. 257 I return, after this digression, to my subject. So far I believe I have demonstrated tolerably conclusively, and in accordance with nature, that the entire growth of the plant consists only of a formation of cells within cells. Let us now pass on to the root. I can contribute but very little to the explanation of this part of the subject; for I have not as yet succeeded in arriving at any satisfactory result, from the somewhat limited researches which I have instituted; for instance, I have been altogether unsuccessful in deciding the question as to whether a fluid is secreted at the extremity of the radicle, in which new cells are developed. On the other hand, it is certain that there exists in the extremity of the root a concavo-convex mass (a meniscus) of cellular tissue, in which the process of cell-formation takes place in the same manner as in the parts of the plants which grow in the ascending direction. A chief cause of the elongation of the root consequently consists in this,—that new cells are continually formed in the interior of the existing cells, on the convex side of that mass of cells, while on the concave side, the cells already formed expand simultaneously, and chiefly indeed in the longitudinal direction, and in this way constantly push the extremity of the root before them.

The third case, the formation of the accidental organs of the plant, I must entirely pass over, as I am altogether unprovided with any personal observations upon the subject. Probably, however, the process here is the same as in the previous cases, for Meyen (Physiologie, vol. i, p. 209) observed the cell-nuclei in germinating tubers of Orchideæ. Analogy also leads to a similar conclusion, since all these parts are nothing more than morphological modifications of organs which have been already treated of in this memoir. The fourth point, however, still remains for discussion, namely, the increase in thickness of plants which form woody stems (Dicotyledons). The origin and signification of cambium is the nut on which so many young phytologists have already broken their milk-teeth, the Gordian knot which so many botanical Alexanders have cut instead of untying, and the enigma, for the solution of which almost all the Coryphæi of our science have laboured with more or less