INTRODUCTION. 3D
the cell-wall cannot be explained as a merely mechanical effect, which would continually tend to render the cell-membrane thinner. It is often even combined with a thickening of the cell-wall, and is probably effected by that process of nutrition called intus-susception. (See Hugo Mohl’s “ Erlauterung und Vertheidigung meiner Ansicht von der Structur der Pflanzen substanzen,” Tubingen, 1836.) The flattening of the cells mayalso be ascribed to the same cause.
With regard to the changes which the cell-contents and cell-wall undergo during vegetation, I only take into consideration the thickening of the latter, as I have but a few isolated observations upon the transformations of the contents of animal cells, which however indicate analogous changes to those of plants. The thickening of the cell-walls takes place, either by the deposition from the original wall, of substances differing from, or more rarely, homogeneous with it, upon the internal surface of the cell, or by an actual thickening of the substance of the cell- wall. The first-mentioned form of deposition occurs in strata, at least this may be distinctly seen in many situations. (See Meyen’s Pflanzen-Physiologie. Bd. 1, tab. I, fig. 4.) Very frequently,—according to Valentin, universally,—-these depositions take place in spiral lines ; this is very distinct, for example, in the spiral canals and spiral cells. The thickening of the cell-membrane itself, although more rare, appears still in some instances indubitable, for instance, in the pollen-tubes, (e. g. Phormium tenax.) Probably that extremely remarkable phenomenon of the motion of the fluid, which has now been observed in a great many cells of plants, is connected with the transformation of the cell-contents. In the Charae, in which it is most distinct, a spiral motion may also be recognized in it. But, for the most part, the currents intersect each other in the most complex manner.
Absorption and Secretion may be classed as external operations of the vegetable cells. The disappearance of the parent cells in which young ones have formed, or of the cell-nucleus and of other structures, affords sufficient examples of absorption. Secretion is exhibited in the exudation of resin in the intercellular canals, and of a fluid containing sugar by the nectar-glands, &c. &c.
In all these processes each cell remains distinct, and main-
