INTRODUCTION. 5 



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 Mold's " Erlauterung und 

 Vertheidigung meiner Ansicht von der Structur der Pflanzen- 

 substanzen/' Tubingen, 1836.) The flattening of the cells may 

 also 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 obser- 

 vations 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 depo- 

 sition 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 deposi- 

 tions 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 in- 

 stances indubitable, for instance, in the pollen-tubes, (e. g. 

 Phormium tenax.) Probably that extremely remarkable phe- 

 nomenon of the motion of the fluid, which has now been ob- 

 served in a great many cells of plants, is connected with the 

 transformation of the cell-contents. In the Charse, in which it 

 is most distinct, a spiral motion ma}' 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 ope- 

 rations 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 intercel- 

 lular canals, and of a fluid containing sugar by the nectar- 

 glands, &c. &c. 



In all these processes each cell remains distinct, and main- 



