INTRODUCTION. D 
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 memer Ansicht von der Structur der Pflanzen- 
substanzen,” Tiibingen, 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 differmg 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 imstance, 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 Charz, 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 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. &e. 
In all these processes each cell remains distinct, and main- 
