30 
MORPHOLOGY OF THE CELL. 
surface-growth of the cell-wall can be regarded only as an intercalation, between its 
already existing particles, of new particles which force the old ones asunder. It is very 
probable that the striation has a genetic connection with this process, similar to that 
which Nageli has shown to exist between the stratification of starch-grains and their 
growth. It was long thought that the growth in thickness of the cell-wall arose from 
the repeated deposition of new concentric layers on its inner side ; in which case the 
innermost layer would always be the youngest. This appeared to be an extremely- 
simple explanation of the stratification of the cell-wall ; and the chemical differentiation 
of thick cell-walls appeared entirely to support this idea. But the increased powers of 
the microscope revealed a fact quite fatal to the theory of apposition ; the stratification 
of thickened cell-walls was shown, as we have seen, to be not a contiguity of similar, but 
an alternation of dissimilar, layers. For reasons which cannot here be discussed, it must 
be concluded that these alternate deposits of more and less watery layers must be the 
result not of an apposition, but of an internal differentiation of the cell-wall already 
formed. The fact is decisive, that on the inner side of every cell-wall there always lies 
a dense layer containing but little water; if growth in thickness took place by successive 
deposition of layers, the innermost and youngest layer would be alternately more and 
less dense, which is not the case. The growth also of such thickening-masses as project 
externally, like the crests and spines of pollen-grains, &c., can only be explained by in- 
tussusception, not by apposition. 
Growth by intussusception can be regarded only as the diffusion of an aqueous 
solution from the protoplasm between the micellae of the cell-wall. What this solution 
is, cannot at present be said with certainty ; probably it contains some carbo-hydrate 
which is easily transformed into cellulose. This substance then forms between the 
micellae of the cell-wall new solid micellae of cellulose. The actual process of growth, 
the internal structure of the cell-wall already described, and certain phenomena which it 
exhibits with polarised light, as well as the swelling of the cell-wall, lead to the conclusion 
Fig. 31. — Hypodermal eel 
from the stem of Pteris aqiii- 
Una, isolated by Schulze's 
maceration. The wall is seen 
in optical longitudinal sec- 
tion; it shows an innermost 
very dense layer, a central 
less dense layer (the dark 
streak to the right below) en- 
closed by two denser layers ; 
these layers are penetrated 
by pit-canals, which are seen 
on the hinder wall in trans- 
verse section. 
Fig. 32. — Striation of the wood-cells of Pinns 
Strohns ; A front view of a young cell ; a fissure 
s runs across the still young bordered pit, cor- 
responding to the spiral striation ; B sectional 
view of the cell-wall with a part of the side view ; 
i the middle lamella of the wall common to two 
cells ; -v V the thickening-layers in contact with 
it ; these are striated, the striation may be recog- 
nised as a formation of layers penetrating the 
whole thickness ; the denser (white) layers pro- 
ject in the form of little knobs. C front vitw of 
a pit; the striation here appears as a star-like 
arrangement of less dense spots (X 800). 
