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 betvyeen 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 



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 Pinus 

 Strobus ; A front view of a young cell ; a fissure 

 J runs across the still young bordered pit, cor- 

 responding to the spiral striation ; R 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 view of 

 a pit ; the striation here appears as a star-like 

 arrangement of less dense spots (X 800). 



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 layer.s, 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 



