THE CELL- WALL. 



(d) hztussuscept'ion as a cause of the gro^vth of the Cell-zvall in surface and thickness. 

 The surface-growth of the cell-wall may now be regarded as an interposition, be- 

 tween its already existing particles, of new particles which force the old ones asunder. 

 It is very probable that the striae have a genetic connexion with this process, 

 similar to that which Nageli has shown to exist between the concentric stratifica- 

 tion of starch-granules 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 the inner side of the cell-wall which was originally thin, so that the innermost 

 layer must always be the youngest. This appeared to be an 

 extremely simple explanation of the stratification of the cell- 

 wall ; and the chemical diflferentiation of thick cell-walls 

 appeared entirely to support this idea. But the increased 

 powers of new microscopes revealed a fact quite fatal to 

 the theory of apposition ; the stratification of thickened cells 

 was shown, in short, as we have seen, to be not a sepa- 

 ration 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, in general, be not the result of an ap- 

 position, but rather only of an internal diff'erentiation of the 

 cell-wall already formed. The fact is, however, decisive, 

 that on the inner side of every cell-wall and on each side of 

 dense ones lies a layer containing but little water ; if growth 

 in thickness took place by successive deposits of layers, the 

 innermost and youngest layer must be alternately denssr and 

 less dense, which is not the case. The growth also of such 

 thicknesses as project outwardly, like the combs and spines 

 of pollen-grains, &c., can only ba explained by intussuscep- 

 tion, not by apposition. 



Growth by deposit can only be considered of this nature : 

 — that an aqueous solution from the protoplasm penetrates, 

 by diffusion, between the molecules of the cell-wall. What 

 kind of solution this is, cannot at present be said with cer- 

 tainty ; probably it contains some carbo-hydrate which is 

 easily transformed into cellulose. This substance then forms 

 between the molecules of the cell- wall new solid molecules 

 of cellulose. The actual process of growth, as well as the 

 internal structure of the cell-wall already described, and cer- 

 tain phenomena which polarised light induces in it, as well 

 as the swelling of the cell-wall, lead to the conclusion that 

 it consists of solid molecules of definite form, each of which 

 is surrounded by an en.velope of water, and separated from 

 the adjoining molecules ; the more watery a layer of cell-wall or a striae is, the smaller, 

 according to the principles laid down by Nageh^, are the solid molecules, the more 

 numerous and the thicker their aqueous envelopes. From this it follows that a 

 certain quantity of water is as indispensable to the growth and the internal organ- 

 isation of the cell- wall as is cellulose itself; this water may be designated 'water of 



B 



Fig. ^4.— Striation of the wood- 

 CiM%oi Piiuts StrobHs; A front 

 view of a young- cell, a fissure 

 runs across the still young bor- 

 dered pit, corresponding' to the 

 spiral striation ; H sectional view 

 of the cell-wall with a part of the 

 side-view ; i the central lamella of 

 the wall common to two cells ; -u -v 

 the thickening -layers in contact 

 with them ; these are striated, 

 the striation may be recognised 

 as a formation of lamells pene- 

 trating the whole thickness ; the 

 denser (clear) lamellre project in 

 the form of little knobs. C front 

 view of a pit ; the striation here 

 appears as a star-like arrangement 

 of less dense places (x8oo). 



magnifying power in isolated wood-cells of Pinus, in bast-fibres, &c. ; one of the examples longest 

 known is the bast-cells of Apocynaceoe provided with power of extension and contraction. (Mohl, 

 Vegetabilische Zelle, Fig. 27.) 



* The theory of the growth of the cell-wall (as of all organised structures) by intussusception 

 was first originated by Nigeli in his great work on Starch-granules (1858). Compare also Sachs, 

 Handbuch der Experimentalphysiologie der Pflanzen, §114. 



