STRATIFICATION OF THE WALL. 



33 



tioned, in which the thickening was supposed to be due to 

 the successive deposition of layers, either inside or outside of 

 the original wall. It is now known that stratification is due 

 to a subsequent 

 change in the 

 amount of water 

 of organization 

 present in partic- 

 ular parts of the 

 wall. When seen 

 with the micro- 

 scope, those layers 

 which contain the 

 most water, and 



j.1 J.T, F 'g- 22. Diagrammatic figure to illustrate Nageli's the- 



COnsequently tne ory of the molecular structure of the cell-wall ; m, m, m, 



Ipaof nollnlr>oo ova the cr ystal molecules ; w. w, w, the layers of water which 



uiuse, ctit; separate the molecules. The water layers are represented 



lpj<3 sfrnno-lv rp as very thin : they are frequently much thicker in propor- 



J11 &V 1C ~ tion to the diameters of the molecules. (NOTR It must 



fractive than be D0nie in mind that this figure is purely diagrammatic.) 



those which contain less water, or which, in other words, are 



denser. 



3Q. Striation. In many cases there is also a similar sepa- 

 ration into more watery and less watery 

 layers at right angles to those just 

 mentioned. There may be one system 

 of such differentiation, giving rise to a 

 transverse striation, which may be an- 

 nular (Fig. 24, c,-d, e) or spiral (a, b) ; 

 or there may be two systems, and then 

 the wall appears to be crossed by two- 

 Fig. 23:-Transverse sec- sets f spirals which run in opposite 



tion of a ba-t fibre of the f li rpo ti nn , aivvrmrl tliP PP!! 



thickened root of Dahlvi UiieLUOllb dlOUnu 1116 C611. 



variabilis ; 1. the cavity : .-, , . . , 



K, pit channels which pen- Wood examples of stratification may be found 



etrate the stratification ; j n the pith-cells of the root of the dahlia, and 



sp, a crack by which an in- . 



ner system of layers has in the epidermal cells of most thick leaves ; and 



-Afte^!a'ch 8 rated ' X8 ' Of8triation in the bast-cells of the periwinkle 

 ( Vinca major), and the wood of the Douglas 



Spruce (Tsuga Uouglam). In many cases it is necessary to treat the 

 specimens with such acids (<2.<7., sulphuric acid) or alkalies (eg., caus- 

 tic potash) as will produce swelling. 



