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 



Mr* 4-1* Fig. 22. Diagrammat.c figure to Illustrate Nagcli's the- 



COllSequeiltly the ory of the molecular structure of the cell-wall ; m, m, m, 



lpaf ppllnlnap QVO the crystal molecules ; w. w, w, the layers of water which 



HUbC, die separate the moli-cules. The water layers' are represented 



] ofrnno-lv VP as very thin ; the v are frequently much thicker m propor- 



16SS Strongly re- tjon to tne diameter8 of Se molecules. <No.-fi must 



fractive than be borne in mind that tnis figure is purely diagrammatic.) 



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



denser. 



39. 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- 

 rig. ss.-Trangverse sec- sets of spirals which run in opposite 



tion of a ba*t fibre of the /U r ppfi'nn<5 arnrmrl flip PA!! 



thickened root of Dahlia, directions arouna tne ceil. 



Goo & examples of stratification may be found 

 in the P ith - cells of the root of the dahlia, and 

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



2T, pit channel* which penl 



-If ter S 8 achs rated ' 



X 8 ' of 8triation in tbe bast-cells of the periwinkle 

 ( Vinca major), and the wood of the Douglas 

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

 specimens with such jjcids (e.g., sulphuric acid) or alknlies (e g., caus- 

 tic potash) as will produce swelling. 



