STRATIFIGATION OF THE WALL. 



33 



Fig. 23. — ^Diagranunatic figare to Illustrate NagcU's the- 

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

 the crystal molecules ; w, w, w, the layei-s of water which 

 separate the molecules. The water layerti are represented 

 as very thin ; thev are frequently much thicker m propor- 

 tion to the diameters of the m^ecules. (Note — It must 

 be borne in mind that this figure is purely diagrammatic.) 



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 



consequently the 



least cellulose, are 



less strongly re- 



fractive than 



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 difEerentiation, 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- 

 sets of spirals which run in opposite 

 directions around the cell. 



Good examples of stratification may be found 

 in tlie pitli-cells of the root of the dahlia, and 

 in the epidermal cells of most thick leaves ; and 

 of striation in the bast?cel]s of the periwinkle 

 ( Vinea major), and the wood of the Douglas 

 Spruce (Tauga Douglam). In many cases it is necessary to treat the 

 specimens with such acids (e.^., sulphuric acid) or alkalies (s.j'., caus- 

 tic potash) as will produce swelling. 



Pig. 33.— Transverse sec- 

 tion of a bast fibre of the 

 thickened root of DaTdia 

 variabilis ; I, the cavity ; 

 .ff, pit channels which pen- 

 etrate the stratification ; 

 sp, a crack by which an in- 

 ner system of layers has 

 become separated. X 800. 

 —After Sachs. 



