Microscopic Structure of the Cell Wall 



19 



thirty layers, with the fibrils in alternate layers at a definite angle 

 to each other (22). In cotton, when the outermost layer of fibrils 

 changes its direction of winding, all the inner layers do likewise. 

 It is readily apparent that these reversals of direction of the cellu- 

 lose fibrils are responsible for the optical differences observed with 



Fig. 7. A single swollen fiber being dissected with microneedles to show 

 its fibrillar structure. Magnification X 500. 



crossed nicols and that the band of extinction is the place at which 

 the reversals occur. It can be shown, moreover, that the number of 

 extinction bands in a single fiber invariably corresponds to the 

 number of fibril reversals. 



Many investigators (3, 8, 14, 15, 16, 17, 18, 22, 24) have called 

 attention to fibrils in the walls of widely different plant cells. There 

 is evidence, also, that the striations detectable in the intact wall are 

 related to the fibrils observed in the swollen material. A compari- 

 son of the work of Anderson and Kerr (2) with that of Berkley (11) , 

 and the work of Astbury and Preston (3) shows, moreover, that 

 the cellulose molecules are oriented approximately parallel to the 

 long axis of the fibrils. Although a variety of physical and chemical 

 treatments reveal the presence of fibrils in the cell wall, probably 

 no method gives a more convincing demonstration of their presence 

 than microdissection. Upon handling cotton fibers with micro- 



