Microscopic Structure of the Cell Wall 



17 



variations in environmental conditions. For certain localities at 

 least, variations in temperature and the width of the growth rings 

 can be correlated (19). Thus, cotton fibers from different plants, 

 but grown under the same conditions, show similar rings, so that 



Fig. 5. A. Mature fiber mounted in water and photographed between crossed 

 nicols to show the bands of extinction where the fibrillar orientation reverses. 

 Magnification X 180. B. A swollen fiber, photographed between crossed nicols, 

 showing a reversal in the direction of orientation of the first layer of fibrils 

 in the secondary wall. Magnification X 420. 



it is possible to cross-date daily growth rings of cotton just as 

 annual rings are cross-dated in the trunks of certain trees. 



Upon examination of single cotton fibers with crossed nicols, 

 much can be learned about the orientation of the cellulose. When 

 the fibers are placed approximately parallel to the plane of light 

 passing through one of the nicol prisms, the high birefringence of 

 the fibers is interrupted by dark extinction bands at irregular 

 intervals along the axis (Fig. 5, A) . These optical variations can be 

 correlated with differences in structure observed in swollen fibers. 

 Upon swelling, the latter clearly reveal a fibrillar structure (Fig. 



