18 



The Structure of Protoplasm 



5,B, and 6,A and B) . The bulk of the fiber is made up of innumer- 

 able fine fibrils oriented at an acute angle with respect to the axis 

 of the fiber. The fibrils make either an "S" or "Z" twist\ reversal of 

 direction (Fig. 5,B) taking place many times in a single fiber. At 

 least in certain types of cotton the direction of orientation of the 

 fibrils in the first-formed layer of the secondary wall is opposite 



Fig. 6. Fibers^ free of wax and pectic substance, swollen in dilute cupram- 

 monium hydroxide solution. A. Showing differences in orientation between 

 the first layer of fibrils and those subsequently deposited in the secondary wall. 

 B. Irregular swelling caused by the constricting influence of the outermost 

 layer of fibrils which has become clumped at various points along the axis of 

 the fiber. Magnification X 420. 



from that in layers which are laid down thereafter (Fig. 6,A) . In 

 other words, if the first layer of fibrils makes an "S" twist, all the 

 fibrils formed in subsequent layers make a "Z" twist. Under cer- 

 tain conditions of swelling, the outer layer of fibrils exerts a 

 constricting influence on the expanding inner layers of cellulose 

 (Fig. 6,A and B) . The fibrillar orientation in cotton seems to be 

 less complicated than in Valonia where the wall consists of some 



' The fibrils are said to show an "S" twist if, when the fiber is held in a vertical 

 position, the spiral of the fibrils conforms in slope to the central portion of the 

 letter "S" and a "Z" twist if the spiral conforms in slope to the central portion of 

 the letter "Z." 



