12 The Structure of Protoplasm 



formed elongate rapidly for a period of 15 to 20 days, when growth 

 in length ceases. In accordance with the description by Anderson 

 and Kerr (2) , the thin wall which encloses the protoplasm of the 

 cotton hair during this period of elongation is called the primary 

 wall. After growth in length ceases the thickness of the wall is 

 increased by a deposition of cellulose which comprises the secondary 

 wall. In discussing the structure of cell walls it is essential to 

 differentiate clearly between the primary and the secondary wall. 

 Since, in cotton fibers, deposition of the latter does not begin until 

 from 15 to 20 days after the fibers originate, young fibers are ideal 

 for studying the structure of the primary wall. 



When young fibers with primary wall only are examined micro- 

 scopically with ordinary light, there is no evidence of structure in 

 the wall. The latter stains deeply and uniformly with ruthenium 

 red which, although not specific for pectic substances, is a satis- 

 factory presumptive test (2) . On examination with crossed nicols, 

 unstained primary walls show a low order of birefringence with 

 colors indicative of a predominantly transverse orientation (11, 17). 

 Although these fibers show only slight birefringence when un- 

 stained, they are clearly anisotropic after treatment with a cellulose 

 dye such as Congo red. This increase in birefringence upon staining 

 is due, apparently, to the double refraction of the dye molecules 

 which are preferentially adsorbed by the small amount of cellulose 

 which is present (12, 21) . Although a number of investigators have 

 suggested that the skeletal material in the primary wall is not 

 cellulose but a closely related substance, X-ray investigations 

 (11, 28), staining reactions (2, 17), and behavior in cuprammonium 

 hydroxide solutions (2, 17) indicate that it is probably cellulosic 

 in nature. 



The cellulose in the primary wall of the cotton fiber has been 

 shown (2, 9, 10, 17) to be present as criss-crossing strands which 

 have a netlike arrangement (Fig. 1, A and B) . The reticulate 

 structure of the cellulose appears to become coarser and to show 

 greater birefringence as the wall increases in age (compare Fig. 

 1, A and B) . Likewise, the angle which the criss-crossing cellulose 

 strands make with the axis of the fiber appears to be greater at the 

 tip than at the base. These facts suggest that growth and expansion 

 of the primary wall involve the separation of fibrils and the deposi- 

 tion of new cellulose strands between them. 



Cellulose is not, however, the only constituent of the primary 

 wall. The latter also contains wax and pectic materials which may 



