14 The Structure of Protoplasm 



When mature cotton fibers are placed in cuprammonium 

 hydroxide solution, or Schweizer's reagent, they immediately begin 

 to swell and twist, the rapid expansion of the cellulose of the 

 secondary wall usually resulting in the rupture of the primary 

 wall. As the cellulose pushes through the tears in the latter, 

 expanded regions resembling beads appear along the axis of the 

 fiber (Fig. 2) . After a short time in the reagent, however, the 



Fig. 2. Mature cotton fiber in cuprammonium hydroxide solution, showing 

 irregular swelling due to the constricting influence of the primary wall. 

 Magnification X 120. 



cellulose dissolves completely, leaving a residue which consists 

 principally of fragments of the primary wall, and to a lesser extent of 

 material from the lumen (16). This residue is isotropic, and chemi- 

 cal analysis indicates that it consists largely of wax and pectic 

 substance (29) . Full-strength cuprammonium reagent dissolves the 

 cellulose of the fibers in a relatively short time, leaving no micro- 

 scopically resolvable particles of cellulose (16) . Dilute solutions 

 of the reagent, on the other hand, swell the fibers greatly, but 



Fig. 3. Cross sections of cotton fibers swollen in cuprammonium hydroxide 

 solution to show the layered structure of the secondary wall. A. Section of a 

 25-day fiber. Magnification X 950. B. Section of a mature fiber from an open 

 boll. Magnification X 420. C. Section of 26-day fiber showing lamellae, photo- 

 graphed between crossed nicols. Magnification X 700. 



