16 



The Structure of Protoplasm 







\ 





dissolution of the cellulose is retarded. Fibers swollen in this way 

 can be used for observing details of structure. 



Swollen cross sections of cotton fibers clearly reveal a lamellate 

 pattern. On treating the swollen sections with a dye such as Congo 

 red, alternating layers appear to be deeply and lightly stained (Fig. 



3, A and B) , and between 



'■"Vl^"**^ ^'****^^ crossed nicols the alternat- 

 ing layers are strong and 

 weakly birefringent (Fig. 

 3,C). This layered struc- 

 ture of the secondary wall 

 can also be observed in 

 longitudinal view (Fig. 4) , 

 where stripes running par- 

 allel to the fiber axis, and 

 extending from the lumen 

 to the primary wall, can be 

 seen. The first layer is laid 

 down on the day when sec- 

 ondary thickening is initi- 

 ated, the number increasing 

 thereafter as the fiber ap- 

 proaches maturity. This in- 

 crease in the number of 

 layers with increase in age 

 of fiber can be seen by com- 

 paring Figure 3, A with 

 Figure 3,B. 



According to Kerr (20), 

 two adjacent layers, one 

 compact and one porous, are deposited every 24 hours during the 

 period of secondary wall deposition. Two adjacent layers together, 

 therefore, constitute a daily "growth ring." The denser of the two 

 layers which comprises each ring was found to be laid down during 

 the day, the other at night. Counts of rings in fibers of different ages 

 indicate that after the start of secondary growth, one ring is laid 

 down per day until the fiber reaches maturity (14, 20) . The width 

 of individual growth rings in unswollen fibers varies from 0.1 to 0.5 

 micron. The width of the growth rings appears to fluctuate with 



Fig. 4. Longitudinal view of the layers 

 in the secondary wall of a swollen 51- 

 day fiber. Magnification X 500. 



