THE CELL WALLS OF PLANTS 79 



can be seen, for example, in a transverse section of wood viewed between 

 crossed nicols on the polarizing microscope: here the two thin layers of 

 the secondary wall appear bright, while the thick layer between them 

 appears dark (Fig. 54, c). In a longitudinal section the thin layers are 

 dark and the thick one bright. This shows that the cellulose chains, 

 although highly variable in orientation, tend to lie more nearly parallel 

 to the cell's longitudinal axis in the thick layer and more nearly at 

 right angles to it in the thin ones. The intercellular substance is dark 

 when viewed at any angle. In the cotton fiber the thin primary wall 

 has two systems of chains following spiral courses in opposite directions 

 in a matrix of pectic and waxy material. The secondary wall is much the 

 same, except that the chains form smaller angles with the longitudinal 

 axis and show more reversals of direction. 



Under some circumstances, as when the cell walls are swollen or 

 dried, coarse fibers become visible in the wall substance. This involves a 

 rupturing of the system of cellulose chains and may not indicate accu- 

 rately the arrangement that the chains had in the untreated wall. When 

 properly handled, the fresh untreated secondary walls of some cells show 

 microscopically visible striations revealing the true orientation of the 

 chains. The orientation may also be shown by the arrangements 

 assumed by iodine crystals induced to form within the wall substance 

 and by the shape of cavities resulting from enzymatic activity when 

 fungus hyphae invade thick secondary walls. 



The relative arrangement of cellulose and lignin in the secondary wall 

 is strikingly shown in the fiber tracheids of certain tropical dicotyledons. 

 When the tracheid is swollen, delignified with cuprammonium hydroxide, 

 and stained with Congo red, the transverse section has the appearance 

 shown in Fig. 55. Comparison with longitudinal sections shows the 

 radiating dark lines to represent a sj^stcm of longitudinally arranged, 

 branching plates having a high proportion of cellulose and a low propor- 

 tion of lignin, the lighter lines between them being regions in which 

 cellulose is less and lignin more abundant. When a fiber tracheid is 

 decellulosed with sulphuric acid and stained with iron alum-hematoxylin, 

 a similar pattern appears, but here the dark regions are those containing 

 a high proportion of lignin and a low proportion of cellulose. Both 

 substances are continuous throughout the wall, but their relative abun- 

 dance varies along different radii. 



Concentric zones appearing in the secondary wall are due in different 

 cases to at least three causes: variations in the cellulose pattern, variations 

 in the intensity of lignification, and the alternation of cellulosic and 

 noncellulosic layers. In the cotton fiber the numerous concentric zones 

 have been correlated with the daily metabolic cycle, a compact anisoti-opic 



