TllK WALLS OF PLANT CELLS 



41 



different jxn-osities. Sneli walls tend to 

 split coneeiitrieally, radio-Iongitndinally 

 or radio-helically (Fi<i-. 28) even in ordi- 

 nary dryin>>-; walls of more nnifoi-m tex- 

 tnre or of raniifyin-z' pjittenis do not. 



By moi-e or less drastic chemieal and 

 mechanical treatments, the secondary wall 

 may be dissected along these predeter- 

 mined planes of cleavage and hydrolysis 

 into fragments of varying shapes and sizes 



Fig. ll. Longitudinal section of a secondary wall, showing cavities produced by wood-destroying 

 fungus. There are two sets of planes of exzymatic hydrolysis, one oriented parallel' to the long axis of 

 „Q *^^® microfibrils of cellulose and the other at an angle of 20 to 25° to this axis (x990). 

 Fig. 22. Longitudinal section through parts of the secondary walls of two adjacent cells, showing three 

 ^ _ orientation of the visible crystals of iodine (x 900). 



Fig. 23. Longitudinal section of a secondary wall showing planes of mechanical cleavage (x700). 

 Fig. 24. Swollen transverse section of a secondary wall of the type illustrated in Fig. 18 (x 1300). 



