68 



PLANT CELLS 



walls it is only rarely that they are subjected to strains in excess of their 



breaking strength. 



Primary cell walls are usually quite elastic. The mesophyll cells of the 



leaves of some species, for example, are known to undergo reversible changes 



in volume of 30 per cent or more in response 

 to changes in turgor pressure. Secondary 

 walls, on the other hand, are elastic only 

 WMthin very narrow limits and break sud- 

 denly when their tensile strength is exceeded. 

 The elasticity of primary walls is undoubt- 

 edly related to the loose open meshwork of 

 cellulose strands that compose it, while the 

 close parallel orientation of the cellulose 

 micelles in secondary walls prevents any ap- 

 preciable elongation without rupture. 



Chemical Constituents of Cell Walls. 

 — Cellulose is by far the most abundant 

 compound found in the cell walls of the 

 higher plants. Associated with cellulose in 

 all the primary cell walls of vascular plants 

 are greater or lesser amounts of pcctic com- 

 pounds. Their presence in the middle la- 

 mella and in the intermicellar spaces of the 

 primary walls has already been noted. The 

 chemistry of the pectic compounds is dis- 

 cussed in Chap. XXH. 



hignin is an important constituent of 

 the walls of most of the cells that make up 

 woody tissues and it also occurs commonly 

 in other thickened walls. Although lignin 

 can be isolated from cell walls by suitable 

 treatments as a brownish amorphous sub- 

 stance it is probably altered considerably in 

 the process. Its structural formula and 

 molecular weight are unknown. Since the 

 lignins produced in different species appear 

 to differ in their chemical properties it seems 

 very probable that lignin is not a compound 



of definite molecular weight but a complex mixture of a number of chemically 



similar substances. The lignin of spruce wood has been extensively studied 



Fig. 15. Diagram illustrating 

 the structure of a thickened plant 

 cell wall. {A) inner layers of 

 secondary wall, (B) second layer 

 of the secondary wall, (C) first 

 la3'er of the secondary wall, (D) 

 cellulose framework of the pri- 

 mary wall, (£■) primary wall of 

 cellulose and pectic compounds. 

 Note that the fibrils may be dif- 

 ferently oriented in different 

 layers of the same wall. 



