160 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



The composition of the actual cell wall of plants varies greatly, 

 as there is a large variety of substances known generically as 

 cellulose, and having the same empirical composition, but 

 which yet exhibit considerable differences in their physical 

 properties and in their behaviour towards reagents. 



Cross and Bevan divided celluloses into three classes 

 according to their behaviour with reagents. The main 

 reagents used in cellulose investigation are strong acids and 

 alkalies, which bring about conversion into sugar by the 

 ordinary hydrolytic change, that is : 



C 6 H 10 5 + H 2 = C 6 H 12 6 



Acetic anhydride combines with any OH groups which 

 may be present according to a general equation : 



2R-OH + ^ 3 } = 2R-OC 2 H 3 + H 2 



^2^3^ ) 



According, therefore, to their behaviour with these and 

 other reagents cellulose bodies are classified as follows : 



1. Those which offer a maximum resistance to hydrolytic 

 action and which contain in their molecule no directly active 

 CO groups, i.e., the CO is not easily oxidised and does not 

 combine, e.g., with phenyl hydrazine. These are represented 

 by the cellulose of cotton fibre. 



2. Those of less resistance to hydrolysis which contain 

 active CO groups, i.e., which will give osazones with phenyl- 

 hydrazine. These are perhaps best regarded as oxycelluloses. 

 They appear to constitute the main mass of the tissue of 

 flowering plants and they exist in conjunction with a sub- 

 stance called lignine in the walls of wheat cells. 



3. Those that hydrolyse with some facility, being more or 

 less soluble in alkalies and easily decomposed by acid, with 

 formation of carbohydrates of low molecular weight. In- 

 cluded among these is the cellulose of the walls of the cells of 

 seeds. It will be remembered that in the preparation of 



