94 Plant Tissue Culture 



Carbohydrates. Carbohydrates make up from 

 0.5 per cent to 10 per cent of the mass of nutrients. 

 They serve two main purposes: as sources of 

 energy for the maintenance of the various living 

 processes in the growing tissues, and as osmotic 

 agents. Apparently the first of these, as energy 

 source, is their major role, for nutrient solutions 

 which have proved satisfactory for plant tissues, 

 in marked contrast to animal tissue culture nutri- 

 ents, are seldom isotonic with the tissues them- 

 selves. The osmotic value of the nutrient seldom 

 exceeds 2 atmospheres for a tissue having an 

 "osmotic value" of 5 to 10 atmospheres (Thiel- 

 man u. Berzin, 1927, 263). This discrepancy may 

 be more apparent than real, since the "osmotic 

 value" usually given for plant tissues is the 

 plasmolytic value and may be greatly in excess of 

 the true osmotic value of the protoplasm itself 

 (Bennet-Clark, Greenwood and Barker, 1936, 238; 

 Mason and Phillis, 1939, 446). That the major 

 role of the carbohydrate is nutritional rather than 

 osmotic is further indicated by the fact that spe- 

 cific carbohydrates, not carbohydrate per se, are 

 required. Glucose and fructose have been em- 

 ployed in some media, particularly those used for 

 the cultivation of tissues of monocotyledonous 

 plants, but with results which have not been en- 

 tirely satisfactory (Kotte, 1922, 51, 52; Robbins, 



