Nutrients 93 



sidered as not undesirable additions. These ele- 

 ments are supplied in the form of inorganic salts 

 (White, 1933, 141, 1937, 142, 143, 1938, 144 ; Berthe- 

 lot, 1934, 136; Bobbins, V. B. White, et al., 1936, 

 139; Bonner and Addicott, 1937, 42). Because of 

 the extremely minute quantities of some of these 

 elements which are required and because of the 

 toxic effects which very small traces of other ele- 

 ments may exert, it is essential that the salts used 

 be of the greatest possible purity. For stock 

 nutrients, the "C.P. Grade" products of most 

 reliable manufacturers are satisfactory, but for 

 critical studies it may be necessary to prepare 

 specially purified salts. Methods of purification 

 for particular purposes, by recrystallization or by 

 other means, have been developed and are to be 

 found in the literature of plant nutrition (see 

 Arnon, 1938, 133 ; Anion and Stout, 1939, 134, 135 ; 

 Stout and Arnon, 1939, 140). Detailed procedures 

 for any given problem will have to be chosen with 

 the particular exigencies of that special problem 

 in mind. Procedures used in the preparation of 

 nutrients will be considered later. 



Organic materials. The organic materials re- 

 quired for the nutrition of plant tissue cultures 

 fall into four main groups : carbohydrates, vita- 

 mins and hormones, amino-acids and other simple 

 nitrogenous substances, and organic complexes of 

 the tissue broth type. 



