Nutrients 97 



parts. In some cases apparently either the thia- 

 zole portion of the thiamin (Robbins and Bartley, 



1937, 223) or both thiazole and pyrimidine (Bon- 

 ner, 1938, 199, 1940, 201; Bonner and Buckman, 



1938, 239) may be substituted for thiamin itself, 

 and there appears to be some evidence that the 

 thiazole can be replaced by certain complex sub- 

 stituted pyridines, but no plant tissues appear to 

 be able to maintain themselves for long without 

 some source of thiamin activity. In many types of 

 tissues, this requirement becomes evident in com- 

 plete failure of the cultures if thiamin or thiazole 

 is not included in the nutrient (Bonner, 1937, 198, 



1938, 199; Robbins and Bartley, 1937, 222, 223; 

 White, 1937, 231, 1939, 182 ; Bonner and Devirian, 



1939, 205). Here the tissues are clearly unable to 

 synthesize thiamin. In other cases, no growth 

 failure may occur in the absence of an external 

 source of thiamin, but thiamin may be recovered 

 in increasing quantities from cultures grown for 

 long periods, showing that the vitamin has been 

 synthesized by the tissues in quantities sufficient 

 to supply their growth requirements (Bonner and 

 Devirian, 1939, 205; Nobecourt, 1940, 252; Mc- 

 Clary, 1940, 248). 



Nicotinic acid appears to be equally important 

 for a somewhat smaller number of plants such as 

 legumes (Bonner, 1938, 200; Addicott and Bonner, 



