98 Plant Tissue Culture 



1938, 197 ; Bonner and Axtman, 1937, 203 ; Bonner 

 and Devirian, 1939, 205). These show a marked 

 failure in the absence of a significant external 

 source of nicotinic acid. Since no means of esti- 

 mating nicotinic acid concentrations in small quan- 

 tities of plant tissues is yet available, it is not yet 

 possible to determine if those plant members 

 which do not require an external source of nico- 

 tinic acid may synthesize it in quantities sufficient 

 to satisfy their needs. It is, therefore, not pos- 

 sible as yet to say if all plant tissues require nico- 

 tinic acid or not. It is only clear that most tissues 

 do not require an external source and that a few 

 tissues de require such a source. 



Pyridoxine (vitamin B 6 ) is also clearly bene- 

 ficial to many plant tissues. It has been reported 

 to be necessary for satisfactory growth of roots 

 of tomato (Robbins and Schmidt, 1939, 61, 224, 225 ; 

 Bobbins, 1940, 202 ; Day 1941, 208 ; Bonner, 1940, 

 201). The evidence in support of this report is 

 not conclusive and has been opposed by other evi- 

 dence (White, 1940, 232). Certainly many plant 

 tissues can dispense with all external sources of 

 this vitamin. As with nicotinic acid, there is no 

 evidence either for or against its synthesis by 

 plant tissues. 



Indole acetic acid has been reported to benefit 

 the growth of roots of maize (Geiger-Huber, 1936, 



