72 THE VITAMINES 



remains to be demonstrated. Still, it is apparent that the vitamine 

 of type A, which is found mostly in seeds and green leaves, is formed 

 in the leaves perhaps from primary products which must be furnished 

 to the plants from some outside source. The question, as to whether 

 old plants that already have leaves need extra vitamine in the same 

 degree, must eventually be answered and may perhaps be solved by 

 the use of methods for sterile growth, as proposed by Grafe (191). 



Since the entire animal world obtains its vitamine requirements 

 either directly or indirectly from plant food, it is especially important 

 to know the vitamine cycle in its entirety. 



