Hoagland —106— Plant Nutrition 



itself, however important it is for secondary reasons. 

 Some discussion has taken place with reference to 

 the value of vitamin substances, especially vitamin Bi, 

 added to the culture medium, but there seems to be 

 fairly general agreement now that additions of vita- 

 mins to a nutrient solution do not have a significantly 

 beneficial effect, except possibly under some excep- 

 tional circumstances. (See plates 20, 21, and 22). 



Of course the water cultures as ordinarily con- 

 ducted are not sterile and the argument might be 

 advanced that microorganisms present produce growth 

 substances necessary for the green plant. This seems 

 improbable. Relatively few organisms develop in the 

 inorganic culture solution, unless considerable organic 

 matter enters the solution through liberation by dying 

 roots. Great difficulty is found in obtaining direct 

 evidence on this question by growing plants under 

 sterile conditions, but cucurbit plants have been de- 

 veloped in this way in some of the experiments of 

 Barker and Broyer (1942), probably for a sufficient 

 period of time to indicate that microorganisms played 

 no indispensable role in the nutrition of the higher 

 plant. In brief — as we should expect — the plants 

 under discussion may be regarded as complete syn- 

 thetic units under normal conditions for growth. 



In making these remarks I do not forget that 

 some at least of the same vitamin substances essential 

 for animals are also essential for plants. The work 

 on the culture of excised root tips shows that for 

 vitamin Bi — possibly for other vitamins. In fact I do 

 not think that it would be too violent an assumption 

 that most or all the vitamins needed by animals, or 

 their precursors, may have a role in plant growth and 

 metabolism. The plant does not synthesize these sub- 

 stances merely for the benefit of the animal, but the 

 important point at the moment is that it does synthe- 

 size them. 



