399] E. E. Free and S. F. Trelease 201 



due to the nitrate in the uranium salt. No stimulation was 

 observed with any other of the elements tested. 



This failure to secure determinable stimulating effects with 

 most of the elements is surprising and is contrary to the re- 

 sults of many previous investigations. It seems possible that 

 it may be due to the fact that the Shive solution, in the con- 

 centration and salt proportions employed, is itself slightly 

 toxic because of its high content of magnesium. This solu- 

 tion, although it gives the best production of dry wefght of 

 tops, produces plants many of which show the morphological 

 modifications characteristic of magnesium poisoning. 2 These 

 observations form one of several bits of evidence which sug- 

 gest that the best growth of a plant, as measured by produc- 

 tion of dry matter, occurs only when the plant is slightly 

 poisoned. It may be a general rule that increased growth 

 is the first response to agents or circumstances which would 

 prove injuriously toxic in greater concentration or on longer 

 exposure. 



"We have found some confirmation of this suggestion in our 

 experiments on the effect of boron on Canada field pea. Using 

 the Shive solutions containing salt proportions other than 

 the ones above referred to, and adding borax to these solu- 

 tions, considerable stimulations were obtained. The experi- 

 ments need to be extended and confirmed, but the present 

 indication is that borax is stimulating in those nutrient solu- 

 tions which contain less magnesium than the one giving great- 

 est dry weight of tops. In other words, slight poisoning, 

 such as that caused by magnesium or boron, is essential for 

 the production of the greatest dry weight of tops. Either 

 magnesium or boron will serve. Probably other poisons 

 would be equally efficacious. 



2 Shive, loo. tit.. (2), p. 370-374. Tottingham, William E., "A 

 quantitative chemical and physiological study of nutrient solutions 

 for plant cultures." Physiol. Res. 1 : 133-245. 1914. 



