266 POPULAR SCIENCE MONTHLY 



line organic compound which, when dissolved in pure water, exhibited 

 a toxic action upon wheat plants, but was relatively harmless to cow- 

 peas. Crystals were similarly obtained from a " cowpea-sick " soil 

 and were found to be harmful to cowpeas, but relatively harmless to 

 wheat plants. Since the same soil in which neither plant was grown 

 yielded none of these substances, it seems only logical to conclude that 

 these substances were formed as a result of the plant growth in that 

 soil. 



Data like those briefly presented above have given strong proof of 

 the existence of excretions from the roots of plants, and show that they 

 may exert a harmful effect upon the growth of succeeding crops of the 

 same kind. Without too liberally interpreting these facts, it may be 

 said that the demonstration of root excretions throws a new light upon 

 the interrelation of soils and plants and promises to afford a solution 

 to some important questions. De Candolle and his contemporaries 

 failed to appreciate the agencies in the soil which effect the destruction 

 of deleterious organic substances, and maintained that, once a crop had 

 been grown, the soil would for a long time be unsuitable to that crop. 

 When the majority of soils are kept in what is ordinarily known as 

 " good tilth " by cultivation and continued rotation of crops, it is 

 improbable that root excretions will accumulate to an extent which 

 would be harmful. Processes of oxidation, brought about by proper 

 cultivation, the oxidizing power of roots, and the action of micro- 

 organisms in the soil are important factors in destroying these dele- 

 terious substances. It has also been shown that substances ordinarily 

 employed as fertilizers have a destructive action upon toxic substances, 

 especially when aided by the action of plant roots. 



It is more than probable that the consideration of the existence of 

 root excretions will also throw considerable light upon the problem of 

 association and migration of species and individuals in the vegetable 

 kingdom. The reasons for the association of certain species in nature 

 have often been studied, but never satisfactorily explained. The same 

 applies to questions of migration and natural succession. It has been 

 shown that such physical factors as light and water play important 

 parts, but they have not been found sufficient to give a satisfactory 

 explanation for all the phenomena. 



It would seem that the methods of soil fertility investigations 

 might be applied with profit to the study of plant ecology. The effect 

 of the root excretions from one species upon another might in certain 

 cases prove to be the controlling factor in association. From the 

 evidence already at hand, it would seem that the biological factors 

 play a definite and considerable role in these phenomena. 



