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 microorganisms in the soil are important factors in destroying these deleterious 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.