THE QUESTION OF ROOT EXCRETIONS 257 



MODERN AND EAELY WOKK UPON THE QUESTION OF 



EOOT EXCRETIONS 



By HOWARD S. REED 



BUREAU OF SOILS U. S DEPARTMENT OF AGRICULTURE, WASHINGTON, D. C. 



AFTER the lapse of over half a century the one-time well-known 

 theory of De Candolle has again come into prominence. The 

 demonstration that De Candolle was essentially correct in his deduc- 

 tions revives interest in a phase of plant physiology which has been 

 comparatively unnoticed for many years. In brief, his theory was 

 that plants excrete from their roots substances which are deleterious 

 to continued growth. These excreted substances were believed to have 

 a deleterious effect when absorbed from the soil by other plants belong- 

 ing to the same order as the plants from which the excretions came, 

 but according to the De Candollian theory the excretions would be 

 harmless or even beneficial to plants belonging to a different order. 



Although this theory was supported by numerous botanists and 

 chemists of the last century, it has come to be known in the literature 

 as " De Candolle's theory of root excretions." There is obviously a 

 twofold reason for this: first of all, the prominence of the man him- 

 self in his own and subsequent times and, secondly, the fact that he 

 used this theory to explain the well-known benefits of crop rotation in 

 agriculture. 



Before the work of De Candolle appeared, Brugmans had alleged 

 that he observed drops of liquid to exude from the roots of Viola 

 arvensis and that he had observed small fragments of material at the 

 extremities of the roots of certain other plants which he regarded as 

 excretions. Although his observations were made without the precau- 

 tions necessary for scientific experiment, they appear to have been quite 

 widely accepted by naturalists of the time, among them such men as 

 von Humboldt and De Candolle. To these pioneer workers the idea 

 that there may be noxious substances present in the soil appeared to be 

 the most direct means of explaining many pertinent problems of plant 

 distribution and of agriculture. 



De Candolle carried his idea further and used it to explain the 

 apparent antagonisms of certain plants and expressed his belief that 

 they injured their neighbors by the substances exuded from their roots 

 into the soil. He cited the case of the cockscomb, which, he said, 

 appears to have a bad effect upon neighboring vegetation, and euphor- 

 bias, which are harmful to the growth of flax, tares to wheat, and 

 thistles to oats. 



