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 deductions 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 belonging 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 himself 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 precautions 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 euphorbias, which are harmful to the growth of flax, tares to wheat, and thistles to oats.