258 POPULAR SCIENCE MONTHLY 



One of De Candolle's contemporaries, Macaire-Prinsep, carried out 

 some experiments, which for a time gave promise of firmly establish- 

 ing the existence of plant (or root) excretions. Macaire-Prinsep took 

 up a number of adolescent plants and after carefully washing their 

 roots, placed them in vessels containing rain water. He reported that 

 after eight days the water in which healthy specimens of Chondrilla 

 had grown had acquired a yellow tint and a strong odor. The water 

 had a bitter taste and gave a precipitate when added to solutions of 

 lead acetate. The same investigator also reported that water contain- 

 ing the excretions from the roots of peas was fatal to other plants of 

 the same species, but not to wheat plants. 



Using these ideas and experiments as a basis, De Candolle formu- 

 lated a theory to account for the well-known benefits of crop rotation. 

 He expressed a belief that the harmful effects observed when the soil 

 is continuously cropped with the same species are due to an accumula- 

 tion of noxious excretions. According to his theory, the substances 

 excreted by the roots of one order of plants were not usually harmful 

 to the roots of plants belonging to other natural orders; in fact, they 

 might be slightly beneficial. In support of this idea he cited the in- 

 controvertible observation of every husbandman that in the majority 

 of cases good yields of all crops are obtained from the soil when plants 

 belonging to different natural orders are grown in succession. 



De Candolle pointed out that there exist, in a state of nature, nat- 

 ural successions of forest trees in the course of which a given species 

 of tree is completely replaced by another species. According to his 

 observations the first species seemed to disappear because its excretions 

 so filled the soil as to render it unsuitable for the longer growth of 

 that species. It is interesting to note in passing that he published an 

 article in the Revue fmngaise showing the harm wrought by the decree 

 of Louis XIV. that forests should be perpetually maintained upon 

 forested lands. 



De Candolle also made the trenchant observation that when shade 

 trees die from any cause it is very difficult to replace them with trees 

 of the same species. 



Using these observations as a foundation, he built up a theory that 

 plants may injure each other by the matter excreted from their roots, 

 and that the success of crop rotation lies in preventing an undue 

 accumulation of excreted material in the soil. He went further and 

 expressed his belief that the excretory matter of the plants of certain 

 families, like the legumes, is not only feebly toxic to themselves, but 

 possesses actual value as a fertilizer for the cereals. 



The ideas and observations of De Candolle were not allowed to 

 pass unchallenged, for within a few years controversial articles began 

 to appear. 



