INFLUENCE OF THE QUALITY OF THE SOIL 441 



that the roots of autotrophic plants are able to excrete substances which 

 act injuriously upon the plants they displace, for the neutralizing power of 

 humus apparently renders any such action impossible ', and, as a matter 

 of fact, carbon dioxide is almost the sole metabolic product which is 

 excreted continually and in considerable amount, and being a gaseous 

 product it can rarely accumulate to any marked extent. Many of the 

 lower heterotrophic plants, however, behave quite differently, and commonly 

 exert more or less marked injurious effects upon one another by means 

 of certain metabolic products which they excrete. 



1 This idea has been frequently put forward in earlier times. See the literature by Mohl, 

 Vegetabilische Zelle, 1851, p. 95, and Sect. 28. 



