92 THE VEGETATIVE FUNCTIONS OF PLANTS 



theory was advocated by the great agricultural chemist, 

 Liebig. Undoubtedly it is correct, as far as it goes, but 

 a more thorough consideration of the question indicates 

 that it is not adequate as a complete explanation. 



(&) Theory of Toxic Excreta. — A second hypothesis is 

 that advanced by certain investigators in the Bureau of 

 Soils of the United States Department of Agriculture.' 

 This is based on the fact that the roots of plants are known 

 to excrete substances which are poisonous or toxic to the 

 species producing them. These toxic substances accumu- 

 late in the soil during a succession of the same kind of 

 crops, and thus gradually render it toxic to that kind of 

 plants. By following with a succession of different kinds 

 of crops the toxic excreta of the first are either removed 

 by seepage of soil-water, or destroyed, either by being 

 oxidized, or by the addition of other substances which 

 render them harmless. By this theory the function of 

 fertilizers is not so much to renew exhausted mineral 

 "nutrients," as to render harmless the accumulated 

 excreta of the previous crop. 



(c) Sanitary Theory. — This theory has been carefully 

 worked out by Professor Bolley, of the North Dakota 

 Agricultural College. By thorough studies of the wheat 

 crop he has been enabled to make the following positive 

 statements:^ Constant or rather constant culture of 

 wheat on the same lands brings about wheat-sickness, or 

 wheat-sick soil. Wheat does not thrive well in the 

 presence of its own dead bodies, no matter how fertile 

 the soil. Constant wheat cropping does not especially 



^ A similar hypothesis was advanced by A. P. DeCandolle in his 

 Physiologic VSgetale, Paris, 1832. 



" The phraseology of Professor Bolley (North Dakota Agric. Coll. 

 Bull. 107), is closely followed. 



