260 POPULAR SCIENCE MONTHLY 



observed. Without appreciating the additional factors which would 

 have to be considered in such an experiment, e. g., absorptive power 

 of the soil, action of microorganisms, oxidation, etc., Mr. Gyde con- 

 cluded that the excretions of plants are not harmful to their kind, 

 but that the necessity for a rotation of crops arises from the depletion 

 of the soil of the mineral plant food constituents. He thus appears 

 to turn, mirabile dictu, from a proposition which was partially proved 

 to one for which he had no proof; neither has conclusive proof been 

 afforded by any subsequent investigator. 



Professor Johnson in " How Crops Grow " has justly remarked 

 that Mr. Gyde's results are not to be regarded as conclusive proofs for 

 or against the existence of root excretions. 



The curiously regular growth of fungi in continually widening 

 circles, known as " fairy rings," presents many questions of scientific 

 interest and it is not surprising to find that the theory of deleterious 

 excretions was called in to explain this phenomenon. If one assumes 

 that harmful excretions are left in the soil by these plants, it is easy 

 to understand how the new and thrifty growth would continually arise 

 on the outer edge of the ring, and thus give rise to the phenomenon 

 observed. 



The subject of " fairy rings " appears to have been studied by Way, 

 who admitted, in a paper published in 1847, " that by far the most 

 scientific and intelligent solution of the question is that which was 

 based upon De Candolle's theory of the excretions of plants." But on 

 account of objections which appeared insuperable to him, he was unable 

 to accept it as a final satisfactory explanation. 



In connection with the decadence of the De Candollian theory, 

 special mention must be made of Liebig and of his attitude toward the 

 question. At first he pronounced this theory of crop rotation to be 

 the only one " resting on a firm basis." He regarded the experiment 

 of Macaire-Prinsep as positive proof that the roots, probably of all 

 plants, expel substances which can not be utilized in metabolism. 



In addition to his extensive investigations upon the chemistry of 

 the soil, Liebig made numerous studies upon the chemistry of the ash 

 constituents of plants. He found that the essential elements were 

 present in the ash of all plants, in quantities which formed a more or 

 less definite ratio for a given plant. Eeasoning from these facts, 

 Liebig developed the idea that each plant requires a certain ratio of 

 mineral constituents in the soil, as well as a certain minimum amount. 

 He held firmly to the idea that plants could no more attain their 

 maximum growth in the absence of a proper ratio of these mineral 

 nutrients than when the total quantity was too small. This theory 

 became known as Liebig' theory of mineral requirements. 



Liebig's explanation of the benefits of crop rotation followed as a 



