X.] EXCRETION OF TOXIC SUBSTANCES 293 



constituents of plant food than would be required for 

 the nutrition of the plant, Whitney and his colleagues 

 have suggested another theory of fertiliser action. 

 According to this point of view, a soil falls off in fertility 

 and ceases to yield normal crops, not because of any 

 lack of plant food brought about by the continuous 

 withdrawal of the original stock in the soil, but because 

 of the accumulation of injurious substances excreted 

 from the plant itself These toxins are specific to each 

 plant but are gradually removed from the soil by 

 processes of decay, so that if a proper rotation of crops 

 be practised, to ensure that the same plant only recurs 

 after an interval long enough to permit of the destruc- 

 tion of its particular self-formed toxin, its yield will be 

 maintained without the intervention of fertilisers. The 

 function of fertilisers is to precipitate or to put out of 

 action these toxins, and various bodies such as lime, 

 green manure, and ferric hydrate are also effective in 

 this direction ; the same result of destruction of the 

 toxins excreted by the plant may even be brought about 

 by minute quantities of certain bodies like pyrogallol. 

 According to this theory the function of fertilisers is to 

 remove toxins rather than to feed the plant : they are 

 only required when the same crop is grown continuously, 

 and the need for them may be obviated by a judicious 

 rotation which permits of the destruction of the toxins 

 by natural causes. Careful consideration will show 

 that this theory can be made to fit a good many of the 

 phenomena of plant nutrition, it would also explain the 

 difficulties experienced in growing certain crops con- 

 tinuously on the same ground ; it is in fact an elaborated 

 revival of one of the earliest explanations of the value 

 of rotations, originally suggested by de Candolle. 

 Furthermore, Whitney's colleagues have succeeded in 

 extracting certain substances from the soil — di-hydroxy- 



