XXI. 



THE EXCEETOEY THEOEY. 



ANOTHER erroneous theory which has been too 

 readily accepted because of its supposed illus- 

 tration of an analogy between plants and animals, 

 is the excretory theory of roots, originated by the 

 elder De Candolle, and elaborated and rendered 

 still more erroneous by others. De CandoUe's 

 theory was that plants, having no power of select- 

 ing their food, absorbed everything soluble that 

 came within reach of their roots, but afterward 

 rejected by the same organs such portions as were 

 not adapted to their use. Maoaire afterward cofi- 

 firmed this theory to the satisfaction of himself and 

 many others, and announced that the material 

 excreted by growing roots was poisonous to other 

 plants of the same kind, but might be harmless, or 

 even beneficial, to plants of a different kind. Many 

 agricultural writers at once adopted this theory as 

 an explanation of the necessity for the rotation of 

 crops, it being assumed that soils which have long 

 supported plants of a given kind become poisoned 

 for plants of that kind by the long continued 

 accumulation, of the root excretions. The value of 



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