POSSIBLE CAUSES OF THE GRASS EFFECT 289 



soil taken was the same in all cases, it was found that after 

 the experiment was concluded, that left in the pots containing 

 the trees with grass over them was actually richer in nitrogenous 

 matter to the extent of about 40 per cent, than that in the pots 

 without grass, though the trees were dying in the former case, 

 and flourishing in the latter. 



It is evident, therefore, that the action of grass on trees is 

 not a question of lack of nourishment : there is ample nourish- 

 ment there, but the trees are incapable of utilising it. This can 

 be due only to some toxic influence interfering with the physio- 

 logical action of the plant, and preventing it from utilising the 

 food which is present. 



