WHY THE LEAVES FALL. 
377 
to be seen whether this separating layer is of universal formation 
in the case of deciduous leaves. For many reasons we are inclined 
to think that it is not formed when leaves fall off prematurely, 
as they do from drought or injury. But supposing it to be of 
general occurrence under what we may term ordinary circum- 
stances, it still remains to be ascertained what are the precise 
conditions inducing the formation of this peculiar layer of tissue. 
It is easy enough to say that the layer in question is formed 
owing to the disproportionate growth of the tissues, those of the 
stem being active, while those of the leaf are obstructed and 
comparatively inert. This may be so, but there is no direct 
evidence on the point as yet. We admit the facts, but the 
precise relation of those facts one to another is not yet fully 
made out ; till this is done, the explanation of the fall of the 
leaf cannot be considered as fully satisfactory. 
