CH. VII] LEAF-FALL. 107 



mere tumbling down of withered leaves. If, during the 

 summer, a branch is half-broken so that it hangs on to the 

 tree by its bark only, its water-supply is cut off and it 

 soon withers and dies. It might have been expected that 

 these withered leaves would fall more easily than normal 

 leaves, but precisely the reverse is the case : they hang 

 on to the tree after all the healthy leaves are cast. This 

 suggests that leaf-fall is an active, not a passive process, a 

 phenomenon of life which can only occur in a living leaf- 

 stalk. This is the case : the leaf falls because a layer of 

 cells (the absciss layer) forms across the base of the stalk 

 specially adapted to allow the leaf to free itself. Beneath 

 this layer cork cells are developed, which serve to cover 

 and protect the wound left by the fall of the leaf. 



