DECAY AND DEATH. 129 



The fall of the leaf in the case of deciduous trees has 

 heen already alluded to. It is only requisite here to say 

 that, under the circumstances, <tbafc it is a natural 

 process ; and it is one that is provided for from the 

 beginning. From a very early stage in the development 

 of the leaf, a special layer of cells has been gradually 

 forming at the base of the leaf -stalk at right angles to 

 the others, which ultimately cuts off the dying and dead 

 leaf -cells from the living tissues of the bark, much as 

 the " drop scene" of a theatre separates the body of the 

 house from the stage at the close of the performance. 

 The leaf is emptied of its contents, and further supplies 

 from below are eventually stopped off by the intervention 

 of the layer of cells above described. A similar process 

 takes place in the disarticulation of branches and of ripe 

 fruits. 



When disease or injury affects the leaves while still 

 growing as in the case of noxious vapors from chemical 

 works or kilns, or in the case of insect injury its effects 

 are naturally most obvious and most severe at the grow- 

 ing points the tips and margins of the leaf ; and when 

 the margins become thus arrested in their growth, while 

 the disc remains in full activity, the result is a cup- 

 shaped appearance or a crumpled surface resulting from 

 the dead or dying portions having lost their elasticity 

 and acting as a curb on the growing portions. Sun-burns 

 and especially the attacks of insects and parasitic fungi 

 are not so much confined to the margins, at least when 

 the leaf is not in a growing state ; they produce their 

 effects in the shape of circular or irregular spots of brown 

 decayed protoplasm. The effects of frost and the reason 

 it kills have been explained in a former page. Nothing, 

 however, can be advanced in explanation of the reasons 

 why some plants of the same species, like the different 

 varieties of wheat, are so much more tender than others. 

 Death by the leaf is rarely immediately fatal, because 



