DECAY AND DEATH. 137 



chlorophyll-containing cells, with abundance of pores and 

 a thick skin, is obviously better able to resist injurious 

 agencies than a thin leaf whose delicate texture speedily 

 withers and falls a prey to adverse circumstances. 



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

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

 that, under the circumstances, that 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 vapours from chemical 

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

 are naturally most obvious and most severe at the growing 

 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 



