DECAY AND DEATH. 129 
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 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 
