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When observed under a microscope they are not unlike a thin 
parchment or small scales of bark. Owing to their presence, 
wind, rain, sun, or dust have no action on the more delicate In- 
ternal living cells, which would soon be destroyed by handling 
or exposure were it not for this natural jacket which covers them. 
This jacket or covering is not, however, absolutely continuous, 
and small microscopic breathing-pores or stomata occur at inter- 
vals on the surface. 
Let us suppose such a leaf to be attacked by a fungus para- 
site. If thin shavings of such a diseased leaf were cut by means 
of a specially made microscopic razor called a “‘ microtome,” and 
one of these thin slices was put under a microscope, we would see 
a section of the leaf showing on the outer edges one row of scales 
of the cuticle, disposed somewhat like bricks or tiles, and in the 
centre a mass of more tender living tissues, disposed somewhat like 
a close-grained sponge, and through which the sap circulates. 
Now in the case of a diseased leaf, we see permeating the 
living tissue of meshwork of filaments which are the roots of the 
fungus, and are called mycelial threads, then, emerging through 
the breathing pores of the plant, slender thread-like stems showing 
on the surface of the leaf, and on each of these thread-like stems 
little branches carrying small sacks full of spores or of seeds of the 
fungus. 
Thus we see that a parasitic fungus robs the plant, throu): 
its mycelial threads, of the sap and the food which is pumped up 
by the plant from its roots to the extremity of its branches for the 
nourishment of its foliage and its fruit. In order to understand 
the usefulness of spraying with fungicides, we must not lose sight 
of these facts. The internal cells of the leaf are so delicate that 
the application to them of a poisonous substance, even though in a 
very diluted form, would prove injurious. But those internal cells 
are covered over and protected by the outside layer of thick-walled 
impermeable cells just mentioned, which would be able to bear the 
effect of the poison in a stronger form. Of course, it will be under- 
stood that a very strong poison would affect these outside cells 
also, but there is a certain strength of the poison, slightly stronger 
than would be injurious to delicate cells, than can be applied to 
them with impunity. It so happens that the cells of fungi are as a 
rule very delicate, and highly susceptible to the destructive in- 
fluence of poisons in, solution ; and, often, such solution as would 
not be injurious to the outside thick-walled cells of leaves are quite 
fatal to fungus cells. 
Tender leaves are, as a rule, for reasons detailed in the fore- 
going paragraph, less resistant than older leaves of the same plants ; 
and, again, some sort of trees, such as the peach or the nectarine, 
having a thinner and more delicate cuticle titan the apple or the 
pear, are sometimes injured by the application of poiscnous liquids 
that would not hurt the more resistant plants. 
