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coating of the surface of leaves by external growths of mildews 
and sooty moulds shuts off the light from the chlorophyl bodies, 
partially preventing photosynthesis, the process of starch forma- 
tion. The presence of internal parasites often excites a morbid 
growth of the plant tissues causing galls, knots or other deformi- 
ties, or they may cause an excessive formation of gums or resins. 
In other cases the parasites may multiply so greatly in the tis- 
sues as to plug the ducts in the vascular bundles, shutting off the 
ascending sap and thus causing the sudden wilting and death of 
the entire top. The rotting of the roots may cause a similar 
wilting. A bacterial parasite causes the fermenting of the sap in 
the soft cambium layer of pear and apple trees causing the sud- 
den death of considerable branches. 
Different fungi have acquired the power of parasitism in dif- 
ferent degrees. The true parasites like the rusts and smuts have 
the power of taking their nourishment directly from the living 
protoplasm of their hosts. In most cases they do not kill the 
tissues in which they are embedded though they may interfere 
seriously with their normal functions. Other fungi that normally 
live on decaying vegetable matter have developed the power 
under certain conditions of penetrating tissues that are still living, 
These are called facultative parasites. They are not able asa 
tule to take nourishment directly from the living protoplasm as 
do the true parasites, but they push their hyphae into or between 
the living cells of the host, and by the secretion of poisonous 
acids and enzymes kill them and render their contents soluble, 
thus causing the actual destruction of the living tissue. Many of 
the species of fungi that are normally strictly saprophytic at times 
develop this power of killing and disintegrating living tissues. 
Most of the timber rots so destructive to forest trees and to struc- 
tural timbers, belong among these facultative parasites, 
It is only after the cause of a disease is thoroughly understood 
that we can begin intelligently to seek a remedy. The annual 
losses from plant diseases are so great as to be beyond computa- 
tion, but it is safe to say that they reach many millions of dollars 
for the State of New York alone. Unfortunately, too, all of these 
losses come from what should be the farmer’s profits, for it costs 
