Plants as Affected by Parasites. 147 
270. Manures are, in part, the Raw Material from 
which the cultivator turns out valuable products. They 
should, therefore, be most carefully preserved and applied. 
Leaching of the manure pile by undue exposure to rain, 
and over-rapid fermentation, by which nitrogen escapes into 
the air as ammonia or other gaseous nitrogen compounds, 
should be stringently avoided. All refuse organic matter 
should, so far as possible, be made to increase the always- 
too-small stock of manure. 
Section VI. PLANTs AS INJURIOUSLY AFFECTED BY 
PARASITES 
The only instance of a beneficial plant parasite of special 
interest to the cultivator, is the bacteroids in the roots of 
leguminous plants, which we have already considered (260). 
Many parasites (24) of harmful insects are beneficial, but 
these are beyond our scope. We need therefore to treat 
here only those parasites that are directly injurious to 
economic plants. 
271. The Injurious Parasites of plants are Very Nu- 
merous and a scientific classification of them is quite be- 
yond the limits of the present work. We shall only en- 
deavor to arrange the different parasites into groups based 
on their manner of working injury, and the methods by 
which they may be controlled. 
With reference to the character of their injury, and the 
preventives used, as well as in their natural characteristics, 
plant parasites are readily separable into two great classes, 
viz., animal and vegetable parasites. These classes will be 
considered in their order, 
