144 Illinois State Laboratory of Natural History. 
sometimes nearly or quite baffled in the struggle bythe me- 
chanical and physiological resistances of the host plant. 
I repeat, we now know that the numerous “rusts,” 
““smuts,” etc., found on the various kinds of vegetation, are 
themselves true plants, and that as such they are limited in 
their development like other organic species by certain condi- 
tions and surroundings. Some of these limitations are well 
known, others are yet to be ascertained. At present there is, 
except in a few cases, not enough of trustworthy information 
to enable us to suggest practical remedies or means of effectu- 
ally destroying the injurious fungi which so reduce the pro- 
ducts of our fields, fruit plantations, and gardens; which so 
disfigure our ornamental trees and defoliate our forests. But 
the difficulty exists not so much in the unconquerable nature 
of the enemies, as in the want of fuller attainable knowledge 
concerning them and their ways. There is reason to predict 
that the time will come when the mastery of man will prevail 
in this as in so many triumphs of the past by the application 
of power made available through persevering research and edu- 
cated perception. 
In some cases, however, we now know practical methods of 
exterminating the parasites, and in other cases of preventing 
their ravages by various processes of cultivation, selection or ap- 
plication. Usually these methods are quite beyond haphazard 
discovery, and often very remote from previous conception. 
For example: the leaves of apple trees are sometimes destroyed 
by a parasitic fungus which shows itself in prominent, scurfy 
bunches occuring here and there on the under side of the af- 
fected leaves, while upon the upper surface of the same spots 
the thickened area has a yellow or sometimes a crimson color. 
The leaf is distorted in shape, shows very evident signs of in- 
jury, and finally becomes ragged and withered. When a large 
number of the leaves of a tree are thus diseased the latter per- 
ceptibly suffers, and though seldom killed outright, after an 
unequal struggle for some years is rendered entirely worthless, 
and may as well be removed by the axe in the hands of the dis- 
appointed proprietor. Now the injury arises from a parasitic 
fungus described below (Gymnosporium macropus), which, in 
an alternation of development, takes a very different form on 
