144 Illinois State Laboratory of Natural History. 



sometimes nearly or quite baffled in the struggle by the me- 

 chanical and physiological resistances of the host plant. 



I repeat, we now know that the numerous u 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, 

 andmay 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 (Gymnospofium nntcropus), which, in 

 an alternation of development, takes a very different form on 



