Plants as Affected by Parasites. 159 



others. The alternating of crops having different food 

 requirements tends to prevent the exhaustion of special 

 food substances. Crop rotation also aids in avoiding 

 damage from injurious insects and fungi. 



268. A Growing Crop Tends to Conserve Fertility, 

 because it reduces drainage by taking up water from 

 the soil, and at the same time, appropriates the avail- 

 able plant food, thus preventing loss of the latter from 

 leaching. 



269. Manures are, in part, the Raw Material from 

 which the cultivator turns out valuable products. They 

 should, therefore, be most carefully preserved and ap- 

 plied. Leaching of the manure pile by undue expos- 

 ure to rain and over-rapid fermentation, by which ni- 

 trogen escapes 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 VII. Plants as Affected by Parasites. 



The only instance of a beneficial plant parasite (24) 

 of special interest to the cultivator, is the micro-organ- 

 isms in the roots of leguminous plants, which we have 

 already considered (259). Many parasites 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. 



270. The Injurious Parasites of plants are Very 

 Numerous and a scientific classification of them is be- 

 yond the limits of this work. We shall only endeavor 



