200 AGRICULTURAL EXPERIMENT STATION. [Jan. 



different cases, being largely controlled by varying con- 

 ditions. 



The great majority of parasitic fungi develop and vege- 

 tate within the tissues of their hosts ; but some forms live 

 and grow supei'ficially, merely sending small branches into 

 the cells of their hosts, for the purpose of absorbing 

 nourishment. These external parasites are, as a rule, much 

 less injurious to the plants they attack than are internal 

 parasites. 



A striking influence is often exerted on the habit of 

 growth of a plant by the attacks of a parasite. Thus, it 

 is often possible to tell which among a number of plants 

 are infected, by their appearing taller or shorter, or slenderer 

 or stouter, than the healthy plants ; or they may appear of a 

 lighter or darker shade of color ; or, as frequently happens, 

 the development of a fungus in the tissues of a plant may 

 cause the aflected parts to become abnormally developed and 

 distorted to such an extent as to attract the attention of even 

 the casual observer. 



Just here should be noted an important fact for the stu- 

 dent of parasitic fungi. As a rule, a given parasite is able 

 to live on only a single host species, or on a few closely 

 related species, seeming to require for its development 

 the special chemical and other conditions afforded by some 

 particular plant or particular group of similar plants. But, 

 on the other hand, closely related parasites may attack 

 widely diflerent plants. For example, there is a very com- 

 mon " rust" which attacks the Canada thistle, and another 

 which is equally common on grasses and grains. The two 

 rusts are very closely related, while the relationship between 

 the thistles and the grasses is very remote. Neither of these 

 rusts can live on the host plant of the other. 



The distinction between parasites and saprophytes, while 

 very useful, must not be made too strict; for there are 

 numerous fungi which, while naturally saprophytes, can 

 assume the role of parasites under certain conditions, and 

 others which may live as saprophytes, for a time at least, 

 though ordinarily obtaining their nourishment parasitically. 

 Many fungi, also, are probably parasites in some and sapro- 

 phytes in other parts of the life-cycle. 



