224 NATURE OF PARASITES 



becomes changed and specialized, although there is no visible evi- 

 dence of this, and so after a time is unable to live upon but one 

 kind of plant. These specialized forms are termed biological spe- 

 cies, in contradistinction to the general or morphological species 

 which include them all. Evidently there is something within the 

 plant that not only attracts these parasites, but also changes them, 

 so that after a time they can only live upon the plant having these 

 substances. If this material is absent from the plant then it is 

 immune and it has been shown in a few cases where individual 

 plants were not subject to a plant disease that this was due to 

 the lack of a substance which the infested plants had or to the 

 presence of a new substance which was repellent to the parasite. 

 The relation between parasite and host is strikingly brought out 

 by Massee's experiment, in which he claims that a purely sa- 

 prophytic fungus was induced to become a destructive parasite 

 upon the leaves of a species of Begonia by injecting the leaves 

 with a sugar solution. The fungus flourished upon the leaves 

 treated in this way and produced spores. These spores were sown 

 upon leaves similarly treated and this was repeated for twelve 

 generations, at which time he states that the spores would grow 

 upon the untreated leaves. Perhaps this is the explanation of some 

 epidemics or the occasional sudden appearance of a plant disease. 

 A variety of circumstances might cause plants to form substances 

 attractive to the parasite or to fail to develop repellent materials. 

 In either case they would become susceptible to the disease. 



The spores are formed in chains (Fig. 146, c) from the end 

 of the erect hyphae that project from the surface of the leaf in 

 thick masses, causing the powdery appearance and the popular 

 name of these parasites. These spores germinate quickly and 

 rapidly spread the fungus. 



The reproductive organs are formed from short branches as 

 shown in Fig. 147, A. The solution of the walls at the point of 

 contact of these organs permits the male gamete to pass over 

 and fuse with the female (Fig. 147, B). The growth of the 

 gametospore forms a limited number of cells and one of them, 

 usually the second from the end, will develop one or several asci 

 (Fig. 147, C D). As in Penicilliuni, this growth becomes en- 



