DEVELOPMENT OF PLANTS 



245 



not infest any of the others. It would seem that the parasite 

 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 

 species, 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 



FIG. 155. FIG. 156. 



FIG. 155. Appearance of one of the powdery mildews, Uncinula, on leaf 

 or elm. 



FIG. 156. Enlarged view of the mycelium, ascocarp, etc., of one of the 

 mildews, Erysiphe: c, erect hyphae forming spores or conidia; /?, haustoria 

 penetrating epidermis of leaf; a, ascocarp or perithecium. 



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 saprophytic 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 



