DISEASE CAUSED BY FUNGI 



199 



to the species may incline it to a greater susceptibility to 

 disease; that is, it may have less power of resistance 

 against external influences injurious to itself. Fungi must 

 always be reckoned amongst such external factors where 



', 



FIG. 50. Rhytisma acerinum, a parasitic fungus form- 

 ing black patches on a living sycamore leaf; 2, section 

 through a portion of a black patch or stroma bearing 

 spermatia or conidial form of fruit ; 3, ascus containing 

 eight needle-shaped spores, and accompanied by two 

 slender curved paraphyses ; 4, a free ascospore. The 

 ascospores are produced in the spring, after the dead 

 leaf has been lying on the ground throughout the winter. 

 Fig. i, one quarter nat. size ; figs. 2-4, highly mag. 



plants are concerned, and many fungi appear to be only 

 able to attack plants that are below the normal in vigour. 

 Other fungi attack plants more especially when life is on 

 the wane, as fading leaves, nearly ripe fruit, etc. 



