198 Parasitic Moulds a7id Mildews. [Sess. 



purpose of a resting-spore, and remains within the dead tissues 

 of the host until liberated by the decay of the latter in winter 

 or spring. In some species the oospore germinates directly 

 by emitting a tube which becomes branched and forms a 

 mycelium ; while in others numerous zoospores are formed 

 within ,the ripe oospore, and are liberated by the rupture of 

 the outer covering which envelops the latter. The zoospores 

 are provided with motile threads, by which in wet weather 

 they are able to move about, but they finally become quiescent 

 and emit a germinating thread. When brought into contact 

 with the host-plant, the thread enters one of the stomata and 

 develops a mycelium. In many species of parasitic Phycomy- 

 cetes, each of the conidia produces several zoospores which 

 resemble those emitted from the oospores, and germinate in a 

 similar manner. Turning now from the Phycomycetes to the 

 parasitic Hyphomycetes, we may observe that the latter are 

 epiphytal, and produce a mycelium which only affects the 

 superficial tissues of the host. They chiefly occur on the 

 leaves of the host-plant, where they either form a coating of 

 hyphae, or give rise to withered or faded spots which bear 

 clusters of conidiophores on their lower surface. The conidia, 

 on germinating, at once develop a mycelium, and never pro- 

 duce zoospores. Many of the so-called " species " of parasitic 

 Hyphomycetes have now been ascertained to be merely an 

 early or conidial stage in the development of certain species 

 of liigher fungi, especially the Ascomycetes. 



While the genera and species of these moulds and mildews 

 can only be satisfactorily determined with the aid of the 

 microscope, it may be observed that the conidiophores of the 

 parasitic Phycomycetes are generally more highly developed 

 and more fully branched than those of the Hyphomycetes ; 

 the conidia are more uniformly of a broadly-elliptical or sub- 

 globose shape ; and the affected tissues of the host are more 

 frequently distorted or blanched than withered. 



For further information regarding parasitic moulds and 

 mildews, and a fuller account of their life-history, reference 

 may be made to Massee's ' British Fungi : Phycomycetes and 

 Ustilagineae,' 1891; Massee's 'Text Book of Plant Diseases 

 caused by Cryptogamic Parasites,' 1899; Massee's 'British 

 Fungus Flora,' vol. iii., 1893; Cooke's 'Fungoid Pests of 



