SAPROPHYTES AND PARASITES 77 



impelled by the vibrating hairs, as if endowed with animal life, 

 hence termed zoospores. Thus it will be observed that some 

 of the species of Fcronospora differ from the true moulds in that 

 the tlireads do not bear veritable conidia but sporangia, whicli 

 contain numerous active zoospores which escape on the rupture 

 of the parent cell and float off on their own account. The 

 ultimate career of these zoospores is usually brief, for after 

 floating a short time they settle down to rest, the cilia or hairs 

 fall away, and a tliin germ tube is projected, which enters a 

 neighbouring stoma in the leaves of the foster plant, and 

 originates a new mycelium, and thus extends the action of the 

 parasite. There is, however, another mode of reproduction 

 which takes place within tlie tissues of the foster plant, by a 

 differentiation of the mycelium and the production of oospores, 

 a kind of resting spores, which hibernate through the winter 

 and provide for the continuance of the parasite in the spring. 

 These oospores are of considerable size, and possess a thick 

 coloured outer coat, and they remain embedded in the old 

 stems, haulms, petioles, or leaves of the host-plant, quiescent 

 throughout the winter, and are only liberated by the decay of 

 the tissues. In the spring, and when uninfected young seedlings 

 of the host-plant may be supposed to be numerous, these 

 oospores awaken to activity, the contents become divided in 

 the same manner as the contents of the conidia were divided, 

 only much more numerous ; then the thick outer coat is 

 ruptured, and a great number of active zoospores emerge, ready 

 to be washed by the spring rains into fovourable positions for 

 germinating and infesting new plants. In this manner the 

 parasite is preserved through the winter, and the perpetuation 

 of the species assured. Whether the zoospores are derived 

 from the sporangia, which are developed on the aerial branches 

 of the hyphae, or whether derived from tlie resting spores, 

 their subsequent history and functions are the same — that is 

 to say, entering the host-plant by means of a germ tube, 

 developing a new mycelium, and producing a new infection. 

 Some species of the Peronosjjoraceac produce simple conidia on 

 the hyphae, which never develop active zoospores, but ger- 

 minate at once. Although, as has been shown, infection may 

 proceed from without inwards, tlie subsequent manifestation of 



