PERO NO SPORES. 263 



process is comparable to that which takes place among the 

 monoecious Saprolegniaceae. The wall of the oospore \&- 

 comes differentiated into two or more layers (as, in fact, .is 

 usual in resting spores), the outer of which (the epispore) is 

 thick, hard, rough, and dark colored, while the inner (the 

 endospore} is thin and transparent (C, D, E, F, Fig. 180). 



347. In their sexual reproduction the species of the genus 

 Cystopus agree with those of Peronospora above described. 

 The various stages are shown in Fig. 180. 



348. The germination of the oospores takes place in some 

 species of the genus Peronospora by the formation of a ger- 

 minating tube, which soon gives rise to a mycelium. In 

 Cystopus, however, the oospore swells, and by the bursting 

 of the epispore the endospore escapes as a loose bladder sur- 

 rounding the protoplasm, which has by this time become di- 

 Tided into a large number of naked masses of protoplasm 

 (E, F, Fig. 180) ; by the bursting of the surrounding mem- 

 brane, these bodies are set free as bi-ciliate swarm-spores (G, 

 Fig. 180), which, after a short period of activity, come to 

 rest, and germinate in exactly the same way as those derived 

 from the conidia. In some species of Peronospora it appears 

 that swarm-spores are developed as in Cystopus, and it ap- 

 pears from the observations of VV. G. Smith, that in the potato 

 fungus (Peronospora infestans} some of the oospores pro- 

 duce swarm-spores, while others send out a germinating 

 tube.* 



349. But little is known regarding the time, as well as 

 the mode of germination of the oospores, but from those ob- 

 served it is probable that it takes place after a period of rest 

 extending from autumn to spring. This is known to be the 

 case in some species of Cystopus, in which the oospores pass 

 the winter in the rotting tissues of its hosts. 



* See a paper " On the Germination of the Resting Spores of Perono- 

 spora Infeatans," by Worthington G. Smith, in Gardeners' Chronicle, 

 July, 1876, and reprinted in " Grevillea," 1876, p. 18. He found that the 

 oospores which germinated first produced swarm-spores like those of 

 Cystopus, while the later ones " protruded a thick and generally jointed 

 thread." Jn his account figures of both modes are given. 



