OOMYCETES 327 



Peronospora (Corda). — The thallus and the sexual organs closely 

 resemble those of Cystopus. In both genera the passage of protoplasm 

 from the antherid into the oogone is not directly visible, and the 

 oogonial periplasm is more abundant than in the other genera. In 

 germinating, the oosperm produces a germ-tube, but the process of germi- 

 nation has not been observed in a number of species, and, as de Bary says 

 (Journ. Roy. Agric. Soc, 1876), it is quite possible that the species of 

 Peronospora which, like Cystopus, produce zoospores from their 'conidia' 

 (zoosporanges) present also the same phenomenon in connection with 

 the oosperms. The sporophores of Peronospora commonly issue from 

 the host-plant through the stomates, and are, for the most part, regu- 

 larly and copiously branched. At the fine points of the branches the 

 non-sexual propagating bodies are produced singly. These are in some 

 species conidiospores germinating by the emission of a germ-tube, and 

 in others zoosporanges producing zoospores. Conidiospores and zoo- 

 sporanges are borne in precisely similar fashion, and present the same 

 appearance up to the. production of the germ-tube or zoospores, as the 

 case may be. The zoospores are formed within, and escape from the 

 original zoosporangial membrane and not from an extruded sac. Inter- 

 mediate between these forms are the plasmatoparous species (P. densa, 

 Rab., and P. pygmaea, Ung.), in which the whole protoplasm escapes 

 from the spore in a mass through the opening of a papilla-like point in 

 the wall, and, at once becoming globular, secretes a cellulose membrane, 

 and subsequently germinates by the emission of a short thick germ-tube. 

 The germ-tubes both of zoospores and of conidiospores penetrate 

 directly the epiderm of the host and the cells underlying it, until an 

 intercellular space is reached. This genus contains a large number of 

 well-known parasites, such as P. viticola (de By.) on the vine, P. nivea 

 (de By.) on Umbelliferae, P. parasitica (de By.) on Cruciferae, P. Schlei- 

 deniana (Ung.) on onions ; P. Viciae (de By.), P. Trifoliorum (de By), 

 &c. Hypertrophy is frequently produced in the host at the time of 

 oosperm-formation, but not so acutely as by Cystopus. The oosperms 

 of several species are unknown, and of these P. Rumicis (Corda) and 

 P. Schachtii (Fiickel) hibernate by means of their perennial mycele, 

 while P. Ficarias (Tul.), the oosperms of which are known, passes the 

 winter in the same way. 



Phytophthora (de By.). — This genus was founded for the reception 

 of P. infestans (de By.), which was formerly placed in Peronospora. 

 Industrious research has failed to discover the sexual organs and 

 oosperms of this species. Mr. Worthington Smith claims to have 

 found them, but the balance of evidence is distinctly against this. 

 The sexual organs of Phytophthora omnivora (de By.) have been 



