326 



FUNGI 



species appear again to have lost the power of producing sexual organs, 

 and this is notably the case in Phytophthora infestans (de B)'.), the 

 potato-disease fungus, which succeeds in hibernating by means of a 

 perennial mycele. In such a case the species is entirely dependent 

 upon the propagating spores for distribution. 



Cystopus (Lev.). — The thallus consists of hyphae inhabiting the 

 intercellular spaces of the tissue of flowering plants, and provided with 

 haustoria. The oosperms are resting-cells which ger- 

 minate in spring by means of the production of zoo- 

 spores in the usual way. The propagating zoospores 

 are borne in zoosporanges at the end of cylindricil or 

 club-shaped zoosporangiophores in vertical series. A 

 small broad swelling first appears at the apex, and 

 then a transverse wall cuts off the upper portion, 

 which rounds off and thus becomes the first and 

 oldest zoosporange of the series. Then another is 

 cut off in the same fashion, while the sporangio- 

 phore elongates. A series or chain is thus produced, 

 each zoosporange joined to its neighbour by a very 

 short and slender connecting stalk. The first cell 

 at the top of the series has a thicker wall than the 

 others, is yellowish in colour, and is, at least in the 

 vast majority of cases, incapable of germination. If 

 germination does take place, a germ-tube is said to 

 be produced, while all the other members of the series 

 give rise to zoospores. These chains of zoosporanges 

 arise in dense masses side by side below the epiderm 

 of the host, which is gradually raptured, permitting 

 their escape, the thick wall of the top member of the 

 series serving as a shield in bursting the epiderm. 

 ^^'^len the zoospores germinate, their germ-tubes enter 

 the host by way of the stomates, by this means attain- 

 ing directly the intercellular spaces. The disease thus 

 set up in the host is not so active as in the case of 

 species of Peronospora, and the parts affected do not perish so rapidly. 

 During the formation of oosperms in Cystopus candidus (Lev.), regions 

 of the host undergo acute hypertrophy. The commonest species of the 

 genus is C. candidus (' white rast '), which attacks a large number of 

 Cruciferae. Cabbages and the Shepherd's Purse (Capsella bursa-pas- 

 toris) suffer conspicuously from it, while the latter is often affected by 

 Peronospora parasitica (de By.) in company with it. Other well-known 

 species are C. Portulacese (Lev.) and C. cubicus (Lev.). 



Fig. 287. — ?Hj mycelial 

 brancli of Cystofius 

 portultuetF'L^v., pro- 

 ducing two sporangio- 

 phores,/, bearing zoo- 

 sporanges, «, in series 

 ( X 390). (After de 

 Bary.) 



