THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 235 



The latter germinate by one or more germ tubes, while the former produce 

 zoospores in a vesicle. In both cases, however, these reproductive bodies 

 are formed in chains so that the conidiospores appear to be abstricted in a 

 similar way to the zoosporangia of Cystopus, but they do not give rise to 

 zoospores as in that genus. 



In Pythium vexans zoosporangia are rarelv formed, but the asexual repro- 

 ductive organs behave as conidiospores, the germ tube being produced in 

 place of the papilla of a zoosporangium. 



Finally in Pythium idtimum zoosporangia are never formed, but spherical 

 terminal conidiospores are produced which germinate by a germ tube in the 

 usual way. 



A similar series can be traced in the genera which comprise the Perono- 

 sporaceae, and we may briefly outline the series here. 



In the genus Basidiophora the sporangia are borne in clusters from the 

 top of a clavate sporangiophore. Each zoosporangium is spherical and 

 possesses a definite apical papilla through which biflagellated, reniform 

 zoospores ultimately emerge. 



In the genus Plasmopara the sporangiophore is complexly branched 

 and the zoosporangia develop terminally on the ultimate branches. Usually 

 the contents of the zoosporangium divide up into separate zoospores which 

 escape, but sometimes the structure forms a germ tube and behaves as a 

 conidiospore. 



In the genus Bremia the sporangiophores are complexly branched, each 

 branch terminating in a saucer-shaped expansion bearing a number of sterig- 

 mata. On each of these a single conidiospore is borne. This conidiospore 

 possesses an apical papilla, and it is through this that the germ tube emerges. 

 Zoospores are never formed. 



Finally in the genus Peronospora itself the branched sporangiophores 

 terminate in long, fine sterigmata, each of which bears a single conidiospore. 

 There is no apical papilla, and the germ tube is produced from an indeter- 

 minate point on the side of the spore. 



From the above evidence, therefore, it seems reasonable to consider that 

 the conidiospore, at least in this group, has been derived from a zoo- 

 sporangium, the contents of which produce a germ tube instead of dividing 

 up into a number of separate zoospores. This change from motile zoospores 

 to conidiospores may have been brought about as an adaptation to drier 

 conditions in which motile zoospores were less successful than aerially 

 distributed conidiospores. 



Cystopus Candidas (Albugo Candida) (The White Rust of Crucifers) 



This Fungus attacks all kinds of cruciferous plants and may sometimes 

 cause swelling and twisting of the stems, especially in young plants. The 

 disease is easily recognizable by the white, smooth, glossy patches which 

 gradually turn powderv and give the Fungus its common name of White 

 Rust (Fig. 227). 



