292 



PART 111. THE CLASSIFICATION OF PLANTS. 



genera (Peronospora, Cystopus) an external coat, the episporium or perinium, is 

 formed round the oospore from the periplasm. 



The germination of the oospore takes place in different ways in different 

 species. In Phytophthora omnivora and Pythium proliferum it gives rise to a 

 small mycelium (promycelium) which produces a few spores, from which sexual 

 plants are developed. In other species (e.g. Cystopus Candidas) the contents of 

 the oospore are set free as a number of zoospores. In yet other species (e.g. 



Pythium de-Baryanum, Pyth- 

 ium Artotrogus, Peronospora 

 Valerianellte), the oospore gives 

 rise to a sexual plant. 



The life-history of most of 

 the Peronosporaceae presents an 

 alternation of generations. In 

 all cases the plant is the game- 

 tophyte. In those species in 

 which the oospore gives rise to 

 a promycelium, the promycel- 

 inm is the sporophyte ; in those 

 in which the oospore gives rise 

 to zoospores, the oospore itself 

 represents the sporophyte ; and, 

 finally, in those in which the 

 oospore at once gives rise to 

 a sexual plant, the sporophyte 

 is altogether unrepresented. 



The Peronosporaceae may be 

 subdivided as follows : 



Planoblastce : the gonidan- 

 gia gives rise to zoo- 

 gonidia on germination : 

 Cystopus, Pythium, Phy- 

 tophthora, etc. 

 Siphoblastce : the gonidan- 

 gia germinate as if they 

 were gonidia, giving rise 

 to a hypha : Perono- 

 spora, Bremia. 

 There is a doubtful genus, 

 named MONOBLEPHAKIS, pro- 

 bably allied to the Perono- 

 sporaceae, which is remarkable 

 in that the male organ is an an- 

 theridium, since it gives rise to a number of uniciliate spermatozoids, and in 

 that the oogonium opens at the apex to permit the entrance of the spermatozoid 

 to the single oosphere. The antheridium is a cell just behind the oogonium, 

 which is terminal. The oosphere is formed from the entire protoplasmic con- 

 tents of the oogonium. The mycelium also bears gonidangia, which give rise to 



FIG. 203. A Surface-view of the epidermis of a 

 Potato-leaf with the gonidiophores of Phytophthora 

 infestans projecting out of the stomata ( x 90). B A 

 ripe gonidangium. C Another undergoing divi- 

 sion. D A zoogonidium. ( x 540 : after Strasburger.) 



