200 THE LOWER FUNGI— PHYCOMYCETES 



indefinite character of its sporangiophore and the successive 

 development of its sporangia. In these respects it differs 

 strikingly from such genera as Peronospora, Plasmopara, and 

 Sclerospora. Its close relationship to Pythium forbids the 

 treatment of the two genera in different families as advocated 

 by Schroter (1893). In fact the two genera stand so close 

 together that at present no satisfactory basis for a generic 

 separation is known (Fitzpatrick, 1923). The recognition of 

 Phytophthora as a genus distinct from Pythium is chiefly in 

 deference to precedent. The merging of the two genera under 

 the older name Pythium will probably take place in time unless 

 future research on the group reveals a more tangible basis for 

 their separation than any known at present. 



De Bary, in founding the genus Phytophthora, realized its 

 close relationship with Pijthium, but felt that the difference 

 in sporangial germination existing in the species known to him 

 served to separate the two. Butler (1907), the foremost student 

 of the Pythiaceae, has accepted de Bary's point of view as a 

 basis for retention of the two generic concepts. He emphasizes 

 the fact that in Pythium germination is always accomplished by 

 the extrusion of the contents of the sporangium into a thin-walled 

 vesicle which finally bursts freeing the spores. Later work by 

 Rosenbaum and others demonstrated that a vesicle is not infre- 

 quently formed in Phytophthora, though it is true that the swarm- 

 spores of this genus usually escape directly from the sporangium. 

 When a vesicle is formed in Phytophthora the swarmspores are 

 said to be fully formed before their migration into the vesicle 

 begins. In Pythium, according to Butler, they flow out in an 

 incompletely differentiated state and attain their final form in 

 the vesicle. Unfortunately, the determination of the exact point 

 at which the final fashioning of the swarmspores is accomplished 

 often proves difficult if not actually impossible. Moreover, in 

 at least one species {Phy. palmivora Butler) germination may be 

 at times typical of Pythium, while at others a vesicle is absent 

 and the swarmspores escape directly from the sporangium. It 

 is clear that this character used alone provides at best an unsatis- 

 factory separation of the two genera. 



The shape of the sporangium in most species of Pythium may 

 be said to be characteristically more nearly spherical than in 

 Phytophthora where it is usually oval to obpyriform and papillate, 

 but so many intergrading conditions exist that shape has only 



