186 THE LOWER FUNGI— PHYCOMYCETES 



being globose knobs in Albugo and elongate branching threads 

 in Peronospora. In some cases (e.g., Phytophthora) the mycelium 

 may be also intracellular, the hyphae entering and passing 

 directly through the host cells. Species characterized by inter- 

 cellular mycelium are usually obligate parasites, while those 

 with intracellular hyphae exhibit facultative parasitism and tend 

 to develop well in artificial culture media. In some species the 

 myceUum is perennial in the host, hibernates in the roots, and 

 results in systemic infection of the parts above ground in the 

 spring. 



Oogonia and antheridia, resembling in general those of the 

 preceding order are formed, but here they seem to be functional 

 in all cases. The content of the oogonium is differentiated at 

 maturity into a peripheral zone of protoplasm (periplasm) and a 

 single central oosphere. After fertilization the oosphere is trans- 

 formed into an oospore, the oogonium being monosporic through- 

 out the group. The oospore usually lies free in the oogonium. 

 It germinates in some forms by swarmspores, in others by a germ 

 tube. 



The order, Peronosporales, as here constituted, contains three 

 families, Albuginaceae, Pythiaceae, and Peronosporaceae. The 

 Pythiaceae are clearly the most primitive of the forms included, 

 and were incorporated by Schroter (1893) in the Saprolegniales. 



Key to Families of Peronosporales 



A. Sporangia borne in a chain at the apex of a short, clavate, usually 



unbranched sporangiophore. Sporangiophores forming a limited 

 sorus beneath the host epidermis, and exposed by its rupture. 



1. Albuginaceae, p. 186 



B. Sporangia usually borne singly at the tips of a branching sporangiophore, 



sometimes intercalary, rarely in chains. Sporangiophores not forming 

 a sorus within the host. 



1. Sporangiophore usually not sharply differentiated from the hyphae 



of the mycelium. Sporangia borne successively. 



2. Pythiaceae, p. 193 



2. Sporangiophore sharply differentiated from the hyphae of the 



mycelium, and bearing the sporangia simvdtaneously at the tips 

 of its branches. 



3. Peronosporaceae, p. 210 



Albuginaceae 



The members of this family occur as parasites on flowering 

 plants, and, as far as is known, their parasitism is in all cases 



