194 THE LOWER FUNGI— PHYCOMYCETES 



two orders. On the one hand, the highest of the Pythiaceae 

 provide a series of closely intergrading forms connecting the 

 family with the Peronosporaceae, while on the other, the lowest 

 of the Pythiaceae are so similar in various respects to members of 

 the Saprolegniales that the two groups tend to merge at the 

 border line. The sexual stage corresponds to that of the Pero- 

 nosporaceae rather than to that of the Saprolegniaceae, but does 

 not differ essentially from that of the Leptomitaceae. In general, 

 the mycelium corresponds to that of the Peronosporaceae in its 

 small diameter and tendency to irregularity. 



Key to Genera of Pythiaceae 



I. Sporangium much broader than the mycehum, strikingly asymmetrical 

 in form, and standing usually with its long axis at right angles to that 

 of the sporangiophore. 



1. Pythiogeton, p. 194 



II. Sporangium not standing with its long axis at right angles to that of the 

 sporangiophore, in most species broader than the mycelium and 

 symmetrical (oval, spherical, obpyriform, etc.), in a few cases (sub- 

 genus Aphragmium of Pythium) filamentous. 



A. Sporangial wall smooth. 



1. Sporangium, in germination, discharging the swarmspores in 



an imperfectly differentiated state into a thin-walled 

 vesicle, which later ruptures allowing the fully formed 

 spores to escape. 



2. Pythium, p. 195 



2. Swarmspores sometimes discharged fully formed into a 



vesicle, but usually freed directly through a pore in the 

 sporangial wall. 



3. Phytophthora, p. 199 



B. Sporangial wall echinulate; germination by germ tube; swarm- 



spores unknown. 



4. Trachysphaera, p. 209 



1. Pythiogeton v. Minden (1916: 228). 



This genus includes three species, P. uiriforme, P. transversum, 

 and P. ramosum, all of them described by von Minden. It is 

 closely related to Pythium, differing chiefly in that the sporangium 

 has the aspect of being attached at its side to the sporangiophore 

 instead of at its base. The long axis of the sporangium hes at a 

 right angle to that of the sporangiophore, and germination 

 takes place at one end of this axis instead of opposite the point 

 of attachment (Fig. 70). In germination, the undifferentiated 

 plasma of the sporangium flows into a thin-walled, tubular, 



