172 THE LOWER FUNGI— PHYCOMYCETES 



cellulose membranes and uniciliate zoospores. As it is known 

 only in the sporangial condition it cannot be assigned to either 

 family with certainty (see p. 130). 



In the habit of growth and general morphology of the thallus 

 the family is allied on one side through Leptomitus and Apodachlya 

 to the Saprolegniaceae, and on the other through Rhipidium and 

 Araiospora to the Blastocladiaceae. In its sexual stage the 

 group shows relationship with the Pythiaceae. The protoplasm 

 of the oogonium is differentiated into ooplasm and periplasm, 

 fertilization occurs, and a single oospore is formed. 



The members of the family are typically aquatic, and occur 

 as saprophytes on vegetable substrata. Few students have con- 

 cerned themselves with the group, the species being in general 

 infrequently collected though apparently not uncommon. The 

 monographic treatment of the group given by Kanouse (1927) 

 is the most complete. 



Key to Genera of Leptomitaceae 



I. Thallus not differentiated into a stout trunk and slender l^ranchcs; 

 composed of cylindrical hyphae which are definitely and characteristi- 

 cally constricted at intervals, and have consequently a segmented 

 appearance. 



A. Zoosporangia cyHndrical, not differing in form from the hyphal 



segments; zoospores escaping singly from the sporangium as in 



Saprolcgnia. 



1. Leptomitus, p. 173 



B. Zoosporangia ellipsoidal to pyriform , considerably broader than 



the segments of the hyphae; zoospores encysting at the 

 mouth of the sporangium as in Achlya. 



2. Apodachlya, p. 173 



II. Thallus differentiated into a stout trunk and slender branches; the 

 latter usually definitely constricted. 



A. Trunk more or less definitely cylindrical, corresponding in form to 

 the narrower apical branches. 



1. Diameter of trunk not strikingly greater than that of the 



branches; sporangia of one sort, smooth and broadly 

 clavate; oogonia pyriform; oospore lacking a cellular 



envelope. 



3. Sapromyces, p. 175 



2. Diameter of trunk much greater tlian that of the branches; 



sporangia of two sorts, one smooth, the other more or less 

 ornamented with spines or finger-like processes; oogonia 

 spherical; oospore surrounded by a cellular envelope. 



4. Araiospora, p. 177 



