110 PHYCOMTCETEAE 



such potential zoosporangia may round up into thick-walled resting 

 spores or chlamydospores. 



Emergence of the zoospores is usually through the softened apex of 

 the zoosporangium. In Saprolegnia, Leptolegnia, and Isoachlya the primary 

 zoospores swim away as soon as released, encysting separately at some 

 distance from the zoosporangium. In Achlya, Aphanomyces, and several 

 other genera the escaping primary zoospores encyst immediately on 

 emerging and form a ball of cells which release the secondary zoospores. 

 In Thraustotheca and other genera the primary zoospores encyst within 

 the zoosporangium and upon rupture of the latter the encysted spores 

 are set free and give rise to the secondary zoospores. In Dictyuchus the 

 encysted spores are polyhedral by mutual pressure and germinate within 

 the zoosporangium by short exit tubes which pierce the zoosporangium 

 walls, thus setting free the secondary zoospores individually. In Aplanes 

 and Geolegnia and some other genera the encysted primary spores germi- 

 nate by germ tubes within the zoosporangium or after the latter has 

 disintegrated. Under varying conditions of culture the same species of 

 Saprolegnia or Achlya may be induced to produce its zoospores in the 

 manner typical of Saprolegnia, Achlya, Thraustotheca, or Aplanes, showing 

 that these modifications are not of very deep fundamental importance. 

 This is corroborated by the fact that Salvin (1942) was able to succeed 

 in attempts at mating Thraustotheca clavata (de Bary) Humphrey with 

 Achlya flagellata Coker, the former producing the antherids and the latter 

 the oogones. The oospores so produced could not be brought to germina- 

 tion by the methods attempted. (Figs. 34-38.) 



Pythiopsis. In this rarely studied genus the sympodially produced 

 sporangia may be ovoid (P. cymosa de Bary) or slender. The mycelium 

 is rather stout, as is typical for the family. The zoospores which escape 

 are of the primary type. After encysting they may germinate by a germ 

 tube or may produce zoospores again, but these are still of the primary 

 type in the two species. The oogone usually has but one egg. The antherids 

 may be up to three in number and may arise from immediately below the 

 oogone. The species of this genus occur in soil or in fresh water. A third 

 species assigned to this genus by Harvey (1925) should be transferred, 

 according to Coker and Matthews (1937) to the genus Isoachlya. 



Saprolegnia. This is the most commonly studied genus of the family. 

 It contains about 20 species, mostly saprophytic, rarely parasitic, on 

 animal or vegetable matter in water or soil. Dead insects in water or even 

 larger animals become surrounded by a fringe of the long external hyphae, 

 the much-branched trophic hyphae being within the animal tissues. These 

 external hyphae are fairly stout (up to 50 to 100 microns in diameter in 

 extreme cases) and more or less straight and but little branched. They 

 terminate in club-shaped zoosporangia within which numerous zoospores 



