ORDKR SAPROLEGNIALES 



115 



v^- 



around carrying within them the encysted primary zoospores. Normally 

 these zoospores encyst within the zoosporangium and by mutual pressure 

 become more or less polyhedral in shape, depending upon their number. 

 The zoosporangial wall may be persistent, in which case the secondary 

 type zoospores escape through short exit papillae which pierce the sur- 

 rounding wall, so that eventually a net-like structure remains. In other 

 species the zoosporangial wall is evanescent early so that the encysted 

 primary zoospores are rounded on 

 their outer face and somewhat flat- 

 tened where they are in contact. 

 They may become separated under 

 pressure. The emerging zoospores 

 in both types are of the secondary 

 type and may encyst and emerge 

 again several times before germi- 

 nating by germ tubes. In the species 

 with persistent zoosporangium walls 

 this is the only type of zoosporan- 

 gium known. In those with evanes- 

 cent walls under certain conditions 

 of culture medium and temperature 

 the first zoosporangia (according 

 to Couch, 1931) empty their zoo- 

 spores through an apical pore where 

 they encyst as in Achlya. The later 

 zoosporangia are as in Dictyuchus. 

 The oogone contains a single egg. 

 The one species with several eggs, 

 D. polysporus Linds., assigned here 

 has been determined by Apinis 

 (1930) to belong to the genus Pro- 

 toachlya. Most of the known species 

 have been isolated from the soil 

 but some occur also in fresh water. Both homothallic and heterothallic 

 species are known (Couch, 1926b; Coker and Braxton, 1926). In Thrausto- 

 theca a similar preliminary production of Achlya-like zoosporangia has 

 been reported (Coker and Couch, 1924). (Fig. 38.) 



Miss Huneycutt (1948) has described a new genus, Aphanodictyon, 

 with the single species A. papillatum. This has the vegetative structure of 

 Aphanomyces, with very slender, branching mycelium. The zoosporangia 

 are globose or subglobose and the primary zoospores encyst within them 

 as in Dictyuchus. The secondary type zoospores escape through short 

 exit tubes. The oogones contain 1-8 eggs (mostly 3-6), which are fertilized 



Fig. 38. Saprolegniales, Family Sap- 

 rolegniaceae. Dictyuchus missouriensis 

 Couch. (A) Early zoosporangia of Achlya 

 type. (B) More enlarged zoosporangium 

 of Dictyuchus type. (Courtesy, Couch: /. 

 Elisha Mitchell Sci. Soc, 46(2) -.225-230.) 



