112 PHYCOMYCETEAE 



are present in no definite arrangement. The primary zoospores squeeze 

 out one by one from the terminal opening and swim away, encysting at a 

 distance. The new zoosporangia are formed by proHferation within the 

 empty ones. Usually a little later, under conditions which can often be 

 controlled in culture, short or long lateral branches arise which swell at 

 the apex into a globular oogone separated by a septum from the main 

 hypha. The oogone may be single or there may be a chain of several 

 oogones. Within the multinucleate oogone the protoplasm cleaves into 

 several or many masses which round up to form the naked eggs 

 (oospheres). These are at first multinucleate but soon all the nuclei but 

 one disintegrate. On longer, usually more slender, branches from the same 

 plant (often arising just below the oogone) or from a different plant the 

 antherids are produced. These are terminal, multinucleate structures, 

 somewhat larger in diameter than the supporting hyphae, from which 

 they are separated by septa. Upon reaching an oogone they become 

 flattened against the outer surface. From the center of the surface of 

 contact a papilla pushes into the oogone, forming the conjugation tube 

 which seeks out an egg or branches so as to reach several eggs. A single 

 sperm nucleus passes into each egg if fertilization actually takes place. 

 In many cases there is no fertilization and the egg becomes a thick-walled 

 oospore, parthenogenetically. After the disintegration of the oogone the 

 oospore may lie dormant in the mud for several months, eventually 

 germinating by a tube which may or may not be terminated by a zoospo- 

 rangium. Reduction division probably occurs as the oospore germinates. 

 (Fig. 35A, B.) 



Achlya, with about 25 species, resembles Saprolegnia in habit and 

 manner of growth. The chief morphological difference is that the hyphae 

 bearing the zoosporangia grow sympodially so that there is no prolifera- 

 tion of zoosporangia. The behavior of the zoospores is also characteristic. 

 The primary zoospores encyst immediately as they emerge from the 

 mouth of the zoosporangium, forming a very typical cluster of encysted 

 spores. Only after 15-45 minutes or longer do these spores give rise to 

 secondary zoospores. The oogones are usually pluriovulate and in most 

 respects resemble those of Saprolegnia. In both Saprolegnia and Achlya 

 and in a few other genera the presence or absence of thin places or "pits" 

 in the oogone walls is of diagnostic importance. Many species of Achlya 

 are parthenogenetic. Both of these genera are usually exceedingly sensi- 

 tive to minute traces of copper salts as discovered by Horn (1904), j'-et 

 Gaumann (1919) found ;S'. monoica Pringsh. growing in the shaft of an 

 abandoned copper mine in Lapland where the dissolved salts gave the 

 water a green color. (Fig. 35C-E.) 



Aphanomyces has slender zoosporangia with but a single row of 

 zoospores which behave on emerging as do those of Achlya. The oogone 



