66 COMPARATIVE MORPHOLOGY OF FUNGI 



before the sporangial opening and are there surrounded by a membrane. 

 No shedding of the membrane follows, and the resting zoospores germi- 

 nate directly to a mycelium (Fig. 39, 9). This emission of naked proto- 

 plasmic portions is reminiscent of diplanetism; but this character too 

 disappears. In Aplanes the zoospore initials, while still in the spo- 

 rangium, surround themselves with a wall and germinate there, piercing 

 the sporangial wall with their germ tubes (Fig. 39, 8). In Geolegnia, 

 the zoospore initials surround themselves with a thick wall and await the 

 decay of the sporangial wall before germinating directly to mycelium 

 (Harvey, 1925). Thus zoospore formation is entirely suppressed. It 

 is noteworthy that these degenerate, functionless sporangia only appear 

 occasionally in the mats; they are replaced by sexual reproduction. 

 Thus Aplanes and Geolegnia seem to be on the point of entirely giving up 

 degenerate asexual reproduction in favor of sexual. 



The sexual organs are developed (as in the Ancylistaceae) as anther- 

 idia and oogonia. In Saprolegnia mixta (Klebs, 1889) and Achlya 

 radiosa (A. decorata) (Obel, 1910), their formation may be hastened by 

 lowering the concentration of the medium or by lack of phosphates. 

 Couch (1926) reports Dictyuchus is heterothallic, physiologically differ 

 entiated strains being necessary to secure production of antheridia and 

 oogonia. The oogonia generally arise terminally on short branches of 

 the main hypha, more seldom intercalary or incased in empty sporangia. 

 Their wall is generally thicker than that of the vegetative hyphae, 

 colorless or yellowish, smooth or covered with stellate projections. The 

 antheridia develop on branches which are more slender and branched 

 than the oogoniophores. If the antheridiophores arise on the same 

 hyphae as the oogonia which they fertilize, the form is called andro- 

 gynous, if they arise on other hyphae, they are called diclinous. 

 Whether androgyny and diclinism correspond to true heterothallism 

 still remains to be investigated. Homothallism has been definitely 

 proved only for the androgynous S. monoica (Kniep, 1919). In certain 

 species, the antheridia arise directly under the oogonia and on the same 

 hypha (hypogyny). In still other species, they are entirely lacking or no 

 longer functional, and the oospores develop parthenogenetically. Klebs 

 (1899) and Kauffman (1908) determined that the forms of development 

 may be entirely influenced by the chemical character of the nutrient 

 solution; thus S. hypogyna, which normally forms hypogynous antheridia, 

 may form them on branches in certain nutrient solutions. 



Dangeard (1891) and Claussen (1908) studied the ontogeny of S. 

 monoica and Davis (1903) the oogonia of S. mixta. The antheridia and 

 oogonia take large masses of nuclei from the hyphae and then are 

 abjointed from the remaining part, the stipe. Before the septum is com- 

 pleted, the cytoplasm and nuclei degenerate in the oogonium, progres- 

 sively from the center toward the periphery. This process continues 



