SAPROLEGNIALES 795 



are to be found in all members of the order. Monoplanetism (monomor- 

 phism) occurs only in the genus Pythiopsis, in which the emerging zoo- 

 spores are of the primary type described below ; that is, after encystment 

 they usually germinate directly to form a mycelium. Diplanetism, as 

 here understood, is the successive formation by a single fungus of two 

 different types of zoospores. The "primary" zoospore is pyriform or 

 "pip-shaped," with two apical flagella. It encysts after swarming and 

 gives rise to a "secondary" zoospore. This is reniform or grape-seed-like 

 and bears two laterally attached oppositely directed flagella. Further 

 encystments of the secondary zoospore have been observed, for example, 

 in Dictyuchus by Weston (1919) and in Achlya by Salvin (1940). When 

 they occur, the succeeding swarmer is always of the secondary type. 

 The number of such "repeated emergences" (Weston, op. cit.) or 

 "repetitional diplanetism" is, as Salvin has emphasized, no doubt often 

 influenced by the environmental conditions (lack of food, cool tem- 

 perature, aeration, and so on) prevailing in the medium. Coker and 

 Matthews (1937) apply the term "monocystic" to zoospores which 

 encyst once before germinating and "dicystic" to those which encyst 

 twice. 



Sexual Reproduction 



Sexual reproduction is accomplished in most genera by the formation 

 of oogonia and antheridia. One or several eggs (oospheres), usually 

 devoid of periplasm, are formed from the contents of the oogonium. 

 This structure, as well as the egg, is at first multinucleate. At maturity, 

 however, only a single nucleus is present in each egg. The antheridial 

 branches, at the tip of which the antheridia develop, may arise from the 

 same or different thalli. If only antheridia are formed by one thallus and 

 oogonia by another (Dictyuchus monosporus [J. N. Couch, 1926b], Ach- 

 lya bisexualis [Coker and A. B. Couch, in Coker, 1927]) the species is 

 heterothallic. If both types of sex organs are produced on a single 

 thallus it is homothallic. In some instances antheridia occur only on a 

 small percentage of oogonia or are completely lacking. When they are 

 not formed the eggs mature parthenogenetically (apogamously). Mo- 

 nandrous or polyandrous conditions occur. In some species well-devel- 

 oped fertilization tubes are formed by the antheridia. Whether or not 



