106 PHYCOMYCETEAE 



encysts after a while and may germinate by a slender hypha or re-escape 

 and re-encyst several times. In some cases the primary zoospore appar- 

 ently produces no flagella but creeps to the opening of the zoosporangium 

 and there encysts, escaping as a secondary zoospore some time later. In 

 still other genera the primary, nonflagellate, zoospores encyst within the 

 zoosporangium as separate round cells or become compacted into poly- 

 hedral cells by mutual pressure. In the latter case they form short exit 

 tubes that pierce the zoosporangial wall so that their contents escape 

 as secondary zoospores. In a few species the zoospores that encyst within 

 the zoosporangium do not escape at all as swimming cells but germinate 

 directly by germ tubes. 



It is customary to refer to the characteristic of forming primary 

 zoospores only, as monoplanetic and of the formation of two successive 

 types of zoospores as diplanetic. More correctly these two terms should 

 be monomorphic and dimorphic. Properly speaking monoplanetic means 

 wandering once, or with only one swimming stage, while diplanetic means 

 with two swimming stages. Since in Pythiopsis the primary type of 

 zoospore may swim and encyst several times and in Achlya, Dictyuchus, 

 and other genera, the secondary type of zoospore may also do the same 

 thing the customary terms are not used in their correct etymological 

 sense. 



Sexual reproduction occurs by the formation of male and female 

 gametangia, antherids and oogones, respectively. They may be terminal 

 on the main hyphae or on short lateral branches, rarely intercalary. The 

 oogones round up and become filled with multinuclear protoplasm and 

 then a cross wall is formed. Most of the nuclei degenerate leaving a much 

 reduced number. A single egg or several eggs may be formed, containing 

 a few nuclei each, of which only one remains sexually functional, the 

 others moving toward the outer wall and usually disintegrating. The 

 whole of the original protoplasm may be used up in the formation of 

 the egg or eggs or a portion of it may be left surrounding a single egg. 

 This is called the periplasm and may contain numerous nuclei which 

 eventually disintegrate. In the genus Araiospora the periplasm becomes 

 divided by radial sheets of protoplasm in which these nuclei lie and then 

 radial cell walls are formed so that the egg is surrounded by a layer of 

 cells. Probably in most cases the periplasm produces much of the exterior 

 ornamentation of the oospore wall. 



The antherids, depending upon the species of the fungus, may be 

 formed on separate plants from those forming the oogone (heterothallic 

 species), or both kinds of sexual organs may be produced on the same 

 mycelium (homothullic species). This distinction is not in all cases sharp 

 as there may be varying degrees of maleness or femaleness, as shown by 

 Bishop (1940). The antherids are terminal on long slender branches or on 



