BLASTOCLADIALES 633 



somes, namely 14. This is the only purely epigynous form and polyploids 

 of it bearing 28 and 50+ (56?) chromosomes are known. As for Kniep's 

 isolate, on which A. javanicus was erected, Wilson (1952) and Emerson 

 and Wilson (1954) conclude from its chromosome complement of 21 

 that it is a natural hybrid {A. arbuscula x A. macrogynus). The nuclear 

 situation in Cystogenes (see p. 625), and Wilson's findings and inter- 

 pretation of Brachyallomyces (p. 627) have already been discussed. 



SYSTEMATIC ACCOUNT 



BLASTOCLADIALES 



Microscopic fresh-water and terricolous saprophytic or parasitic fun- 

 gi; the thallus, usually walled, coenocytic, eucarpic, sometimes with 

 pseudosepta, true cross walls formed for the most part only to delimit 

 the reproductive organs, composed of a system of branched rhizoids 

 that anchor it to the substratum and either a single reproductive 

 rudiment or, more commonly, a basal cell which bears one or more 

 reproductive structures directly on its surface or on lobes or on extensive 

 nonseptate or pseudoseptate dichotomously, sympodially, or umbel- 

 lately branched hyphae, sterile setae present or absent, the walls some- 

 times giving a reaction for chitin, never for cellulose, protoplasm 

 variable in aspect, frequently alveolately or reticulately vacuolate; plant 

 sometimes differentiated into similar sporophyte and gametophyte 

 phases, the asexual plant bearing one or more inoperculate, uni- or multi- 

 porous thin-walled zoosporangia and thick-walled, usually punctate, 

 generally brownish resting spores borne singly within and usually 

 completely filling the terminal segments of the hyphae, the sexual plant 

 monoecious or dioecious, bearing one or more thin-walled inoperculate 

 uni- or multiporous gametangia; zoospores posteriorly uniflagellate, 

 with a conspicuous subtriangular nuclear cap, and sometimes a side 

 body, movement swimming or amoeboid, germinating directly and 

 bipolarly to form the asexual plant; resting spores upon germination 

 producing either (a) posteriorly uniflagellate planonts which give rise 

 directly to new asexual plants or to sexual plants, or (b) posteriorly bi- 

 flagellate planonts which immediately encyst, each of the cysts func- 

 tioning as a gametangium and forming endogenously four uniflagellate 



