BLAST OCLADl ALES 81 



the phenomerxon as sexual and asexual phases of a single gener- 

 ation. 



In A. cy St genus the resistant sporangium cracks open on ger- 

 mination, and the content becomes a group of encysted cells at 

 the point of discharge [AlcCranie (1942)]. After a period semi- 

 amoeboid srametes, each with a sins^le flafjellum, emerg^e from 

 these cysts. They conjugate in pairs, forming biflagellate, motile 

 zygotes of a size like that of the uniflagellate zoospores from the 

 thin-\valled sporangia. Conjugation in this species is therefore 

 isos^amous. Presumably, the life cycle of A. vwnilifoiims is 

 similar to that of A. cystogemis. There is no alternation of sexual 

 and asexual phases as in ^. javaniciis or A. arbiisciila; instead, the 

 hyphae bear both thin-\yalled and resistant sporangia, the btter 

 s^erminating to produce isogamous gametes. 



In the remaining species, A. miomahis, there is no indication of 

 gamete or cyst formation, the zoospores from resistant sporangia 

 deyeloping directly into plants bearing thin-\yalled and thick- 

 \yalled sporangia. Sorgel (1937) has reported this occurrence in 

 A. arbitsciila as \yell, so perhaps this life cycle is a mere yariant; 

 at any rate three distinct life cycles are known in the genus. A 

 precise eyaluation of this situation is at present impossible in the 

 face of such complexities as the occasional presence of resistant 

 sporangia on the gamete-bearing plants [Sorgel (1937), Indoh 

 (1940), Emerson (1941)] and the discrepancies in the yarious 

 cytoloorical accounts of the Hfe history [Hatch (1938)]. 



Two additional genera of somewhat greater simplicity in thal- 

 lus structure haye recently been described; they are Blastocla- 

 diella [Matthews (1937)]' and Sphaerocladia [Stiiben (1939)]. 

 In these genera the thallus is much reduced, bearing^ but a single 

 sporangium, either a zoosporangium or a resistant sporangium. 

 Likewise the male and female gametangia are borne on separate 

 thalli, so that these genera are heterothallic, whereas Allomyces 

 is homothallic. The gametes themselyes are isogamous [Harder 

 and Sorgel (1938)]. 



The simpHcit)^ of thallus structure and sexuality in Blasto- 

 cladiella and Sphaerocladia suggests their relationship with the 

 chytrids. It is therefore possible to postulate an ascending series 

 from the chytrids through these isogamous forms to Allomyces, 



