78 AQUATIC PHYCOMYCETES 



that meiosis occurs at the first division of the diploid nucleus during 

 germination of the zygote, and sex genotypically determined, making 

 sporangia and conjugating thalli haplonts, and (2) on meiosis oc- 

 curring at germination of zygote but sex phenotypically segregated at 

 close of sporangial and zoosporic period. 



In Rhizidium windermerense, Canter (1950c) states that contact is 

 made between two encysted zoospores (sexual thalli) by means of fine 

 threads ("conjugation tubes") which they produce. These meet at their 

 tips and a swelling is formed at their junction which is the incipient 

 resting spore. The contents of both thalli pass into the developing 

 zygote, which increases in size, and a well-developed rhizoidal system 

 is then formed. Because of the small size of the conjugating thalli, 

 it is evident the zygotic plant functions vegetatively for a time. 



Among the operculate chytrids sexual reproduction has been re- 

 ported with certainty in Chytridium and Zygorhizidium of the Chytrid- 

 iaceae and in Zygochytrium of the Megachytriaceae. It may possibly 

 occur also in Tetrachytrium (see pp. 599, 601) of the latter family. 

 In Zygorhizidium (Lowenthal, 1905; Scherffel, 1925b; Canter, 1950c) 

 epibiotic thalli of different sizes are formed on the algal cell. Each of 

 the smaller thalli produces a tube which makes contact with the lateral 

 wall of a larger plant and fuses with it (Fig. 4 P, p. 72). The contents 

 of the small ("male") thallus are then conveyed into the larger ("fe- 

 male"), which subsequently becomes transformed into a thick-walled 

 resting spore (Fig. 4 Q). It is probable that upon germination this 

 resting structure produces zoospores. A cytological examination of 

 Zygorhizidium by Lowenthal revealed that the two gametangia were 

 uninucleate. The female possessed a relatively large centrally disposed 

 nucleus imbedded in the fatty cytoplasm. That of the male was dis- 

 tinctly smaller. After migration of all or nearly all of the male gametic 

 material a wall was formed which separated the tube from the zygote. 

 Plasmogamy took place at once, but the two nuclei remained separate 

 even after the formation of the thick wall of the resting spore. It is 

 possible that fusion occurs at germination. 



Certain aspects of the sexuality of Zygorhizidium are puzzling. From 

 the accounts of both Lowenthal and Scherffel two kinds of sporangia 

 are produced, large, "typical." ones about 15 [jl in diameter and "dwarf" 



