CHYTRIDIALES 69 



conditions, with the purpose of ultimately reaching a suitable substra- 

 tum. In connection with these purely morphological observations on 

 vegetative "rejuvenation" by means of the fusion of two protoplasts 

 it is of interest to quote the remarks of Wager (1913) concerning the 

 so-called "double fusion" in Polyphagia (see also "Sexual Reproduc- 

 tion," p. 75): "The double fusion which takes place in the life-cycle 

 of Polyphagus is clearly bound up with this dual function of the nucleus, 

 the chromidial fusion in the zygote promoting vegetative growth, whilst 

 the nuclear fusion in the sporangium precedes the formation of the 

 spores. The importation of two nuclei into the zygote appears therefore 

 to be primarily for the purpose of increasing its vegetative activity. ..." 

 The emergence of a motile swarmer from a cyst has been observed 

 in Achlyogeton and, possibly, in Phlyctidium (Atkinson, 1894). 



Sexual Reproduction 



Although much has been learned about the chytrids, particularly 

 in recent years, the sexual reproduction of most species is still unknown; 

 in some chytrids, indeed, such reproduction is suspected to be entirely 

 lacking. It has been established, however, that sexuality when it does 

 exist is of an extremely varied character. In all well-authenticated oc- 

 currences the zygote becomes transformed into a resting spore, which 

 upon germination produces swarmers. From this fact it is probable 

 that the thalli are haploid and that, as in many of the algae, the sexually 

 formed resting spore represents the only diploid structure in the life 

 cycle. 



Among the inoperculate chytrids as in certain species of Olpiclium, 

 which are parasitic upon terrestrial phanerogams (Kusano, 1912, 1929) 

 the fusion of isogamous (like) planogametes (swimming gametes) has 

 been observed (Fig. 4 A, p. 72). After this fusion the biflagellate zy- 

 gote encysts on the surface of the host, penetrates it, and forms endo- 

 biotically a thick-walled resting spore. Among the alga-inhabiting mem- 

 bers of the Olpidiaceae no sexual process has as yet been convincingly 

 demonstrated. Observations on certain terrestrial chytrids belonging 

 to the genus Synchytrium (Curtis, 1921; Kusano, 1930a) show unques- 

 tionably that fusion of isoplanogametes precedes resting-spore for- 

 mation. There is strong but not conclusive evidence (Couch, 1931) that 



