CHYTRIDIALES 193 



rather than zoospores, the sori in these cases are here termed "com- 

 pound" to distinguish them from the similar "simple" ones that form 

 zoospores directly. 



As regards sexuality, the striking similarity of the swarmers from 

 compound sori to those in the "Cystogenes" species of Allomyces 

 (p. 625) was remarked by Canter and is re-emphasized here. It is en- 

 tirely conceivable that the secondary zoospores are, indeed, isogamous 

 gametes, such as are unquestionably formed in certain species of 

 Synchytrium. 



In commenting upon the phylogenetic implications of the flagellate 

 sporangial stage ("primary zoospores") Canter (/. c. p. 92) states: 



"Such a phase is normally present in M[icromycopsis] fischeri and 

 Endodesmidium formosum. The latter may be regarded as the most 

 primitive type, the primary non-swarming swarmers being formed 

 separately. Micromycopsis fischeri, however, differs from Endodesmid- 

 ium in the sorus being divided into sporangia each of which liberates 

 five or more of these primary zoospores, which after liberation behave 

 in a similar manner. Passing to Micromycopsis cristata this flagellate 

 sporangial stage is rarely recorded and more typically the behavior is 

 as in M. zygnaemicola and Micromyces spp., where there is apparently 

 only one sporangial stage liberating the normal chytridiaceous zoo- 

 spores, which correspond to the secondary zoospores of the former types. 



"It thus appears that during the course of development of these 

 organisms the primary non-swarming zoospore stage and the sub- 

 sequent formation of secondary sporangia is suppressed and we get, 

 as in most species of Synchytrium, a sorus of sporangia immediately 

 giving rise to chytridiaceous zoospores. S. fulgens Schroter may be 

 considered as representing the culmination of suppression in which 

 the sorus of sporangia is formed within the prosorus." 



Karling (1953) presents reasons for uniting Synchytrium and Micro- 

 myces, citing the similarities of the two as outlined by Couch (1931), 

 and merges them, erecting a subgenus Microsynchytrium for the dis- 

 position of Micromyces. He does not, however, include either of the 

 obviously closely related genera Endodesmidium or Micromycopsis (here 

 treated under Micromyces). From personal observations on both the 

 algal parasites and Synchytrium and from the observations of others, 



