CHYTRIDIALES 79 



ones 4-5.4 u. in diameter. The latter are considered to be "male" thalli 

 which either have not succeeded in making contact with a receptive 

 plant after forming a conjugation tube or have at once been transformed 

 into sporangia. A further distinction between such sporangia and typical 

 ones, according to Scherffel, is the lack of rhizoids on the endobiotic 

 knob and the failure to produce bending of the host cell. In some 

 specimens at least the "dwarfs" attained nearly the size of the usual 

 sporangia (see Lowenthal, 1905: figs. 17 and 26). Presumably both 

 nonsexual and sexual individuals have been derived from ordinary 

 zoospores which came from sporangia of typical size. The question 

 arises, then, whether environmental conditions determine the subse- 

 quent nature of the thallus formed by the zoospore (nonsexual sporan- 

 gium or gametangium), or whether there are inherent differences in 

 the swarmers which are produced at the germination of the resting 

 spore. In favor of the former interpretation is the fact (observed by 

 both Lowenthal and Scherffel) that dwarf thalli on which a conjugation 

 tube has already developed may function as sporangia. Since both non- 

 sexual and sexual structures are formed at the same time and since 

 a cytological examination shows that sporangia and receptive thalli 

 of the same size differ in their nuclear condition, the former being multi- 

 nucleate, the latter uninucleate, the evidence is not conclusive. 



A type of sexuality which resembles that seen in certain species of 

 Rhizophydium was found in Chytridium sexuale by Koch (1951). A 

 swarmer comes to rest on the host wall (Vaucheria), encysts, and intro- 

 duces into the alga a germ tube which expands into an apophysis. The 

 epibiotic spore cyst then enlarges and elongates and, while this is yet 

 relatively undeveloped, a motile male gamete encysts on the thallus 

 surface and empties its contents through a short germ tube into the 

 young plant. The combined protoplasts migrate into the endobiotic 

 apophysis, which increases in size and is converted into a resting spore. 

 No structural differences were observed between the zoospores which 

 formed receptive thalli and those which functioned as males. Indeed, 

 whether a thallus became a zoosporangium or a receptive plant seemed 

 entirely dependent upon whether or not engagement with a male cell 

 took place; however, Koch indicated there was no proof of this. 

 The authenticity of Sorokin's (1874a) account of the sexual process 



