CHYTRIDIALES 11 



walled spiny resting spore (Fig. 4 Y), and fusion of the nuclei follows. 

 The curious sexual reproduction in Rhizoclosmatium and Siphonaria 

 involves passage of gametic material through anastomosed rhizoids. 

 Siphonaria is, however, the only one in which the passage of gametic 

 material and subsequent fusion have actually been seen (Petersen, 1903; 

 Sparrow, 1937a; Karling, 1945d). Because of the tenuousness of the 

 parts in members of other genera observations on such processes 

 are difficult even at high magnifications. In Siphonaria variabilis two 

 (sometimes three) thalli of unequal size make contact by the fusion of 

 elements of their rhizoidal systems. The contents of the smaller plant 

 migrate into the larger one (Fig. 4 V), which then lays down a thick 

 wall around the reproductive rudiment and becomes a resting spore 

 (Fig. 4 W). Many times active contact of the two thalli appears to be 

 accomplished solely by the efforts of the receptive thallus. The rhizoids 

 of this plant seem to seek out the other thallus and, often, instead of 

 fusing with some element of the vegetative system, make direct contact 

 with the main body. Whether or not the two types of thalli develop 

 from swarmers formed in the same sporangium is not known. It has 

 been frequently observed in Siphonaria, however, that sporangia of 

 two very different size classes are produced. 



In Siphonaria petersenii (Karling, 1945d), the male thallus is recog- 

 nizable by a single apical spine which usually forms on it (Fig. 26 D, 

 p. 422). Variation in the degree of development of the male's rhizoidal 

 system is evident. Sometimes rhizoids are absent and the male body 

 either makes direct contact with the female or is joined to it by a broad 

 tube. Similar variation in the spatial relations of the two conjugants 

 occurs in S. sparrowii. In this species, however, as in S. variabilis, no 

 structural differences are found in the two types of thalli. Fusion of 

 the protoplasts through anastomosed rhizoids or a long conjugation 

 tube is very common in S. sparrowii. Furthermore, as many as six male 

 thalli may be connected to one resting spore. Since fusions in Sipho- 

 naria always resulted in resting-spore formation. Karling {op. cit.) 

 considers them to be syngamous rather than merely vegetative even 

 though karyogamy has not as yet been observed. Karling presents 

 two theoretical life-cycle diagrams based (1) on the very probable 

 supposition that karyogamy takes place shortly after plasmogamy and 



