ORDER CHYTRIDIALES 



57 



external cell becomes a thick-walled resting spore which produces its 

 swarm spores only after some time. Couch (1932) has shown for R. couchii 

 Sparrow that these resting spores arise as follows: A zoospore "comes to 

 rest on the host, penetrates the wall, and develops apparently just like a 

 sporangium. Later another spore comes and attaches itself to the larger 

 body. . . . The smaller cell discharges its entire contents into the larger. 

 . . . This now secretes around itself a thick wall and goes into the resting 

 state." In R. ovatum Couch the same author (1935) reports that the male 

 cell encysts on the algal host (Stigeodonium) and produces its rhizoids. 

 The female cell comes to rest upon the male cell and both cells enlarge, 

 the Avail between becoming perforated. The male nucleus passes into the 

 larger female cell and unites with its nucleus. This cell becomes a thick- 

 walled resting spore. (Fig. 15.) 



Rhizophlydis differs from Rhizophydium mainly in that the rhizoids 

 are numerous and arise from more than one point on the sporangial wall. 

 The four or five species are parasitic on fresh-water algae or saprophytic 

 on insect exuviae and vegetable debris. R. petersenii Sparrow can be 

 cultivated on cellophane or filter paper in pure water cultures. Karlingia 

 rosea (de Bary and Wor.) Johanson which differs in the production of an 

 operculum near the base of each of the several exit papillae was formerly 

 assigned to this genus under the name R. rosea. This species grows 

 saprophytically on organic matter in the soil, the large sporangia some- 

 times exceeding 0.1 mm. in diameter. These become rose-colored and give 

 a rosy tinge to the soil. This species also may be cultivated on cellophane 

 and various kinds of vegetable matter such as sterilized onion skin, grass 

 leaves, etc. The position of the operculum at the base of the exit papilla 

 instead of at its apex is of interest. 



Ohelidium grows upon the exuviae of aquatic insects. Its external body 

 bears a spine and a cup or funnel-like base, and the branching rhizoids 

 arise from a small apophysis. The zoospores escape from a lateral, ino- 

 perculate opening below the spine. No resting stage is known. It occurs 

 in Europe and Asia and Sparrow (1938) reports 0. mucronatum Now. 

 also from the United States. Siphonaria forms a round or ellipsoid 

 zoosporangium and strongly developed rhizoids. One species has spines 

 laterally and apically but does not possess the cup or funnel-like base of 

 the preceding genus. Sexual reproduction occurs by the union of two 

 thalli through a short or long slender tube, the female thallus becoming 

 a thick-walled resting spore. (Fig. 16.) 



The genus Polyphagus is usually placed in this family but its rela- 

 tionship to the other genera is doubtful. P. euglenae Now. is parasitic on 

 species of Euglena and other one-celled green organisms. Instead of being 

 an internal parasite it lives externally. According to Wager (1913), the 

 germinating zoospore sends out in various directions slender processes 



