CHYTRIDIALES 111 



and Lund believe that they have strong, if indirect, evidence that Rhi- 

 zophydium planktonicum is not a saprophyte, merely attacking dead 

 or moribund cells, but a true parasite. 



Canter and Lund (1948) also point out that one can follow the nat- 

 ural sequence of an epidemic by an analysis of the stage of develop- 

 ment of each fungal cell, whether it is encysted zoospore, sporangium, 

 empty sporangium, or resting spore. Early in an epidemic, encysted 

 zoospores which are about to or are just penetrating the diatom host, 

 predominate. At this stage the alga is little affected and, even though 

 a high percentage of infection exists, the host population does not 

 decrease. When, for unknown reasons, the period of active parasite 

 multiplication comes to an end, the number of zoospores decreases, 

 but that of the sporangia resulting from the infective units produced 

 during multiplication continues to increase. In the final stage, when 

 very few zoospores or sporangia remain to accomplish further infection, 

 the number of empty sporangia reaches a maximum, whereas the count 

 of algal cells falls sharply. Although the influences governing the 

 formation of the parasite's resting spores are not known, the appear- 



Exptanation of Figure 11 



A-B. Zygorhizidium planktonicum Canter: A, young sporangium with rhi- 

 zoidal system; B, resting spore with small, conjugating male plant; on 

 Asterionella formosa. C—E. Zygorhizidium parvum Canter: C, sporangium; 

 D, resting spore with small conjugating male plant; E, another; on Kirchner- 

 iella. F-G. Zygorhizidium parallelosede Canter: F, immature resting spore 

 with small conjugating male plant; G, immature sporangium; on Ankistrodes- 

 mus sp. H. Podochytrium cornutum Sparrow, developing sporangia on Stepha- 

 nodiscus niagarae. I. Chytriomyces tabellariae (C. Schroter) Canter, mature 

 operculate sporangium on Tabellaria flocculosa. J. Rhizidium windermerense 

 Canter, empty sporangium showing broad rhizoidal axes; on Gemellicystis 

 neglecta. K. Rhizosiphon crass um Scherffel, nearly mature sporangium with 

 persistent cyst and germ tube of zoospore; on Anabaena sp. L. Rhizosiphon 

 anabaenae (Rodhe and Skuja) Canter, sporangium on heterocyst of Ana- 

 baena sphaerica.M. ? Rhizosiphon akinetum Canter, two sporangia, the young- 

 er with persistent zoospore cyst and germ tube; on akinetes of Anabaena 

 affinis var. intermedia. 



(A-B, Canter and Lund, 1953; C-E, Canter, 1950c; F-G, Canter, 1954; 

 H, Sparrow, 1951; /, Canter, 1949a; /, Canter, 1950c; K-L, Canter, 1951; 

 M, Canter, 1954) 



