CHYTRIDIALES 109 



edly with the course of the epidemic. At the start of the autumn growth 

 period the cell count for each colony rose from 6.4 to over 8 and then 

 declined slightly, but only to the still high level of 7.5. By November 

 14, however, it had fallen very low, to 4.6. With the end of the Rhi- 

 zophydium epidemic and the onset of flood conditions this average 

 rose rapidly. Thus, here and also in the epidemics of 1945, 1947, and 

 1948, a rise in fungus count was followed by a fall not only in number 

 of Asterionella cells but in cell-per-colony average. This decline lasted 

 until the end of the period or as long as the lake level remained low. 

 Throughout the period the concentrations of nitrates, phosphates, and 

 silica were suitable for active growth of Asterionella. In 1947 a similar 

 epidemic occurred in the same period and under closely similar con- 

 ditions, but it was of shorter duration and the maximum infection 

 (38 per cent of the Asterionella cells) was higher., 



Although similar epidemics were recorded on other parts of the lake 

 and at other times in the three years observed, none occurred during 

 the main part of the spring-growth period possibly because of the host's 

 fast growth. Furthermore, only once was there an instance of an epi- 

 demic after the Asterionella maximum and even this one took place 

 before the typical catastrophic fall in numbers of live cells began. Canter 



Explanation of Figure 10 



A. Rhizophydium fragilariae Canter, mature and discharged sporangia on 

 a filament of Fragilaria sp. B. Rhizophydium ephippium Canter, discharging 

 sporangium on Stylosphaeridium stipitatum. C. Rhizophydium hyalobryonis 

 Canter, sporangium on Hyalobryon mucicola. D-E. Rhizophydium difficile 

 Canter: D, immature sporangium; E, resting spore with mucilaginous en- 

 velopment ; both on Staurastrum faculiferum. F. Rhizophydium megarrhizum 

 Sparrow, immature and discharging sporangium on Oscillatoria sp. (del. 

 Ingold). G. Rhizophydium uniguttulatum Canter, discharging sporangium in 

 Gemellicystis neglecta. H-I. Rhizophydium planktonieum Canter: H, thalli 

 and discharged sporangia; /, mature sporangium, both on Asterionella sp. 

 J-K. Rhizophydium oblongum Canter: J, a mature sporangium; K, another, 

 showing rhizoidal system, both on Dinobryon sp. L. Rhizophydium fulgens 

 Canter, mature and discharged sporangium on Gemellicystis neglecta. M. 

 Chytridium versatile ScherfTel, sporangium on Fragilaria crotonensis. 



(A-B, Canter, 1950c; C, Canter, 1951; D-E, Canter, 1954; F, Canter 

 and Lund, 1951 ; G, Canter, 1954; H-I, Canter and Lund, 1948; J-K, Canter, 

 1954; L, Canter, 1951 ; M, Canter, 1950c). 



