CHYTRIDIALES 57 



with differences in size of the algal cell on which the thallus was devel- 

 oping. Though the largest sporangia were formed on the alga having 

 the largest cells (Rhizoclonium), the smallest were not formed on that 

 with the smallest cells {Oedogonium), but, rather, on Spirogyra. Evi- 

 dently in this instance the type of nutrient material or its availability 

 to the fungus was a factor. A similar lack of constant correlation 

 between the size of the sporangia and the cell of the substratum has 

 been observed in other chytrids, notably in Endochytrium operculatum 

 (Karling, 1937a). In this fungus mature almost spherical sporangia 

 ranged from 5 to 140 \x and pyriform ones from 5 by 7 to 60 by 150 [x. 

 Though range in size was to a certain extent proportional to the diam- 

 eter of the cell of the substratum, small sporangia might be found 

 in large cells or vice versa. E. operculatum also demonstrates the varia- 

 tion in the number of zoospores which may be produced by individual 

 sporangia of the same species. Fewer than twenty were found in small 

 sporangia, whereas thousands were estimated to be present in the larg- 

 est. Haskins (1939) has estimated that up to 70,000 zoospores are 

 formed in large sporangia of " Rhizophlyctis peter seriH." 



Discharge of the zoospores. — In inoperculate chytrids there are form- 

 ed one or more discharge papillae on the sporangium, some time 

 previous to the cleavage of the zoospores. In certain species these may 

 make their appearance when the thallus is relatively immature. In 

 Siphonaria variabilis, to cite an extreme example, the papilla is clearly 

 visible as one of the first-formed elements of the young thallus (Sparrow, 

 1937a). The degree of development of the papilla varies considerably 

 among the different species. In Obelidium mucronatum if a papilla is 

 present at all it must be very slightly developed, since it is not visible 

 at ordinary magnifications on the mature plant (Sparrow, 1938d). In 

 Rhizophydium sphaerotheca (Zopf, 1887), Rhizosiphon crassum (Scherffel, 

 1926a), and Phlyctochytrium chaetiferum (Karling, 1937c), on the other 

 hand, the papilla is a pronounced and prominent structure. In most 

 of the epibiotic chytrids it protrudes directly from the wall of the 

 sporangium. In others it is slightly elevated, and in Rhizophydium am- 

 pullaceum (Sorokin, 1874b) it is formed at the tip of a tubular prolon- 

 gation. The endophytic chytrids for the most part produce a more or 

 less well-developed tube which penetrates the wall of the substratum 



