ORDER CHYTRIDIALES 



65 



Fig. 19. Chytridiales, Family Physodermataceae. Physoderma zeae-maydis Shaw. 

 (A) Resting sporangium, side view. (B) Resting sporangium germinating to form 

 zoosporangium. (C) Young ephemeral zoosporangia. (D) Emptied ephemeral zoo- 

 sporangia, one with beginning of proliferation. (E) Swarm spore from ephemeral 

 zoosporangium. (F) Swarm spore from resting spore sporangium. (Courtesy, Sparrow: 

 Am. J. Botany, 34(2):94-97.) 



the genera are kept distinct, especially since phytopathological literature 

 maintains this separation. They differ from the foregoing polycentric 

 forms in that the primary infection produces an external sporangium with 

 rhizoids penetrating the epidermal cells of the host. This rather flattened, 

 or even slipper-shaped, zoosporangium opens without an operculum and 

 sets free numerous swarm cells. Successive sporangia may arise by 

 proliferation within the empty walls. The polycentric stage is originated 

 in a manner characteristic of the Cladochytriaceae, probably by infection 

 from these swarm cells 



In Physoderma macular e Wallr., Clinton (1902) has shown that the 

 germinating zoospore sends rhizoids into the host cell (submerged leaf of 

 Alisma) and enlarges externally to become a zoosporangium much in the 

 manner of Rhizophydium. After the zoospores escape through an exit 

 papilla a second zoosporangium may be formed within the empty wall of 

 the first, and so on two to four times. Other zoospores settling on older 

 leaves, or perhaps upon leaves not permanently emersed, send in a fine 

 filament which enlarges to become a storage cell (or "Sammelzelle") and 

 produces haustorial processes or fine filaments which may penetrate to 

 other cells and there in their turn produce similar cells, and so on. From 

 each such cell there may arise a bud which eventually becomes larger 

 than the original cell. This is a resting sporangium and is thick-walled 

 and somewhat flattened on one side. After a period of rest it enlarges and 



