396 



SPORE GERMINATION 



100 



8 12 



Time, hours 



Figure 3. The rate of germination of mature (curve 1) and immature (curve 2) 

 uredospores of Phragmidium mucronatum. Redrawn from Cochrane (57), by per- 

 mission of the Cornell University Agricultural Experiment Station. 



when newly formed (42, 145, 240). Conidia of powdery mildew fungi 

 do not germinate until detached from the chain in which they are 

 formed, possibly because they are impermeable to gases until exposed 

 (33), or because the parent mycelium forms a diffusible germination 

 inhibitor (70); this is not, therefore, comparable to immaturity. 



Many phycomycetes form thick-walled resting spores; these are prob- 

 ably always the site of meiosis, and at least part of their apparent resting 

 period represents the time required for cytological processes (81). 

 Whether there is in addition a further period of purely physiological 

 dormancy is still uncertain. There can be no doubt that the environ- 

 ment, especially temperature, influences the length of the resting pe- 

 riod in some members of the Saprolegniales (246) and Peronosporales 

 (3, 17, 18). However, it is not yet possible to define the locus of this 

 effect. Many oospores, it should be noted, germinate immediately 

 after their formation (157, 321), as do also the resistant sporangia of 

 BJastocladiella sp. (45). 



The rest period of resistant sporangia of Allomyces spp. is shortened 

 dramatically by certain cultural conditions and by indoleacetic acid 

 (176, 177). In this and related genera the strength of the sporangium 

 wall may be one of the decisive factors in germination (176, 270) — 

 sporangia grown under conditions in which the wall is thin germinate 

 without a rest period. 



