BIOLOGICAL EFFECTS 413 



of strongly fermentative species have a very low oxygen requirement 

 and those of less strongly fermentative species a higher one; all, how- 

 ever, probably require some oxygen. In this study of the Mucorales 

 (304), it was also found that the swelling of the spore of Mucor spp. can 

 occur without oxygen, i.e., oxygen is required only at a later stage in 

 development. 



Pure oxygen is at least slowly toxic to spores of Streptomyces scabies 

 (242); in short-term experiments inhibition of germination in Botrytis 

 cinerea by high oxygen pressure is measurable but small (37). 



It appears, to conclude, that spore germination usually requires at 

 least some oxygen, but that the atmospheric level is more than ade- 

 quate. In nature, it seems unlikely that spore germination is prevented 

 by oxygen deficits except under such extreme conditions as flooded soil 

 or heavily polluted water. 



11. CARBON DIOXIDE AND GERMINATION 



From the observations cited in Chapter 7, that carbon dioxide often 

 accelerates the early growth of fungi, we might guess that spore germi- 

 nation is accelerated by carbon dioxide. This proves to be true in 

 Aspergillus niger (223). Effects of high levels of carbon dioxide in 

 increasing chlamydospore germination have been interpreted, however, 

 as arising primarily from the effect of the dissolved gas on the acidity 

 of the medium (218). 



At high concentrations, carbon dioxide is partially or even com- 

 pletely inhibitory to spore germination. Brown (37) determined that 

 the spores of seven different fungi, if sown in water, are completely 

 inhibited by an atmosphere of 20-30 per cent carbon dioxide; sensitiv- 

 ity is markedly less if the spores are sown in a plant decoction. Other 

 reports agree that carbon dioxide is inhibitory and that the level at 

 which germination is affected may be low (180, 266) or high (139, 169). 

 Inhibition at concentrations of 1 per cent or so is probably specific; 

 effects of the higher concentrations could be indirect, e.g., on acidity 

 or on oxygen supply. 



12. BIOLOGICAL EFFECTS ON SPORE GERMINATION 



It is commonly observed that excessive crowding of spores on a sur- 

 face results in reduced germination; the phenomenon is known in many 

 fungi (27, 37, 70, 73, 148, 160, 172) and may be described by the general 

 term self-inhibition. In Neurospora crassa, crowding of conidia re- 

 duces the germination rate, but not total germination (240). These 



