INFLUENCE OF LIGHT ON GERMINATION 231 



for germination, as do also Peronospora parasitica and P. trifolio- 

 rz/777, which germinate by formation of germ tubes. 



INFLUENCE OF CARBON DIOXIDE ON GERMINATION 



The observations of Brown (1922) on volatile materials pro- 

 duced by apples and potatoes in storage led him to conclude that 

 volatile substances may have considerable influence in control 

 of organisms which produce decay. Ethyl acetate, a common 

 fruity ester evolved by apples, exerted either a stimulatory or an 

 inhibitory effect on the germination of Botrytis cinerea, depend- 

 ing upon the concentration. Volatile substances from leaves of 

 apple, Ruta, Eucalyptus and other aromatic plants increased germi- 

 nation of this fungus, whereas vapors from potato tubers and 

 onions were inhibitory. Platz, Durrell, and Howe (1934) con- 

 cluded that stimulation of germination of Ustilago zeae in the 

 presence of plant tissues is the result of increased carbon dioxide 

 tension, the carbon dioxide being generated by the plant tissues. 

 The presence of corn leaves in their germination chambers in- 

 creased the carbon dioxide content to 15%, the optimum for ger- 

 mination of the corn smut. Platz, Durrell, and Howe reported 

 that carbon dioxide acts by changing the reaction, and that 15% 

 carbon dioxide in the air produces hydrogen-ion concentrations 

 ranging from pH 4.9 to 5.6, which is optimum for U. zeae. 



INFLUENCE OF LIGHT ON GERMINATION 



Too little is known regarding the effect of radiations on the 

 germination of fungi, and published reports frequently contain 

 conflicting conclusions. De Bary [Doran (1922), p. 333] and 

 Farlow [Doran (1922), p. 333] state that light inhibits germina- 

 tion of spores of Oomycetes. Melhus (1915), on the other hand, 

 found that light does not inhibit germination of sporangia of 

 Phytophthora infestans. Doran (1922) noted that Alternaria 

 solani and conidia of Sclerotinia fructigena germinate equally well 

 in direct light, diffuse light, or darkness. Dillon-Weston (1932) 

 germinated urediniospores of Puccinia graminis avenae, P. graminis 

 tritici, and P. coronata under standardized Wratten green and blue 

 filters, which permit the passage of wavelengths of 450 to 555 m^, 



