358 REPRODUCTION 



and cannot be ascribed to temperature effects (262). The effective 

 wavelength is still in doubt (88, 232), but it is probably longer than 

 the well-known "germicidal" wavelength of 254 m^. Ultraviolet 

 light, of course, in high doses represses both sporulation and growth. 



Most of the work on induction and stimulation by visible light has 

 been at best semi-quantitative. The available data on the action 

 spectrum indicate fairly consistently that the effective wavelengths are 

 in the blue region of the spectrum (37, 83, 117, 198, 248, 250a, 263, 

 277, 319). Morphogenetic action on Coprinus lagopus is maximal at 

 440-460 m^ (255), close to the peak of the action spectrum of photo- 

 tropism. 



The amount of light needed for inductive response is similarly un- 

 certain; it is much less than normal daylight exposure (105, 168). The 

 maximal effect of light on fruiting in Coprinus lagopus is exerted by 

 exposure for 5 seconds at 0.1 foot-candles intensity (196). 



So much attention has been focused on the positive effects of light 

 that the possibly more important inhibitory action may be overlooked. 

 Induction of conidiation in Choanephora cucubitarum requires a dark 

 as well as a light period (18), and light of wavelength below 580 m/A 

 appears to be inhibitory (80). Dark periods are required for maximum 

 production of ascospores by Diaporthe phaseolorum (286) and of coni- 

 diophores by Alternaria brassicae (312). For the Myxomycete Didym- 

 ium eunigripes, Leith (189, 190) presents evidence for both induc- 

 tion and inhibition of sporangial formation by different wavelengths. 

 Finally, the type of zonation in which spore production is greater dur- 

 ing dark periods (see below) is most easily interpreted as resulting 

 from a light inhibition of sporulation. 



The diurnal cycle of Pseudoperonospora humuli and other downy 

 mildew fungi in nature involves both light inhibition and light stimu- 

 lation, although effects on the host metabolism cannot be excluded 

 (315). Other diurnal cycles in nature (314, 316) and the frequently 

 observed nocturnal production of conidia (161, 162) may also result 

 from light inhibition, although the dominant factor in noctural forma- 

 tion of spores by Sclerospora spp. has been thought to be water (306, 

 307). 



Zonation — the formation of fruiting structures in concentric rings 

 on an agar surface or on the host plant — is a common phenomenon 

 and may best be considered at this point, since light is usually the 

 primary environmental factor. The occurrence of light-induced zona- 

 tion is reviewed by Hall (214) and by Hafiz (123). A simple explana- 

 tion is rendered difficult by the observation that two extreme types of 

 response to an alternation of light and dark periods are known, viz.: 



