MORPHOGENY REACTIONS 125 



effects upon reproduction. Unfortunately many of the results, 

 especially those with visible light, are not reproducible because 

 account has not been taken of three correlated factors: (a) quan- 

 tity or intensity of light, (b) quality of light or wavelength of 

 radiations, and (c) duration of exposure or total radiation. Even 

 in some studies employing monochromators intensity was not 

 kept constant or was not measured when quality was changed. 

 It is all too apparent, furthermore, that such other factors as age 

 of the culture, hydrogen-ion concentration, temperature, and 

 screening effects from the culture media and from the massing 

 of hyphae must be considered in experimentation on the reactiv- 

 ity of fungi to radiation. Since this has not always been done, 

 a barrage of criticisms may be levelled against the experimental 

 procedures and consequently against the conclusions. In the 

 account that follows some of the extensive publications in this 

 field are brought together, but many meritorious ones have been 

 excluded from the discussion. Although certain generalizations 

 may be drawn from these publications, none appears to rest on 

 too secure a basis, and the only broad statement warranted ap- 

 pears to be that much further work is needed. Such studies 

 should be attempted, however, only by mycologists well grounded 

 in the physical principles of light and other radiations. Much of 

 value on the effects of radiation will be found in the summary 

 by Smith (1936). 



MORPHOGENIC REACTIONS 



A survey of studies to determine whether light is required for 

 the development of fructifications by fungi reveals that light 

 exerts a profound influence upon some species but that others 

 appear to be completely indifferent to it. Several early mycolo- 

 gists noted that in the complete absence of light the stipes of 

 some species of Hymenomycetes are longer than normal, that 

 other species, normally sessile, develop stipes, and that other 

 species, normally pileate, branch in clavarioid fashion. Coprinus 

 stercorarius, in darkness, has been observed to grow stipes 2 to 

 3 ft long. Buller (1906) noted that Poly poms squamosw, kept 

 in the dark, developed coal-black stag-horn-shaped sterile stro- 

 mata, 15 cm tall and having white tips, in place of normal stalked 

 pilei. The formation of the pileus is entirely conditioned by the 



