360 REPRODUCTION 



A unified theory should take as its principal postulate that light is 

 inhibitory to one or more regulatory systems which in the normal 

 culture prevent or retard reproduction and favor continued vegetative 

 growth. That is to say, there is in truth no such thing as stimulation 

 by light in the induction of reproductive activity; instead, growth is 

 checked and the chain of events so initiated leads to reproduction il 

 other factors are not limiting. Alternative explanations are possible, 

 and several have held specifically that stimuli for reproduction do not 

 check growth; nevertheless, the weight of the evidence favors the hy- 

 pothesis presented. Confirmation must wait upon studies which deter- 

 mine accurately and quantitatively the effects of light on both repro- 

 duction and growth simultaneously. 



Phototropic responses are common in fungi; examples include the 

 orientation of basidiomycete sporophores (37, 278, 298) and of asci or 

 ascocarps (8, 41, 53). Gametic phototaxis has been reported (180). 

 The most dramatic and the most thoroughly investigated phototropic 

 reaction is that of the sporangiophores of Phycomyces spp. and Pilobo- 

 his spp., observed also in others of the Mucorales (96). 



Consideration of phototropism in Phycomyces spp. should begin with 

 the observation of Blaauw (32), confirmed by Castle (68), that a 

 sporangiophore illuminated equally from two sides exhibits a measur- 

 able temporary acceleration in vertical growth, the "light growth reac- 

 tion." A more recent study of this phenomenon (86) brings out sev- 

 eral important aspects: (1) the response probably involves no net 

 increase in total growth; (2) the effect of the light stimulus depends in 

 a predictable fashion on the level of illumination prior to the appli- 

 cation of the stimulus; (3) there is a latent period of about 2.5 seconds; 

 and (4) the magnitude of the increased growth is proportional to the 

 strength of the stimulus. This last observation is important in show- 

 ing that the light growth reaction is not an all-or-none, or threshold, 

 reaction. 



Granted that illumination can increase growth, the fact of photo- 

 tropism — unequal growth — requires some mechanism whereby the 

 sporangiophore wall distal to the light grows more rapidly than the 

 proximal wall. Castle (71, 72) argues that the size, shape, and refrac- 

 tive index of the sporangiophore combine in the "lens effect" to con- 

 centrate unilateral illumination in such a way that the distal region of 

 the protoplasm receives more light than the proximal. This is con- 

 sistent with the observation that sporangiophores immersed in paraf- 

 fin oil become negatively phototropic (14, 48), with the effect of ultra- 

 violet light (66a), and with the demonstration by Banbury (14) that 



