LIGHT 359 



Type 1. The fungus speculates well in darkness, much less or not 

 at all during the light period. Examples are Cephalothecium roseum 

 (145) and Sclerotinia fructicola (124). 



Type 2. The organism, e.g., Fusarium spp. (29, 124, 320), sporulates 

 poorly in the dark and forms a zone of spores after even a brief ex- 

 posure to light. 



It seems necessary to explain Type 1 reactions as the result of light 

 inhibition of sporulation. Type 2 responses are more difficult; we 

 may propose, however, that light inhibits mycelial growth — reports 

 of such inhibition are reviewed in Chapter 13 — and that stimulation 

 to sporulation results from the check upon vegetative growth. The 

 light effect in these fungi is exerted only upon actively growing hyphal 

 tips (29, 45, 264). 



Temperature alternations also induce the formation of zones of 

 reproductive structures (94, 123, 131, 250a, 320); it appears, especially 

 from the data of Ellis (94) and Hafiz (123), that the immediate stimu- 

 lus for sporulation is a check in growth. 



Zonation of reproductive structures in some fungi is dependent 

 neither upon light nor upon temperature fluctuations (40, 75, 146, 

 206); presumably nutritional factors are involved here, but they are 

 not understood. The pH of the medium has an effect (212, 240). 



Any unitary hypothesis to explain the action of light on sporulation 

 will have to take account of several facts, viz.: 



1. The indifference of many fungi to light stimuli. 



2. The high specificity of the response. Thus, the formation of 

 conidia of Choanephora encurbitarum is governed by light, but the 

 development of the sporangia is independent of light (18), and the 

 formation of zygospores is inhibited by light at low temperature (20). 



3. The fact that light may either stimulate or inhibit sporulation. 



4. The partial replacement of the light effect by high temperature 

 (29, 37, 185, 212), by periodic exposure to air (206), by treatment with 

 oxidizing agents (82), or by mechanical agitation (37, 275). 



5. The observation of Houston and Oswald (159) that Helmintho- 

 sporium gramineum forms spores in the dark if growing on host tissue 

 but requires light for sporulation on agar media. 



6. The extreme sensitivity of the response (29, 168), which suggests 

 that an enzymatic mechanism is triggered by the stimulus. 



7. The frequent efficacy of ultraviolet radiation, known to be in- 

 jurious to all organisms in sufficient dose. Sporulation of Alternaria 

 solani is induced by either ultraviolet or mechanical damage (74). 



