LIGHT 357 



Sclerotium development is, of course, often a prelude to sexual repro- 

 duction and may be expected to be influenced by the same factors, 

 allowing for a higher nutritional level entailed by the larger mass (1, 

 249, 252, 290). The "sclerotia" of myxomycetes are purely resting 

 structures and appear to develop primarily as a response to desicca- 

 tion (110, 154, 166). 



The geotropism of large sporophores is an established fact of ob- 

 servation (27, 49, 50, 278), but its physiological correlates have not yet 

 been uncovered. 



Finally, in the list of omissions, purely genetic problems are not here 

 considered. These are, of course, most important to a thorough under- 

 standing of reproduction, but they lie outside the field of physiology. 



1. LIGHT 



The reactions of fungi to visible and ultraviolet light are of three 

 principal types: inductive effects, inhibition, and trophic responses. 

 Inductive effects include both absolute light requirements for initiation 

 or maturation of reproductive structures and quantitative responses, 

 i.e., an increase in the number of reproductive structures upon illumi- 

 nation. 



Light is absolutely required for the formation of various types of 

 reproductive organs in certain species. As examples we may cite the 

 requirement of light for the formation of sporophores in the Agari- 

 cales (226, 253), of perithecia (264), of apothecia (248, 283, 319), of 

 basidiocarps in the Gasteromycetes (298), of pycnidia (105, 183, 185), 

 of sporangia in the Mucorales (221), of conidia in the Mucorales (18) 

 and Fungi Imperfecti (97, 159), and of Myxomycete sporangia (117). 

 Smith (263) reviews still other examples. 



As might be expected, the effect of light is not always exerted at the 

 same point in these diverse fungi: to take two extremes, light is re- 

 quired for the first steps in sexual reproduction in Pyronema con- 

 fluens (248) and for all stages in Polyporus arcularius (110a), but only 

 for pileus development in some Agaricales (27, 106). Whether all 

 developmental stages of the Coprinus lagopus sporophore require light 

 is in dispute (196, 294). 



Light of the visible range often has the effect of increasing the num- 

 ber of reproductive structures formed (82, 89, 120, 124, 130, 261). 

 More striking effects of this type are induced by ultraviolet light in 

 certain ascomycetes and Fungi Imperfecti; both sexual and asexual 

 structures are affected (11, 66, 160, 198, 232, 262, 271, 272, 273, 271). 

 Ultraviolet effects are exerted in the cell, not in the medium (74, 271), 



