362 REPRODUCTION 



men ted "ocellus" at the base of the subsporangium is believed to be 

 the photosensitive area, but it should be noted that photosensitivity 

 is apparent before the formation of the subsporangium (222) and that 

 other phototropic forms do not have an "ocellus." 



Other light responses of Pilobolus spp. may be briefly noted. Light 

 induces the formation of the trophocyst, or sporangial primordium, of 

 some isolates; Page (221) raises the possibility of a flavin receptor in 

 this reaction. Second, alternations of light and dark, or of tempera- 

 ture, set up a periodic rhythm in the development of sporangia (175, 

 196). This rhythm, once established, persists for two or three 24-hour 

 cycles after the removal of the stimulus (254, 291). Finally, the forcible 

 discharge of the sporangium is hastened by light (55). 



Light has other effects on the reproductive apparatus, notably a 

 positive effect on the size and septation of macrospores of Fusarium 

 spp. (130, 264). Pigment production is also enhanced by light (189, 

 264), but attempts to connect this phenomenon with induction of 

 sporangia in Didymium eunigripes were unsuccessful (189). 



Exposure to light, unless appropriate filters and cooling devices are 

 used, means an increase in temperature. Thus, the apparent attrac- 

 tion of myxomycete pseudoplasmodia to light is a temperature effect; 

 the plasmodium moves toward a source of heat along a temperature 

 gradient as small as 0.05°C per centimeter (35). Similarly, a negative 

 phototropic response of Phycomyces sp. to intense light was shown by 

 Castle (70) to be a heat effect. Many reports on light effects have 

 not included quantitative determinations of the ambient temperature; 

 possibly some of the apparent complexities may disappear when such 

 data become available. 



2. TEMPERATURE 



Only two generalizations can be formulated with respect to tempera- 

 ture and sporulation. First, the temperature range permitting repro- 

 duction is usually narrower than that permitting growth. Second, each 

 spore form has its own temperature optimum, which may or may not 

 coincide with that of vegetative growth or of other spore forms of the 

 same species. 



The relative narrowness of the temperature limits of reproduction 

 is virtually universal in the fungi. Examples include both sexual and 

 asexual spores in the phycomycetes (174) and ascomycetes (148, 194, 

 281), sporophores of basidiomycetes (5, 182), and spores of the Fungi 

 Imperfecti (2, 204, 287). Usually the differences in range are not 



