316 PHYSIOLOGY OF THE FUNGI 



at the time of irradiation had an effect on the number of perithecia formed. 

 Colonies 4 days old when irradiated produced perithecia, which were most 

 abundant on mycelium 1 day old at the time of irradiation. Irradiation 

 of colonies 12 days old led to the formation of but few perithecia. No 

 evidence was obtained that irradiation of the medium alone had any 

 effect on perithecium formation. A species of Coniothyrium which 

 formed pj^cnidia only after the cultures were very old was stimulated to 

 produce pycnidia within 3 days after irradiation. This work of Stevens 

 is apparently the first which demonstrated that ultraviolet radiation 

 stimulated sporulation by fungi. 



Spore production by Macrosporium tomato and Fusarium cepae was 

 greatly increased by the proper exposure to ultraviolet radiation (Ramsey 

 and Bailey, 1930). A 12- to 15-fold increase in the numbers of spores 

 produced by these two species was obtained by the optimum exposure. 

 These investigators also showed that irradiation of the medium before 

 inoculation had no subsequent effect on sporulation by these two fungi. 

 The range of wave lengths which stimulated the most abundant sporula- 

 tion was found to be 2,300 to 2,800 A. Smith (1935) points out that 

 many workers have neglected the precaution of controlling the tempera- 

 ture of cultures during irradiation. She found it necessary to control 

 the temperature of the cultures of Fusarium eumartii in order to separate 

 the effects of increased temperature and ultraviolet radiation. 



Ultraviolet radiation stimulated or depressed sporulation of Diaporthe 

 phaseolorum var. hatatatis depending on the medium used (Timnick 

 et al., 1951). Neither stromata nor perithecia were formed on casein 

 hydrolysate-glucose medium, unless the cultures were irradiated. Cul- 

 tures grown on potato-glucose agar produced stromata and long-beaked 

 perithecia without irradiation. Irradiation of cultures on potato-glucose 

 medium resulted in the formation of fewer and smaller short-beaked 

 perithecia. Although the mode of action of ultraviolet radiation in 

 stimulating sporulation is unknown, long exposures are known to be 

 lethal. We may assume that even short exposures injure or kill some of 

 the exposed cells. Perhaps some substance is thereby released which 

 stimulates sporulation. The presence of such a substance in the potato- 

 glucose medium might explain why irradiation was not necessary for the 

 production of perithecia by D. phaseolorum var. hatatatis on this medium 



Aeration. Although the fungi are aerobic organisms, the amount of 

 free oxygen that they need to carry out their life processes varies from 

 fungus to fungus. The amount of oxygen required is less for growth than 

 for reproduction. The aquatic fungi would be expected to grow and 

 reproduce in a more limited supply of oxygen than terrestial forms. 

 While many aquatic Phycomycetes produce their spores under water, a 

 large number of fungi fail to fruit until some aerial mycelium has been 



