898 BIOLOGICAL EFFECTS OF RADIATION 



21) estimated the amount of light necessary to produce a just perceptible 

 increase in the growth rate. They found that the increase in sensitivity 

 is a simple logarithmic function of the time that the fungi are kept in 

 the dark, but Wiechulla (194) found that this holds only for certain 

 energy values. He determined that the amount of light necessary to 

 produce a change in the growth rate is between ^^ and 3^^ mcs. A 

 certain amount of time elapses between the first reception of light and 

 stimulation of the growth rate. Castle (21) found that this reaction time 

 is compound, consisting of an exposure period and a latent period. In 

 the latent period he included both the period in which photochemical 

 action occurs and any "action time" necessary for the response. During 

 the latent period light is not necessary. The duration of the latent 

 period is constant for a particular intensity unless the duration of exposure 

 is reduced below a certain minimum. Below this minimum the reaction 

 time lengthens progressively as the time of exposure decreases. Castle 

 (22) also found that there may be a "dark-growth" response as well as a 

 "light-growth" response. Sporangiophores adapted to light respond 

 to a sudden darkening by a temporary decrease in the rate of elongation 

 after a latent period of several minutes. The reaction time of the 

 "dark-growth" response is compound like that of the "light-growth" 

 response. The rate of dark adaptation was found to be proportional 

 to the logarithm of the preceding light intensity and the amount of dark 

 adaptation which takes place before the response occurs is always 

 constant. 



SPORES 



Production. — A small ampunt of evidence has accumulated to indicate 

 that light may affect spore characteristics. It was noted by von Miiggen- 

 burg (128) that hyphae appearing on the cut half of an apple have 

 crescent-shaped conidia in the dark and club-shaped ones in the light. 

 Appel and Wollenweber (cf. Morris and Nutting, 127) also found that 

 when Fusarium cultures are grown in the dark the conidia are variously 

 shaped and unevenly septate. There are also reports of spores produced 

 by irradiation which would not normally occur. Ramsey and Bailey 

 (154) reported that conidia were produced in a strain of Fusarium coeru- 

 leum when the cultures were exposed to ultra-violet radiation transmitted 

 from a quartz-mercury-vapor arc and filtered through vita glass. This 

 strain had never previously produced conidia. They also found that 

 cultures of Fusarium argilaceum may produce only chlamydospores after 

 irradiation. Stevens (173, 174, 176, 178) exposed a large number of 

 fungus species to the full radiation of a quartz-mercury-vapor lamp. 

 He found, however, that ultra-violet may initiate the development of 

 reproductive structures in great numbers where they would not occur 

 without irradiation, but in no case was he able to initiate structures 



