Pileus formation inFavolus arculariusis induced by light, but no photoinduction occurred in young epileate stipes. The stipes usually had to attain a length of about 5 mm to be photosensitive. Synchronous pileus formation could be induced by exposure to light using epileate stipes which had been preincubated in darkness for 48 to 72 hr. The pileus primordium formed about 24 hr after the start of illumination, however, continuous illumination was not necessary to produce this effect. A dark period given between 1 and 8 hr after the start of illumination did not retard pileus formation. The photoinduction of pileus formation involved two light-requiring processes, one occurring during the first hour (the first light process) and the other from the 8th to the 24th hr (the second light process). The photoresponse in the first light process was saturated with 5 lux of light, but a light intensity below 1 lux was essentially ineffective. On the other hand, the reaction in the second light process could be started by less than 2 lux, and was accelerated by increasing the light intensities up to about 150 lux. Further increases in light intensity did not improve any significant effect.
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