1114 BIOLOGICAL EFFECTS OF RADIATION 



merit when subsequently exposed to sunlight. Pearce and Streeter (27) 

 studied the development of color in apples under various filters using 

 sunlight as a source. They found that the region between X3600 and 

 4500 A was effective in producing color. The optimum they deter- 



o 



mined at or near X4100 A. They concluded that ordinary glass did not 

 inhibit color development since it transmitted this region well. They 

 found that apples were severely injured by an exposure twice daily over 

 a nine-day period to the ultra-violet emitted by a mercury-vapor lamp 

 (Alpine sun lamp) and concluded that, contrary to Magness' observa- 

 tions, the ultra-violet did not induce color formation. Arthur (1) 

 studied the development of pigment in Mcintosh apples under various 

 light sources and at various temperatures. Arc lamps and a special 

 incandescent filament lamp having a high ultra-violet output were found 

 effective in developing color on apples when these were held at an air 

 temperature of 2°C. After a continuous exposure to any of these light 

 sources for a five-day period the fruits developed necrotic areas on the 

 side exposed. It was shown that this injury could be produced by infra- 

 red alone. This has been discussed in greater detail in another section 

 of this work (Arthur, Paper XXV). Using glass and dyed cellophane 

 filters the most effective region for producing pigment was found to be 

 the visible blue-violet and the ultra-violet of sunlight which is not 

 transmitted by ordinary window glass, that is, the region between 

 X3130 and 2900 A. The ultra-violet of wave-length shorter than 2900 A, 

 emitted by a mercury-vapor arc in quartz, injured the epidermal cells in 

 a 30-min. exposure so that no pigment was formed when exposed subse- 

 quently to sunlight. The ideal light source for developing color on 

 apples was found to be one which had considerable energy in the blue- 



o 



violet and ultra-violet regions to the limit for sunlight, that is, to X2900 A. 

 Energy in the red and infra-red was not only unnecessary but injurious 

 to the fruit. The source nearest this ideal which has been found to 

 date is the 50-in. mercury-vapor arc in Uviol glass placed at a distance 

 of about 16 in. from the fruit and u.sed in conjunction with a Corex D 

 filter to decrease the ultra-violet output of wave-length shorter than 

 2900 A. The best air temperature was found to be 15°C. Apples 

 colored at a much slower rate or not at all at 3°C. and at 20°C. When 

 placed under the lamp in the manner described, the internal temperature 

 of the apples was found to be 21°C. Other light sources having a greater 

 output of energy would result in greater internal heating and would no 

 doubt require a much lower air temperature for the maximimi rate of 

 pigment formation. 



The rate of pigment production was greatest on fruit picked green on 

 August 25. This fruit was well colored after 40 hr. exposure. The rate 

 of color production fell off from August 25 in fruit either picked or loft 

 upon the tree. The green peel, when removed from apples and floated 



