868 BIOLOGICAL EFFECTS OF RADIATION 



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3130 A there were also present some of the shorter lethal rays, if the 

 mercury arc was the source of radiation. In the use of artificial radiation 

 they had a controlled light source with considerable intensity in the ultra- 

 violet region. They used cellophane filters especially developed for the 

 experiment (Withrow, 122) the transmission curves of which showed 

 sharp cut-offs at the desired wave-lengths, a quality said not to be 

 possessed by many glass filters which have been used. Their results 



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indicated to them that "the removal of the 2900 to 3100 A ultra-violet 

 region is detrimental to the growth of tomato and Coleus plants, and that 

 the inclusion of a small amount of lethal radiation of shorter wave-length 



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than 2900 A is sufficient to mask the beneficial effect of the 2900 to 



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3100 A region." This was indicated by the greater height and growth 

 rate, greater internodal length, greater stem diameter, greater number and 

 average area of leaves, and greater fresh and dry weights of those plants 



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which received radiation in the region 2900 to 3100 A, with no shorter 

 ultra-violet present. The final statement in the paper, however, reads 

 that "because of the limited number of plants used and the inadequate 

 growth conditions, especially with regard to intensity of illumination, 

 these results are offered simply as preliminary data, indicating the possible 



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growth-promoting action of the 2900 to 3100 A ultra-violet region." 



Unfortunately the total light intensity was only about 30 foot-candles, 

 an intensity so low that it resulted in etiolation of the plants. None of 

 the plants, therefore, was normal, except the so-called controls which 

 were kept in the diffused light of a window sill with a southern exposure. 

 These "controls" were not used in any of the measurements, and justly 

 so since they were under conditions totally different from those of all 

 other plants. While the cellophane filters used in this experiment were 

 said to be stable, they were also said to show slow deterioration under 

 intense irradiation. No further indication is given of the degree of 

 stability of the filters. No measurements of total energy under the 

 various screens are given. Such measurements would have been of 

 particular value since the illumination intensity was probably below the 

 minimum for normal growth and hence small differences in the different 

 compartments might have caused pronounced effects on the plants. 



The data recorded represent for tomato comparisons of measurements 

 on 2, 6, and 14 plants, respectively, and the 14 plants under supposedly 

 identical conditions show average measurements per pot to be more 

 variable than average measurements of plants under different conditions. 

 The tomato data are obviously of little value. The Coleus data were 

 obtained by picking out three "representative" plants from each of five 

 pots of indefinite numbers of unthinned plants under each condition. 

 The photographs of these plants show decided crowding in the pots, 

 which is very unfortunate, particularly when the illumination intensity 

 was extremely low. It is difficult to understand why so many plants 



