772 BIOLOGICAL EFFECTS OF RADIATION 



light of a neon lamp. None of these workers has given sufficient data 

 concerning the light conditions operating to enable one properly to 

 evaluate the results with respect to quality of light. 



Special emphasis was given to determinations of fresh and dry 

 weight and chemical composition in the experiments of Popp (34^) 

 already referred to. In general, when wave-lengths shorter than 5290 A 

 were eliminated (house 5), the most marked differences occurred in 

 fresh weight, dry weight, and composition. Reductions in absolute 

 amounts of all substances were more marked than differences in the 

 relative percentages of constituents determined by analysis. A con- 

 siderable decrease in the amount of starch and total carbohydrates 

 was noted for most plants, and an increase in the amount of total nitro- 

 gen. In both stems and leaves of tobacco plants and in entire tops of 

 sunflower plants, total soluble nitrogen was highest in houses 4 and 5, 

 that is, in the absence of the blue- violet end of the spectrum. 



Almost without exception the fresh weight and the dry weight of 

 the plants as a whole, or of any part of the plants, were lowest in houses 

 4 and 5. With the exception of soy beans, the percentage of moisture 

 in the plants in these two houses was greater than that of the plants in 

 the other houses. The difference in soy beans was due to their greater 

 maturity. Carrots, petunias, sunflowers, and Coleus had the greatest 

 fresh weight in house 3 which eliminated only ultra-violet, while tobacco, 

 four-o'clocks, tomatoes, and Sudan grass had the greatest fresh weight 

 in the full-spectrum house — house 2. On the basis of dry weight, 

 the amount of growth made in all plants when the blue-violet end was 

 eliminated was decidedly less in spite of the fact that the total intensity 

 under these conditions was little different from that of the full-spectrum 

 house. 



Shirley (43) calculated, from Popp's data, the dry weights which 

 the plants would have attained if they had been grown under 100 per cent 

 light intensity, and from this the dry weight per unit intensity for the 

 various regions of the spectrum. From this he concluded that plants 

 grown under the complete solar spectrum had the advantage over the 

 others, that is, that the plants receiving the full spectrum produced a 

 greater dry weight per unit intensity than did the plants in any other 

 house. This calculation and conclusion, however, are based on the false 

 assumption that the rate of increase in dry weight would remain directly 

 proportional to intensity up to 100 per cent, or full intensity. There is 

 no justification for this assumption, since full daylight intensity is prob- 

 ably considerably above the maximum for dry-weight production in 

 plants. Shirley himself reports in the same paper, from his own intensity 

 studies, "At low light intensities the dry weight produced by the plants 

 studied is almost directly proportional to the intensity received up to 

 about 20 per cent of full summer sunlight. At higher intensities the 



