750 BIOLOGICAL EFFECTS OF RADIATION 



The effects of light intensity on the structure of plants has received 

 a great deal of study. The differences between shade leaves and sun 

 leaves has been pointed out by many investigators. This work is 

 reviewed in the general textbooks of botany and plant physiology and 

 especially by Goebel (40), Lundeg&rdh (64), MacDougal (66), and Haber- 

 landt (44). In practically all this work no quantitative studies have been 

 made. Studies of the structure of plants grown under known light 

 conditions have been made by Hasselbring (47), Popp (75), Pfeiffer (74), 

 Hoffmann (49), and Shirley (94). Practically all these investigators, 

 however, have used the sun as a source of radiation. The features 

 observed by these workers are those described in the introduction and 

 text of this section. 



LIGHT INTENSITY AND MINERAL NUTRITION 



Stutzer and Goy (103) made analyses of the nitrogen, ash, and nicotine 

 content of tobacco grown under shades transmitting about 70 and about 

 6 per cent light. Yield of leaves was higher in the 70 per cent light. 

 Total nitrogen, ash, and potassium increased with decreasing light, while 

 nicotine decreased. 



Studying the chemical composition of blue-stem wheat which had 

 ripened under the shade of 16-ounce duck and comparing the analyses 

 with those of plants ripened in full sunlight, Thatcher and Watkins (107) 

 found the shaded kernels higher in protein and lower in starch. The 

 ash and moisture content of the kernels seemed not to be affected. Fur- 

 ther studies by Thatcher (106) of wheat, oats, barley, field peas, and 

 emmer gave similar results. In this case the plants were subjected to 

 shading for a longer period. Shading increased the percentage of mineral 

 and nitrogenous matter, but decreased the percentage of dry matter and 

 stored carbohydrates. 



Kraybill (56) found that shading apple trees caused an increase in the 

 percentage of total nitrogen, a decrease in carbohydrates, and an increase 

 in dry matter. This is opposite to the findings of other workers on her- 

 baceous plants but agrees with some of the writer's unpublished results 

 on shaded conifers. Vinson (117) found a higher ratio of nitrogen to 

 carbohydrates in shaded plants. Auchter et al. (6) found that shading 

 apple trees to 5 per cent of normal sunlight caused a lower starch content 

 of terminals and spurs, evidently a result of reduced photosynthesis. 

 The shaded trees and shaded halves of trees were lower in percentages 

 of dry matter and generally lower in carbohydrates but higher in nitrogen. 



Surveying the influence of light intensity and light quality upon the 

 assimilation of nitrates in wheat, Tottingham and Lowsma (109) deter- 

 mined that those plants exposed to only 1}^ hr. of direct sunlight on clear 

 days, as contrasted with plants kept in the greenhouse, had more protein 



