EFFECTS OF LIGHT INTENSITY 747 



both the tops and the beans; Ukewise, the yield of seed was considerably 

 reduced. 



The effect on plants of shading to 44, 18, and 15 per cent of full sur;- 

 light in New Hampshire was studied by Gourley (41) and Gourley and 

 Nightingale (42). Shaded leaves of the Carman peach tree averaged 

 69 per cent, and Elberta 59 per cent larger in area than unshaded leaves. 

 Oldenburg apple-tree leaves from the shade were over three times as large 

 as those developed outside the shade. Similar changes were observed 

 in strawberry, asters, lettuce, buckwheat, geranium, and eggplant. The 

 typical thin leaves with single layers of palisade cells and loosely organized 

 parenchyma were produced in shade. The internodes were longer, but 

 branches and spurs fewer, and root systems were restricted. 



With seedlings of Douglas fir and Engelmann spruce grown in Utah 

 at 7000 ft. elevation, under lath screens which provided }4, 3^, and % 

 shade, Korstian (55) obtained best growth and survival with )^ shade. 

 The osmotic concentration of the sap as measured by freezing-point 

 depression was 16.7 atmospheres under half shade as compared with 

 19.1 in full sunlight. The palisade tissue and spongy parenchyma were 

 only weakly developed in the shade, and the needles were much thinner 

 and less in cross-sectional area. 



Apple trees shaded to about 5 per cent light intensity are described 

 by Auchter et al. (6) to have larger and thinner leaves, long, spindly 

 branches which tended to curl and twist at the ends, loose leaf structure, 

 and early leaf fall. Where one-half of the tree was shaded and the 

 other not, the shaded half developed Hke trees completely shaded, and 

 the unshaded half developed like normal trees, except that during the 

 second season the shaded half produced branches shorter than those on 

 the completely shaded trees. He suggested that shading may be the 

 primary cause of the poor development and dying of lower and inner 

 branches of dense trees. 



Coniferous seedlings under different intensities of artificial illumina- 

 tion increased in size with increasing light intensity (Bates, 7; Bates and 

 Roeser, 8). The minimum amount of light necessary to maintain the 

 seedlings for 11 months varied from about 1 to 6 per cent of total sunlight. 

 In making his measurements, however. Bates compared the total radia- 

 tion delivered by the lamp with the total radiation of the sun. Since 

 only about 15 per cent of the radiation given by an electric lamp lies in 

 the visible region, as contrasted with approximately 50 per cent for sun- 

 light, these values are undoubtedly high. 



Popp (75) grew soy beans with different degrees of shading. Initial 

 stem elongation was greatest with decreasing light intensity, but the 

 final height was greater in moderate light intensity. The best plants 

 were produced under full sunlight. Leguminous plants which under full 



