748 BIOLOGICAL EFFECTS OF RADIATION 



sunlight have no tendency to twme, such as bush and hma beans, (117) 

 and soy beans (75), may become twiners in reduced hght intensity. 



ZilUch (122) grew plants in the open and under lattice shades which 

 covered 3^, 3^, and ^ of the area. The corresponding light intensities 

 were 100, 75, 50, and 33 per cent, respectively. With temperature and 

 humidity conditions remaining essentially uniform, most of his plants 

 showed the usual morphological characteristics developed in the shade, 

 though weeds were less affected than cultivated plants. In some plants 

 flowering was delayed by shading, as was also the ripening of the fruit. 

 Fresh weight and the number of seeds per plant were often at a maximum 

 at one-fourth to one-half shade for weeds, but the size of seeds was largest 

 with three-fourths shade. Cultivated plants, peas, and barley showed a 

 decrease in yield of seeds and also in dry weight, with decrease in light 

 intensity. 



Shirley (94) grew a number of plants under different light intensities 

 using both artificial light and sunlight. Dry weight increased with 

 increasing light intensities from 30 to 700 foot-candles, an increase more 

 or less in direct proportionality to intensity. The plants grown under 

 artificial light were in the path of an atmosphere delivered from an 

 air-conditioning machine which rendered temperature and humidity 

 constant. Plants grown in the greenhouse under different shades 

 generally attained a maximum dry weight at the highest light intensity 

 available. Temperature and moisture conditions were, in general, 

 favorable for growth in the greenhouse. When the plants were grown in 

 shades out-of-doors where temperature conditions are not always favor- 

 able, the dry weight did not always show an increase with increased light. 

 For several plants there appeared to be an optimum value below that of 

 full sunlight. Later in the season, however, the same species showed 

 greatest dry weight in full sunlight. The percentage of dry matter always 

 increased with increasing light intensity, as also the ratio of the roots to 

 the tops. Height growth decreased with increasing light when the light 

 values were high, but at lower intensities height growth increased witli 

 light. Leaf structure tended to become more compact with increasing 

 light. Plants grown under from 1 to 20 per cent light developed only 

 one layer of palisade tissue, whereas those grown at 70 per cent had 

 two distinct layers. The cells increased in size from low light intensities 

 up to a certain optimum, at which maximum leaf area was developed, and 

 thereafter there was a decrease in size. 



Hoffman (49) compared plants grown in Vienna in a shaded court with 

 those on a roof garden. The light intensity in the court was only about 

 one-sixth of that on the roof garden. She made a careful study of leaf 

 area and leaf anatomy of the plants used. Leaf areas and width of 

 the epidermal cells were greater in the shade while leaf thickness, length 

 of epidermal cells, and the cross section of vascular bundles were greater 



