EFFECTS OF LIGHT INTEXSITY 737 



nutritional balance. For instance, seedlings from seeds very low in 

 protein but high in carbohydrates, when grown without nitrates, attain 

 greater size and weight in darkness than in light. When nitrates are 

 supplied, the reverse condition prevails. Nitrates were assimilated in 

 darkness and were converted into growth-promoting substances ; however, 

 the extent of this process was limited by the carbohydrate supply. 

 Nitrate utilization was always greater when the plants were provided 

 with both light and carbon dioxide. Shoot growth, at the expense of 

 root growth, was favored by a relatively low ratio of carbohydrates to 

 nitrogen, while with high ratios the relationship was reversed. This 

 held true regardless of whether the carbohydrates and nitrogen were 

 supplied by the seed or by photosynthesis and nitrates. A limited nitro- 

 gen supply caused seedlings to mature rapidly in light, while an abundant 

 nitrogen supply favored vegetative activity. In all cases, secondary 

 thickening of stems, roots, and cell walls was favored by light. Reserve 

 carbohydrates were accumulated more rapidly in seedlings with a limited 

 nitrogen supply. An increase in carbon dioxide content above that of 

 normal air tended to accentuate the effects observed from photosynthesis, 

 viz.; increased root growth, especially in plants from high-protein seeds 

 but without nitrates; increased shoot growth, when nitrates were sup- 

 plied, which was more pronounced with seedlings from low-protein seeds 

 when deprived of external nitrogen. Cow peas and soy beans grown in 

 light without carbon dioxide had roots of almost the same weight as plants 

 grown in darkness, while high- and low-protein corn seedlings had greater 

 root development in light, as did also sunflower and muskmelon seedlings. 

 Leaf development was greater in light than in darkness, even though 

 carbon dioxide was excluded in the former. Best development occurred 

 when both light and carbon dioxide were added. 



Mason (67) reports interesting studies on the effect of light on the 

 growth of the date palm. The leaves of this plant elongate rapidly 

 at night but scarcely at all during the day. Under bright sunlight no 

 growth could be detected. On cloudy days partial growth occurred. 

 Mason enclosed these leaves in light-tight boxes and found that the 

 growth could be started or stopped at will by closing the box or opening 

 it to direct sunlight. In order to demonstrate the complete dependence 

 of growth upon light, he placed electric lamps in the box and 

 illuminated the plants at various times during the day and night. At 

 first only small incandescent lamps were used. Normal growth occurred 

 under such conditions regardless of the time when the plants were 

 illuminated. When illumination from lamps of 1800 watts total capacity 

 was provided, growth was greatly inhibited but not entirely stopped. 

 When a mercury arc in a lead glass tube was used, growth was completely 

 inhibited. Evidently the failure of ordinary incandescent lamps to 

 inhibit growth was due to their low intensity in the blue and near the 



