328 GROWTH OF PLANTS 



for six months in light intensities scarcely sufficient to increase the dry- 

 weight. On the other hand, sunflower under this condition perishes within 

 two or three weeks. At lower light intensities up to 20 per cent of full 

 summer sunlight the increase in dry weight is proportional to the light 

 intensity. At higher intensities the increased dry weight does not keep 

 pace with the increased intensities. In shade plants the increase in dry 

 weight begins to fall behind the increased intensities at lower light levels 

 than in sun plants. With increase in light intensity go an increase in dry 

 matter of tops, ratio of dry matter of roots to shoots, density of growth, 

 strength of stem, and thickness of leaf. Leaf area and height of plant attain 

 the maximum at about 20 per cent of full summer sunlight. 



Chlorophyll concentration increases with decreasing light intensities 

 until the low light intensities threaten survival. Still further decreases in 

 intensity gave a decrease in chlorophyll concentration. Both of these find- 

 ings are confirmed by Guthrie ^^ in his study of many factors affecting the 

 synthesis and decomposition of chloroplast pigments, which is to be dis- 

 cussed later in this chapter. The time of maximum flowering and fruiting 

 was delayed by low intensities and flowering did not occur in intensities 

 of 8 per cent of full summer sunlight. The entire solar spectrum, including 

 the ultraviolet, was more effective than any portion of it and the blue 

 region is more effective than the red. 



In cases where the water supply is such as not to give waterlogging of 

 the soil or to reach ihe wilting coefficient, light intensity is usually the 

 limiting factor to vegetative growth under a forest canopy ; quality of light 

 is not a serious limiting factor. 



Light Measurement 



Shirley ^* developed a thermoelectric radiometer for ecological use on 

 land and in water. It is rugged, light and compact, simple in construction 

 and operation, gives readings within 7 seconds, and is sensitive to 0.1 per 

 cent of full summer sunlight. 



Effect of Radiation on Transpiration 



A study ^^ was made of the effect of artificial illumination of varying 

 intensity and quality, including in some cases only infrared, on the rate of 

 transpiration of tobacco plants. The experiments were run at two ranges 

 of temperature (73° to 78° F [23° to 26° C] and 98° to 100° F [37° to 38°C]) 

 and with 1000- watt lamps of both high efficiency (105-volt lamps on 

 120- volt lines) and low efficiency (120- volt lamps on 120-volt lines). A 

 considerable range of humidity was also used. At the lower temperatures 

 the radiant energy (visible and infrared) had to rise 2.3 times to double 

 transpiration. This held within the range of humidity 50 to 88 per cent. 

 At the higher temperatures rise in humidity lowered transpiration some- 

 what. At the lower temperatures the complete spectrum (visible and 



