EFFECTS OF LIGHT INTENSITY 753 



shade, rospoctively. This increased evaporation occurred in spite of the 

 fact that the leaf surface in each case was greater in the shade. Similar 

 responses were found to hold for the castor bean (73). Maximow 

 and Lebedincev (68) found that light stimulated the development of 

 water-conducting vessels. 



Lachenmeier (57) tested water intake and transpiration of Veronica 

 Beccabunga, Hieracium pilosella, and Mijosotis palustris under constant 

 conditions of temperature, humidity, and light. With light of 8 lux, 

 transpiration was but little higher than in darkness; with 100 lux some- 

 what higher; while with 30,000 lux a decided increase occurred. Veronica 

 more than doubled the rate in darkness. Furthermore, with continuous 

 illumination the transpiration rate increased up to a maximum at the 

 end of 1 hour and remained constant thereafter. For Hieracium at 

 30,000 lux the transpiration rate immediately increased threefold but 

 later fell off to about 2 to 2^ times the rate in darkness. Other plants, 

 including excised shoots, showed similar correlation between light 

 intensity and transpiration. 



Chodat and Kann (20) noticed that the rate of transpiration of alpine 

 plants decreased at high light intensities. Chodat (19) attributes this 

 decrease to a change in cellular permeability under the influence of the 

 high content of blue and ultra-violet light at high altitudes. Since this 

 explanation is at variance with the results of Priestley and Lepeschkin, 

 who found that light increased the permeability of protoplasm, it should 

 not be accepted without direct experimental proof. Possibly some other 

 factor is responsible, such as the closing of stomata, due to a deficit in 

 water supply. Mittmeyer (69) showed that the daily cuticular trans- 

 piration of xerophytes followed light intensity more closely than evapora- 

 tion. In xerophytes cuticular transpiration accounts for two-thirds to 

 three-fourths of the total, and in mesophytes one-half or more. 



A careful study of the effect of radiation on transpiration was recently 

 conducted by Arthur and Stewart (1). Temperature, humidity, and 

 light conditions w^ere carefully maintained. The loss of water in 12 hr. 

 per square inch of leaf surface at 73° to 78°F. and 50 per cent relative 

 humidity was: in darkness, 0.05 to 0.07 gm.; at a radiation intensity of 

 0.28 gm. cal./cm. Vmin., 0.60 to 0.63 gm. ; at 0.65 gm. cal./cm.Vmin., 1.00 

 to 1.29 gm. At 88 per cent relative humidity the transpiration was: in 

 darkness, 0.03 to 0.05 gm.; at 0.28 gm. cal./cm.Vmin., 0.56 gm.; at 

 0.65 gm. cal./cm.Vmin., 0.93 to 1.17 gm. Increasing the temperature to 

 98° to 100°F. caused a loss at 0.65 gm. cal./cm.Vmin. of 2.67-2.82 gm. at 

 68 per cent relative humidity. There was, therefore, a very definite 

 dependence of transpiration upon radiation intensity. 



Sunlight may at times be so high as to cause injury to plants, partic- 

 ularly young seedlings. This is generally due to excessive heat or to 

 excessive transpiration. Such injuries have been reported in coniferous 



