SOME INFRA-RED EFFECTS ON GREEN PLANTS 



847 



excess energy received in this region is of special interest. Johnston (14) 

 observed that plants grown in chambers where infra-red radiation was 

 present were found to be more economical in their use of water than those 

 grown with visible only. This indicates that plants lose less water under 

 infra-red than under visible radiation. In order to obtain more data on 

 transpiration as related to both infra-red and visible energy a study was 

 made by Arthur and Stewart (3) of water losses from tobacco plants 

 grown under various conditions of temperature, humidity, and radiation 

 intensity with both the visible plus infra-red and infra-red regions only. 

 In this work 1000- or 1500-watt tungsten-filament lamps were used and 

 standard air-conditioning machin- 

 ery served to control temperature 

 and humidity. Coming's heat- 

 transmitting glass was used to 

 absorb the entire visible region. 

 Plants were potted in metal con- 

 tainers and were sealed in with a 

 paraffin mixture so that all water 

 loss was through the leaf and stem 

 surfaces of the plant. Using the 

 lamp without a filter (visible and 

 infra-red) within a temperature 

 range of 73° to 78°F., it was found 

 that doubling the total energy 

 increases the rate of water loss by 

 1.74. This was found to be inde- 

 pendent of humidity within a range 



of 50 to 88 per cent relative. At an energy level of 0.22-gm. cal./cm^./min. 

 the loss at this temperature was almost twice as great under infra-red 

 plus visible conditions as under infra-red alone. At a higher energy 

 level (0.65 to 0.72 gm. cal.) and at the same temperature the loss under 

 visible plus infra-red was 2.5 times that under infra-red alone. When the 

 temperature was increased to the range 98° to 100°F., the infra-red rate 

 of loss increased rapidly as compared with that under visible plus infra-red 

 so that the visible loss was only 1.3 times that of the infra-red. High 

 humidity (87 per cent relative) at this high temperature produced a 

 slight decrease in water loss in both infra-red and visible plus infra- 

 red conditions and produced great injury on most of the leaves. In 

 Fig. 2 are shown two tobacco plants which were used in this transpira- 

 tion study. The plant at the left shows the leaf injury which developed 

 after exposure to infra-red at high temperature and high humidity. The 

 plant at the right is the normal plant. The water loss in darkness was 

 increased first by the increase in temperature and again by the increase in 

 humidity. At the lower temperature range the amount lost in darkness 



Fig. 2. — Tobacco plants sealed in porce- 

 lain enamel cups for transpiration study. 

 Left, plant injured by combination of high 

 infra-red, high humidity, and high tempera- 

 ture. Right, normal plant. 



