XXV 

 SOME INFRA-RED EFFECTS ON GREEN PLANTS 



John M. Arthur 

 Boyce Thompson Institute for Plant Research, Yonkers, New York 



Energy absorption of a green leaf. Absorption by chlorophyll and the possibility of 

 photosynthesis. Transpiration in the infra-red. Injury from infra-red. References. 



In general more than 50 per cent of the total energy of sunlight is in 

 the infra-red region. This percentage varies owing to the selective 

 absorption of the atmosphere, presence of clouds, and the distance the 

 solar rays must travel through the atmosphere to reach the observer. In 

 artificial light sources the percentage output of infra-red is higher. This 

 value in case of the incandescent-filament lamp decreases with the 

 increasing efficiency of the lamp, that is, with an increasing filament 

 temperature. In the 1000-watt tungsten-filament lamp with an efficiency 

 of 20.5 lumens per watt and 1000 hr. normal life the infra-red output is 

 approximately 88 per cent of the total according to Forsythe and Watson 

 (11). The carbon arc has approximately 75 per cent of the total energy 

 output in the infra-red region according to the data of Coblentz and 

 others (9) and of Karrer (15). 



ENERGY ABSORPTION OF A GREEN LEAF 



Both sunlight and common artificial light sources have more than 

 50 per cent of the entire radiant-energy output in the infra-red. It is, 

 therefore, relevant to inquire into the known effects of this region on 

 plants. First, it is important to determine whether infra-red is absorbed 

 by green plant leaves, as energy must be absorbed in any region before 

 it can accomplish a result. Very little study has been made of the 

 infra-red absorption or transmission of leaves. Some reflection measure- 

 ments have been made by Coblentz (8). He found that reflection 

 decreased steadily from X8000 to 30,000 A. A red oak leaf reflected 

 18 per cent at X8000 and about 8 per cent at 30,000 A. Allowing the 

 leaf to dry overnight caused a decrease in reflection due, Coblentz 

 believes, to the loss of water which increases rapidly in absorption 

 beyond X14,000 A and decreases the amount of radiation which can 

 return by internal reflection. Leaves of chestnut, oak, ash, locust, and 

 pokeberry were all found to have a lower infra-red reflection than red oak. 



841 



