846 BIOLOGICAL EFFECTS OF RADIATION 



Although the dry-weight production was greater for plants receiving 

 infra-red, these plants were not so green as those receiving only visible 

 light. This destruction of chlorophyll may, however, be ascribed to the 

 red region as well as to the infra-red. Johnston points out that in the 

 region of the strongest chlorophyll-absorption bands the plants grown 

 in the distribution including the infra-red received some three times 

 greater intensity of radiation. It is probable that this action on chloro- 

 phyll would occur in the region of greatest absorption. Guthrie (13) 

 has shown that plants grown without the blue-violet region of sunlight 

 also produce less chlorophyll. There is some indication, therefore, that 

 even the visible red, where blue-violet intensity is low, is injurious to 

 chlorophyll. Johnston has definitely shown that normal-appearing 

 tomato plants can be grown without the infra-red. The increase in the 

 dry weight of tissue produced under the infra-red transmitting filter is 

 no doubt due to the greater energy transmitted in the red region of the 

 visible spectrum. 



It follows from this discussion that, while there is an absorption of 

 chlorophyll in this region and the possibility of some starch formation, as 

 indicated by the work of Ursprung, photosynthesis in the infra-red is 

 negligible and during a considerable period of exposure of green plants 

 to this energy it contributes little or nothing to the dry weight of plant 

 tissue produced. 



TRANSPIRATION IN THE INFRA-RED 



The work of Brown and Escombe (4) indicates that by far the greatest 

 consumption of total energy received by the green leaf is in transpiration. 

 The amount used in this way, they found, amounted to as much as 60 per 

 cent of the total received. It is probable, therefore, that the evaporation 

 of water performs the important function of dissipating excess energy in 

 both visible and infra-red radiation. This is the method which the 

 animal uses to maintain a body temperature often several degrees below 

 that of the surrounding air. Such a cooling mechanism is even more 

 important in the case of plants, as these must remain fixed in the soil and 

 exposed to any fluctuating temperature and light intensity which the 

 vagaries of a given climate can produce, and at the same time hold the 

 temperature of the leaves and stems below the thermal death point. 



Older literature on the subject of plant transpiration, based on a study 

 of water losses from plants growing under field conditions, showed that 

 transpiration rose to a maximum each day and fell off to a very low value 

 at night. Stomata were found quite generally open in light and closed in 

 darkness. There was some indication, therefore, of a regulation of water 

 loss by stomatal movement. It has been pointed out already that 

 stomata do not open under infra-red. The possibility, therefore, of 

 plants losing sufficient water through cuticular transpiration to eliminate 



