848 BIOLOGICAL EFFECTS OF RADIATION 



was almost negligible. Under the most favorable conditions for water 

 loss (high temperature, high radiation, and low humidity) the water 

 loss per square inch of leaf area (calculated on one surface per leaf only) 

 reached a maximum value of 2.82 gm. in 12 hr. under visible plus infra-red, 

 and 2.09 gm. under infra-red only. If this high rate were maintained in 

 large tobacco plants with 3000 in.^ of surface the water loss in 12 hr. for 

 visible plus infra-red might amount to 8 1. as compared with 6 1. for infra- 

 red only. This study was made with plants reduced mainly to three 

 leaves. It is probable that the rate would fall off considerably as more 

 leaves were added since not all leaves would be well exposed to the light. 



The percentage of energy received which was eliminated again by 

 evaporation of water in the infra-red region was calculated as approxi- 

 mately 72.5 per cent. This calculation is based upon an incident energy 

 value of 0.65 gm. cal./cm.^/min. and a loss of 20 per cent by reflection 

 and a second loss of 30 per cent by transmission using the figure 2.09 gm. 

 of water lost per square inch of leaf surface in 12 hr. and assuming the 

 value 585.3 as the latent heat of vaporization of water at 20°C. 



It is seen from the foregoing study that, as the thermal death point of 

 leaves is approached through increases of temperature and radiation 

 intensity in either visible or infra-red, transpiration increases rapidly, 

 and cuticular transpiration in the infra-red becomes a large part of the 

 total transpiration (stomatal plus cuticular) under visible-radiation 

 conditions. The fact that the cuticular transpiration rate does not quite 

 overtake stomatal plus cuticular rate probably accounts for the more 

 severe injury obtained under infra-red at high temperature as compared 

 with visible plus infra-red. 



Curtis (10) and Clum (6, 7) have challenged the idea which has grown 

 among plant physiologists that transpiration cools plants by the evapora- 

 tion of water. Curtis admits that the evaporation of great quantities 

 of water is an efficient method of eliminating large amounts of excess 

 energy received, but states that no evidence has been obtained which 

 shows that stopping transpiration actually produces a rise in the tempera- 

 ture of plant leaves. Clum found that when plant leaves were coated 

 with vaselin, only a slight rise in temperature could be observed when 

 thermocouples were inserted into the leaf lamina. Arthur and Stewart (3) 

 observed that when tobacco leaves which had been thoroughly coated 

 with vaselin on both sides were placed in a cellophane envelope, water 

 vapor escaped from the leaf through the vaselin and condensed on the 

 inner surfaces of the cellophane envelope. When the leaf was placed in 

 such an envelope and exposed to a total energy value of 1.6 gm. cal.- 

 /cm.2/min. using a 1000-watt lamp, the leaf temperature rose from 87° to 

 127°F. in an exposure of 4 min. The cellophane envelope they concluded 

 effectually prevented all cooling of the leaf by the evaporation of water, 

 as the water evaporated was again condensed, resulting in low-energy 



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