132 PHOTOSYNTHESIS 



of the evaix)i-ation of water, 3, is the water lost by transpiration per unit 

 area and unit time Q; 592.6 calories is the latent heat of vaporization 

 of one gram of water at 20\ The internal work from evaporation of 

 water thus becomes 592.6 Q calories. The energy of photosynthesis 

 can be calculated from the quantity of carbon dioxide absorbed. The 

 heat of combustion of a hexose carbohydrate is 3760 calories per gram, 

 and one cc. of carbon dioxide corresponds to 0.001336 grams hexose 

 N.T.P., so that the conversion of one cc. of carbon dioxide into hexose 

 sugar corresponds to the absorption of 0.001336 X 3760 = 5.02 calories. 

 With the volume of carbon dioxide absorbed in photosynthesis per square 

 centimeter per minute designated by c, the energy absorbed in photo- 

 synthesis is expressed by 5.02 c per square cm. per minute. Alterations 

 in external conditions afifect mainly the thermal emissivity of the leaf. 

 Thus, the emissivity of a leaf in still air is doubled by an air current 

 of 44.2 meters per minute. This and other influences which affect the 

 emissivity of the leaf make Brown and Escombe's approach to this prob- 

 lem exceedingly complex and indicate how, under natural conditions, the 

 thermal relations of a leaf with its surroundings must change from 

 moment to moment. Brown and Escombe's method involves a number 

 of determinations which are exceedingly difhcult to make so that it has 

 found little application in further investigations. 



One example of Brown and Escombe's results will indicate the differ- 

 ence between the energy used in photosynthesis and that of transpiration : 



Energy used for photosynthesis 0.66 



" transpiration 48.39 



Total energy expended in internal work 49.05 



Solar energy transmitted by leaf "^r 'cc 



Energy loss by "thermal transmission" 19.55 



100.00 



External conditions such as temperature and humidity greatly influ- 

 ence the results so that wide variations in the various items are obtained. 

 We shall enter into these more full}- in the consideration of the Energy 

 Relations of Photosynthesis. We wish here to emphasize that the tem- 

 perature of the leaf and its surroundings greatly affect the performance 

 of the leaf as to its photosynthetic function. In plants well adapted for 

 transpiration the superfluous or excessive radiant energy absorbed by 

 the leaf can be readily dissipated through the evaporation of water. 

 Transpiration rates of 500 to 1000 cc. of water per sq. meter per hour 

 are not unusual; this would correspond to the dissipation of about 0.5 

 to 1.0 calory per sq. cm. per minute. With light intensity of about 1 

 calory per sq. cm. per minute incident on the leaf it can readily be seen 

 that the plant may be kept at the temperature of the surroundings or 

 absorb heat therefrom. In plants specially adapted to resist transpira- 

 tion as in the succulents, thermal emission of absorbed radiant energy 



