320 PHOTOSYNTHESIS 



reversed sign, equals the heat of formation. The heat of combustion of 

 glucose was taken as 3760 calories per gram, and as 1 cc. of carbon diox- 

 ide corresponds to 0.001336 gram of a hexose, this quantity of carbon 

 dioxide absorbed in photosynthesis would correspond to an absorption 

 of 0.001336 X 3760 = 5.02 calories. The energy used in photosynthe- 

 sis expressed in calories per square centimeter per minute is 5.02 c. The 

 total internal work to be ascribed to the vaporization of water and to 

 photosynthesis may be expressed by 



592.6 0+ 5.02 c = V^ -\-w. 



The term "work" is perhaps not altogether fortunate and is to be taken 

 as a generalized expression. 



When the energy used in vaporization of water and in photosynthesis 

 (W -\- zv) equals the total solar radiation absorbed by the leaf, Ra, there 

 is a steady thermal state. This is rarely the case ; however, Ra is usually 

 either greater or less than (W + zv) . When Ra is greater than (W + w) 

 the excess of radiant energy absorbed will raise the temperature of the 

 leaf above that of its surroundings. The thermal steady state can then 

 be represented by 



Ra= (W + w) -l-r 



and when Ra is less than ( W -(- zv) heat must be absorbed from the sur- 

 roundings and the steady state becomes 



Ra + r = W + w 



While Ra and (W-)-ttf) are values determined experimentally, r is a 

 difference value. The actual difference of temperature between the leaf 

 and its surroundings can be determined from r if the thermal emissivity of 

 the leaf is known. Brown and Wilson ** determined the thermal emissivity 

 of the leaves of different plants. In Table 34 are given some of these 

 values. 



TABLE 34 



Thermal Emissivity of Leaves ok Various .Species of Plants under Still Air 



Conditions. (Brown and Wilson.) 



Thermal Emissivity 



in Calories per 



Minute per sq. cm. 



of Surface for 1° C. 



Plant Temperature Excess 



Liriodendron tulipifera 0.01194 



0.01274 



Helianthus multiflorus 0.01499 



Tropeolum majus 0.01427 



Tilia europea 0.01598 



Brown and Wilson also determined the thermal emissivity of leaves 

 in moving air. Lhider these conditions the rate of cooling is consider- 



Brown and Wilson, Proc. Roy. Soc, 76 B, 122 (1905). 



e 



