1212 THE TEMPERATURE FACTOR CHAP. 31 



same plant, e. g., the chloroplasts, which absorb Ught, may be warmer than 

 the epidermis of the leaf, which is cooled by evaporation. 



Fifty years ago, Timiriazev (1903) thought that local heating of chloroplasts by 

 light absorption may be sufficient for thermodynamic reversal of combustion processes, 

 and that ttiis may explain photosynthesis. However, it has since become clear that the 

 mechanism of activation of chemical reactions by light is different from that of activa- 

 tion by heat. The former is based on the formation of electronically excited molecules, 

 or of free radicals, wliile the latter depends on the production of molecules with liigh 

 kinetic energies. 



Unlike warm-blooded animals, plants have no mechanism for automatic 

 regulation of oxidation and transpiration processes, capal)le of maintaining 

 the organism at a constant temperature. However, they, too, continuously 

 produce heat by exothermal metabolic processes, and a plant enclosed in a 

 dark vessel full of saturated water vapor (to prevent both transpiration and 

 photosynthesis) warms itself up until the internal evolution of heat is com- 

 pensated by increased conduction losses to the gas and by thermal radia- 

 tion losses to the walls. 



If plants are allowed to absorb hght, and to transpire, the above three 

 items of energy exchange are augmented by two additional terms: energy 

 supply by light absorption and energy losses by transpiration. A certain 

 fraction of the former is lost, from the point of view of heat balance, by 

 conversion into chemical energy (photosynthesis). 



Brown and Escombe (1905) and Brown and Wilson (1905) made the 

 first attempt to calculate the stationary temperature of plants by esti- 

 mating all these energy terms. They concluded that the stationary tem- 

 perature of ordinary leaves, enclosed in a dark space saturated with water 

 vapor, can rise only a few hundredths of a degree above the temperature of 

 the medium. The difference may be somewhat larger in plant organs in 

 which the volume/surface ratio is small, such as fruits, stalks and succulent 

 leaves. 



In strongly illuminated leaves, the conversion of light into heat is by far 

 the largest item on the credit side of the heat balance, this physical process 

 outweighing by far the chemical heat production by respiration and other 

 exothermal metabolic processes. On the debit side, too, the two physical 

 energy-consuming processes, transpiration and heat transfer (the latter 

 we consider to include thermal conduction, convection and radiation losses), 

 account for a much larger amount of energy than the chemical process of 

 photosynthesis. Photosynthesis is most important in weak light (of the 

 order of 1000 lux), where it may consume 30% or more of absorbed Ught en- 

 ergy; but in direct sunlight this proportion does not exceed 5% (c/. chapter 

 28). Consequently, the internal temperature of leaves exposed to the 

 sun can be attributed (in the first approximation) to the balance of three 

 physical processes: light absorption, transpiration and heat transfer. 



