CONTROLLED ENVIRONMENTAL CONDITIONS 329 



infrared) gives 2.5 times the transpiration that the infrared alone gives, 

 although the infrared represents over 90 per cent of the energy. At the 

 higher temperatures the complete spectrum gives only 1.3 times the trans- 

 piration induced by the infrared alone. The authors conclude that all 

 transpiration in the infrared is cuticular, for the stomates are completely 

 closed in darkness. They emphasize strongly the significance of transpira- 

 tion as a cooling factor which protects the leaf against overheating under 

 strong irradiation. The fact that leaves carefully covered with Vaseline on 

 both sides endure considerable irradiation without injury has been offered 

 as an argument against the importance of transpiration as a coolmg factor. 

 The authors dispose of this argument by showing that Vaseline-treated 

 leaves transpire rather freely when ii-radiated. They did find that transpira- 

 tion was effectively stopped by enclosing the leaf in a snug-fitting cellophane 

 envelope. Leaf temperatures of enclosed leaves at high radiation values 

 (1.6 gram calories) rose from 87° to 127° F (31° to 53° C) in an exposure 

 of four minutes, and the leaves were badly injured. 



Miller,^^' p"*^^ in his critical review of the literature on transpiration in 

 his text, says, "As previously mentioned, however, the average leaves are 

 cooled by transpiration rarely more than 2° to 5° C — a difference which, 

 so far as our knowledge of protoplasm goes, could be of no marked benefit 

 in preventing injurious effects." In this statement does not Miller confuse 

 the cooling effect of transpiration mth the number of degrees of tempera- 

 ture below that of the atmosphere that the leaf sometimes attains under 

 irradiation, at which time the cooling effect of transpiration is in full opera- 

 tion? Must we not stop transpiration entirely under irradiation to learn 

 how much the cooling effect of transpiration really is, just as Arthur and 

 Stewart have done? There is one adverse criticism of the Arthur-Stewart 

 determination: they fail to show that the tight-fitting cellophane bag did 

 not interfere with the loss of heat by thermal emissivity as well as by 

 vaporization of water. 



It seems to the writer that the plant physiologist who cannot see any 

 advantage in transpiration as a cooling factor for the green leaf under 

 irradiation ought to do as Bro^\^l and Escombe ^^ did much earlier, namely, 

 consider the leaf as a physical apparatus. The green leaf receives energy 

 by absorption of radiant energy; indeed under high illumination in absence 

 of the cooling processes of transpiration and thermal emissivity, it would 

 absorb enough energy to raise it to the thermal death point within a few 

 minutes. If the leaf has a lower temperature than the surrounding air it 

 also gains energy by thermal emissivity, and a slight amount of heat is also 

 added to the leaf by its o^vn respiration. It loses heat not only by vapori- 

 zation of water but by thermal emissivity as well, if the leaf has a higher 

 temperature than the surrounding air; also there is a slight use of energy 

 in C-synthesis. Brown and Escombe show that the great loss of energy by 

 the leaf under irradiation is by transpiration if the leaf has about the tem- 

 perature of the surrounding air; but the loss by thermal emissivity becomes 



