INTERNAL TEMPERATURE OF PLANTS 1213 



A controversy has arisen as to the relative importance of the two last- 

 named processes. Brown and Escombe (1905) considered transpiration 

 I)y far the most important of the heat-dissipating processes; and this 

 view was supported by the experiments of Shull(1919), Eaton and Belden 

 (1929), Arthur and Stewart (1933) and Clements (1934). Eaton and 

 Beldon (1929) pointed out that only transpiration can account for the fact 

 that sun-exposed leaves often are cooler than the air. 



Smith (1909) and Clum (1926), on the other hand, came to the con- 

 clusion that transpiration has a relatively small effect in preventing an ex- 

 cessive rise of temperature in strongly illuminated leaves. According to 

 Clum, the leaves in which transpiration is prevented by a layer of vaseline 

 are, in direct sunlight, only 2 or 3° C. warmer than similar, freely trans- 

 piring leaves. He therefore suggested that heat transfer is the main 

 factor responsible for keeping the temperature of sun-exposed leaves 

 within narrow, comfortable limits. Watson (1933, 1934) pointed out that 

 even if it were true that heat transfer normally dissipates less energy than 

 transpiration, its relative importance must increase with increasing dilTer- 

 ence in temperature between leaf and air. The low estimate of the energy 

 loss by heat transfer made by Brown and Escombe might have been due, 

 at least in part, to the neglect of infrared reradiation; an item, the impor- 

 tance of which was pointed out in particular by Curtis (1936 ^■2). 



The concept of transpiration as the all-important heat-dissipating and 

 temperature-regulating process in green leaves, which was for a time gener- 

 ally accepted in plant physiology, needs correction. The combined heat- 

 dissipating effect of convection currents and thermal radiation may some 

 time equal or even exceed that of transpiration. The relative importance 

 of the several heat-dissipating processes depends on the structure of the 

 leaf, and on atmospheric conditions, such as wind, exposure to cold surfaces 

 and the humidity of the air. Because of these special conditions (and also 

 because of the use of insufficiently reliable experimental methods), the data 

 given in the literature for the temperature of illuminated leaves vary widely. 



Table 31.1 contains the most important results. A discussion of the 

 experimental methods can be found, e. g., in Miller's Plant Physiology (1931) 

 and in an article by Seybold and Brambring (1933). 



The earliest investigations were carried out by means of mercury thermometers with 

 bulbs pressed against, or wrapped into, leaves. Later, small thermoelements were 

 substituted for thermometers; some authors used them to measure the surface tempera- 

 ture of the leaves, whereas others attempted to introduce them into the leaves to deter- 

 mine the internal temperature of the latter. 



Table 31.1 shows that Shreve (1919), Miller and Saunders (1923), 

 Eaton and Belden (1929), and Seybold and Brambring (1933) found only 



