TEMPERATURE EFFECTS ON TRANSPIRATION 197 



In like manner a warm stove loses heat by invisible infrared radiation to its 

 environment. Radiation occurring from leaves is also in the infrared range 

 of wave lengths. 



The temperature of a leaf exposed to direct sunlight or strong artificial 

 illumination almost invariably exceeds that of the atmosphere. Thermal 

 absorption — in this case direct absorption of solar radiations — proceeds under 

 such conditions at a rapid rate. A portion of the absorbed radiant energy is 

 dissipated (usually) by transpiration, and a portion is lost from the leaf by 

 thermal emission. As already shown in Chap. XII transpiration alone is never 

 adequate to dispose of the energy absorbed if the leaves are exposed to strong 

 insolation. Thermal emission under these conditions will involve loss of heat 

 both by radiation and by conduction. In general the greater the excess of leaf 

 temperature over that of the atmosphere the larger the proportion of heat loss 

 by thermal emission as contrasted with transpiration (Watson, 1934). 



On a cloudy day the temperature of leaves seldom deviates very greatly 

 from that of the enveloping atmosphere. Heat exchanges between a leaf and 

 its environment under such conditions probably occur principally by conduc- 

 tion. Similarly at night leaf temperatures usually do not deviate greatly from 

 those of the surrounding atmosphere. 



Leaves sometimes have a lower temperature than the surrounding atmos- 

 phere. This is often true, for example, of leaves which are transpiring fairly 

 rapidly but which are not exposed to direct sunlight. Such leaves are often 

 cooler by several degrees centigrade than the atmosphere. It is also possible 

 that leaves sometimes lose heat energ\' by direct radiation to their environ- 

 ment rapidly enough to result in a lowering of their temperature below that of 

 the atmosphere (Curtis, 1936a). This is especially likely to occur on clear 

 nights when the vapor pressure of the atmosphere is low. These conditions 

 favor direct radiation from the leaves to the sky, i.e. to relatively cold gases, 

 especially carbon dioxide and water-vapor. 



The temperature of leaves usually fluctuates from minute to minute, 

 especially during daylight hours. Minor fluctuations are due largely to 

 shifts in wind velocity. The thermal emissivity of leaves exposed to direct 

 sunlight is invariably increased by an increase in wind velocity. When the 

 sunlight is intermittent, frequent changes in leaf temperatures also occur. 

 Each time the sun is obscured by a cloud a sudden drop in leaf temperature 

 usually takes place. Contrariwise, when the sun emerges from behind a cloud 

 there is usually a distinct and abrupt rise in leaf temperature. 



The factors controlling the internal temperatures of the other organs of 

 plants are in general similar to those influencing leaf temperatures. The 

 temperatures of fleshy leaves, fruits, tree trunks, succulent stems such as those 



