1 68 THE LOSS OF WATER FROM PLANTS . 



between 50 and 60° C. it should not require more than two minutes to heat 

 the leaves of most plants to a lethal temperature. Actual measurements of 

 leaf temperatures, however, show that they seldom even approach their ther- 

 mal death points. Leaf temperatures usually do not exceed atmospheric 

 temperatures by more than a few degrees centigrade. Evidently some efficient 

 energy-dissipating mechanism is at work which prevents the accumulation of 

 heat energy in leaves. 



Since transpiration is an energy-consuming process it has often been as- 

 sumed that it is in the evaporation of water from the leaves that most of the 

 energy absorbed by them is dissipated. We might well inquire, therefore, 

 regarding the possible efficiency of transpiration as an energy-dissipating proc- 

 ess. The evaporation of a gram of water at 20° C. requires 584 g.-cal. For 

 the dissipation of 0.65 g.-cal. of heat, therefore, the evaporation of o.ooii g. 

 ^0.65' 



of water per square centimeter of leaf surface per minute is required. 



This is equivalent to 6.6 g. (o.OOil X lOO X 60) of water per square deci- 

 meter of leaf surface per hour, a rate of transpiration which is seldom if ever 

 attained by plants for any sustained period of time. Evidently transpiration, 

 even when occurring at its maximum rate, can at the most account for a 

 dissipation of only part of the radiant energy which is absorbed by leaves in 

 direct sunlight. 



The fact that transpiration is often inadequate in direct sunlight to account 

 for the dissipation of all the absorbed radiant energy leads naturally to the 

 question of whether it is at all essential for this process. Observations have 

 shown that leaves in which the transpiration rate is greatly reduced, as for 

 example those in which the stomates are plugged with vaseline, leaves in a 

 wilted condition, or the leaves of plants in xeric habitats during dry seasons, 

 in which the occurrence of any appreciable amount of transpiration is pre- 

 cluded by the lack of soil water, seldom have temperatures which are anywhere 

 near the thermal death point, even when exposed to direct sunlight. Although 

 transpiration often accounts for the dissipation of some or even most of the 

 absorbed radiation, in so doing it apparently plays no essential role since 

 absorbed radiation can be dissipated by purely physical means. As soon as 

 the temperature of a leaf exceeds that of the surrounding atmosphere, it will 

 lose heat to the atmosphere in the same way that any other object heated 

 above its environmental temperature does, i.e. by one or more of the purely 

 physical processes of conduction, radiation, and convection. The term thermal 

 emissivity is frequently applied to this physical loss of heat energy by leaves 

 and other objects. As will be shown in Chap. XIV, the thermal emissivity 



