196 FACTORS AFFECTING TRANSPIRATION 



Theoretically the temperature of a leaf may be regarded as conditioned 

 by four different influences: (i) thermal absorption, (2) thermal emission, 

 (3) internal endothermic (energy-storing) processes, such as photosynthesis 

 and transpiration, and (4) internal exothermic (energy-releasing) processes 

 such as respiration. The influence of this last factor upon leaf temperatures 

 is practically always negligible and will be disregarded. {Cf. Chap. XXIX 

 for examples in which internally produced heat of respiration does influence 

 the temperature of plant organs.) Similarly the quantity of energy trans- 

 formed in photosynthesis is relatively so small that it need not be considered 

 in evaluating the factors determining the temperature of leaves. 



Thermal emission refers to the loss of heat from a leaf by the processes of 

 conduction, convection, and radiation (Brown and Escombe, 1905). Thermal 

 absorption refers to the gain of energy by a leaf by these same physical 

 processes. 



Heat transmission which is brought about by intermolecular contacts is 

 known as (thermal) conduction. The greater the temperature difference 

 between a leaf and its environment the more rapidly conduction of heat will 

 occur from the leaf to the gases of the atmosphere, if its temperature is the 

 higher, or in the opposite direction if the temperature of the atmosphere is the 

 higher. 



Whenever loss of heat is occurring by conduction from a leaf to adjacent 

 gas molecules of the atmosphere convection currents (Chap. VII) are set up 

 in the atmosphere in the vicinity of the leaf. Cooler gas will displace the gas 

 in the vicinity of the leaf surfaces which has become warmed as a result of 

 thermal conduction from the leaf. This accelerates the rate at which conduc- 

 tion can occur from the leaf. 



Radiation is the transfer of radiant energy across space. The best known 

 tj^pes of radiant energy are light, infrared radiations ("heat waves") and 

 ultraviolet radiations. Radiant energy is commonly pictured as being propa- 

 gated across space in the form of undulatory waves (see Chap. XIX for dis- 

 cussion of another concept of the nature of radiant energy). Radiation 

 occurs from the molecules of one body to those of another only if the radiat- 

 ing body is at a higher temperature than the receiving body. Thus light, 

 infrared and ultraviolet radiation are transferred from the sun to the earth. 



metals, for example copper and constantan (an alloy of copper and nickel). A 

 difference of electrical potential is set up between two wires thus brought into 

 intimate contact. The magnitude of this electrical potential is very nearly di- 

 rectly proportional to the temperature of the thermocouple. In actual practice 

 two junctions are generally used, one being inserted into the leaf blade, the other 

 being kept at a standard temperature, usually 0° C. The difference in potential 

 between the two thermocouples is generally measured with a potentiometer. 



