330 PHYSIOLOGY 



low barometer may coincide with low humidity and therefore intense 

 light, the excessive evaporation often becomes a powerful factor in 

 dwarfing plants and in controlling their distribution. 



Temperature. The temperature of the plant itself tends normally 

 to equal that of the air, since its extended surface permits quick gain 

 or loss of heat toward equilibrium. A rise of temperature in the air, 

 therefore, is quickly followed by a rise of temperature in the plant, and 

 (even with no change in the relative humidity of the air) by increased 

 evaporation. But the temperature of the plant depends also upon the 

 energy absorbed by the green pigment in diffuse light or direct sunlight. 

 In diffuse light the greater part of this energy is used in food making, 

 and only a small portion exerts a heating effect. But in sunlight two 

 thirds to three fourths of that absorbed is free to heat the tissues, and as 

 soon as that begins, evaporation is thereby much accelerated. This 

 tends to dissipate the heat. 



It has been proposed to call the evaporation due to the excess of energy absorbed 

 by the chlorophyll, chloro-vaporization. The term has its only value in promoting 

 recognition of the fact; but chloro vaporization cannot be distinguished practically 

 from the rest. 



Were it not for this transfer of energy to the water vapor, the tempera- 

 ture of the tissues would rise to the danger point, or at least to a degree 

 which retards food making. When transpiration is greatly reduced by 

 enclosing a shoot in a glass chamber whose air quickly becomes nearly 

 saturated while the light is absorbed, death quickly ensues. The 

 " scalding " of leaves by sunshine after a summer shower is an example 

 of the same effect. If a plant derives no other advantage from tran- 

 spiration, this prevention of injury by overheating in direct sunlight 

 is certainly one. For even temporary interference with food making 

 might be serious, and permanent stoppage of it by the killing of any 

 considerable area of leaves might be fatal to the whole plant. How- 

 ever possible it might be for plants to meet this difficulty by other 

 methods, if transpiration could be eliminated for other reasons, under 

 the present organization transpiration is of real advantage in this 

 particular. 



Amount transpired. Because of the extreme variation, from zero 

 to the maximum, a quantitative statement of the amount of evaporation 

 is of little value, though a voluminous literature records an enormous 

 number of observations and calculations. The following will serve as 

 illustrative examples. 



