CHAPTER II — THE MATERIAL OUTGO OF PLANTS 

 i. TRANSPIRATION 



The term transpiration. — Frequent reference has already been made 

 to the most important outgo of material from the plant body — the water 

 evaporated from the aerial parts. This was long ago called trau.s pi ra- 

 tion, after the analogy of the exhalation of water vapor from the lungs, 

 with whose movements, however, it has nothing in common. It is 

 considered by many to be a function of the aerial parts, something 

 which they actively do, in which case a special name would be quite 

 appropriate. It is better, however, to look upon it as a process in which 

 they are passive. In this case evaporation is no more a " function " of 

 a wet leaf than it is of a wet towel, and the need of a special term is less 

 evident. Yet the word is convenient as a short form for the expression, 

 the evaporation of water from live plants. 



Evaporation. — When a dish of water is exposed to air which contains 

 less water vapor than it can hold, more water particles will fly off into 

 the air in a given time than will fall into the water from the air ; hence 

 the volume of liquid will be diminished; the water evaporates. The 

 rate of evaporation is determined by the temperature of the water, the 

 temperature and pressure of the air, and the relative amount of water 

 vapor in the air (humidity). Decreased humidity, higher temperature, 

 or lower pressure increases the rate of evaporation, and vice versa. The 

 presence of any solutes in the water retards evaporation. Likewise 

 water adherent to any substance, or imbibed by it, is held there and 

 evaporates less readily than if in contact with water particles only. 

 Thus the water evaporates from a dish of wet sand or from a wet towel 

 or sponge more slowly than from an equal surface of free water. 



Since the actual exposed surface may be greatly increased by spreading out the 

 water over sand grains or linen fibers, the evaporation from a given area of the 

 material is not comparable with that from an equal area of water. 



Because the evaporation from a green leaf and that from a like area of water are 

 not equal is no reason for giving a special name to the evaporation from leaves, 

 as has been urged. If it were, we should need one term for evaporation from a 



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