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 transpira- 
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 cne term for evaporation from a 
323 
