THE LEAF og 
and the amount of mineral matter in plants. It would appear 
that plants are frequently subjected to higher rates of tran- 
spiration than are necessary for the movement of sufficient 
dissolved material through the xylem. | 
While some transpiration would appear to be beneficial, par- 
ticularly to tall plants, because it accelerates the movement of 
dissolved material from the roots to the leaves, there are at least 
many small plants for whose development it would seem to be 
unnecessary, as is shown by the fact that they -will grow in an 
atmosphere kept as nearly saturated with water as is possible 
even under experimental conditions. In the absence of tran- 
spiration the movement of dissolved materials can take place 
through the plant slowly by means of diffusion. 
Factors influencing transpiration. The rate of transpiration 
varies greatly with changes in the environment. The factors 
that affect transpiration are the same as those that influence 
the rate of evaporation in general. 
The effects of light, heat, wind, and humidity on ordinary 
evaporation are matters of common knowledge. We know that 
if we put a wet handkerchief in a warm place it will dry much 
more rapidly than it would in a cold place. The drying of the 
handkerchief is, of course, simply the evaporation of water from 
the handkerchief. We know also that wet clothes will dry more 
rapidly in sunlight than in a dark or shaded place, and, other 
conditions being equal, more rapidly on a windy day than on a 
still day. Likewise they will dry more rapidly on a dry day 
than on a very moist one. 
Transpiration varies with the temperature, because water 
vaporizes more rapidly at high temperatures than at low temper- 
atures, and because warm air absorbs more moisture than cool 
air does. Light increases transpiration, owing to the heat pro- 
duced by the light. Wind increases transpiration in two ways: 
Diffusion of water vapor from the stomata tends to saturate the 
atmosphere around the leaves; wind removes this moist air and 
replaces it by drier air. Wind also, by causing the leaves to shake 
and bend, increases the movement of gases in the intercellular 
