THE MATERIAL OUTGO OF PLANTS 329 
in which the guard cells are immobile, or respond very sluggishly to 
external stimuli. Further, the more exact become the studies on plants 
of desert regions, where the need of an effective regulating mechan- 
ism seems most obvious, the less efficient do the stomata appear. 
Rather it appears that they are scarcely more than a tardily acting mech- 
anism which may save the plant in extremity, but does not produce any 
exact adjustment. 
Factors in transpiration. The amount of water lost from a given 
surface of plant tissue is extremely variable. The humidity of the air, 
its temperature and pressure, which also affect humidity, and the tem- 
perature of the plant are the chief factors which cause the rate of evapora- 
tion to vary. The simplest mode of determining evaporation quanti- 
tatively is by weighing potted plants at intervals, having prevented 
evaporation from the surface of pot and soil by some impervious cover- 
ing of rubber, metal, or wax. It is not justifiable, however, to apply 
these data to plants in nature. 
Humidity. In a saturated atmosphere there can be no water loss. 
Yet experimentally this is very difficult to establish. The reason is to 
be sought in two directions. First, it has been found practically im- 
possible to maintain an atmosphere absolutely saturated at all times, 
for that means an invariable temperature, which, under other conditions 
necessary to the experiment, is unattainable. Second, even were the 
proper external conditions attained, the plant by respiration would be 
a little warmer than the air, and the air next the plant, therefore, would 
not be quite saturated; so some small amount of evaporation might take 
place. Yet during rain, mist, or fog, practically no evaporation occurs; 
and as the humidity decreases from 100 per cent to the 70 per cent of a 
moderate day or to the 50 per cent of a dry day, evaporation increases. 
As the humidity fluctuates from day to day or even from hour to hour, 
the evaporation varies likewise. The most marked changes in relative 
humidity are due to the rising or falling temperature of the air.. As 
temperature rises, relative humidity becomes less, the heat energy im- 
parted to the plant is greater, and evaporation is increased by both 
causes. 
Barometric pressure. As the air pressure is reduced the boiling point 
of water falls; so fluctuations in the barometer indicate inverse changes 
in the rate of transpiration. Yet these variations at any locality are 
insignificant; the reduction in air pressure becomes important only in 
comparing plants at high and low altitudes. In alpine regions, where 
