70 A TEXTBOOK OF GENERAL BOTANY 
plant loses more water from its leaves than is absorbed by its 
roots, the water content of the cells of the leaf, including the 
guard cells, is lowered and the guard cells become less and 
less turgid. As this process continues the stomata close. The 
rate of transpiration is regulated to a considerable extent by the 
opening and closing of stomata. The closing of stomata does 
not, however, entirely stop transpiration, as a small amount of 
transpiration takes place through 
the cuticle of the epidermal cells. 
Daily march of transpiration. 
The rate of transpiration is de- 
pendent to a considerable extent 
on the rate of evaporation. There- 
fore we shall consider first the 
variations in the rate of evapora- 
tion at different times during the 
day and night. During the night 
the rate of evaporation is usually 
very low, as at this time the air is 
much colder than during the day 
Fig. 538. Arrangement to imitate and the accelerating influence of 
the bending of the ventral walls SUlight is absent. The air is also 
of a stoma much more nearly saturated with 
water during the night than dur- 
ing the day, as cold air can hold less water than warm air. 
Thus, although there may be the same amount of water in the 
air during the day and during the night, the air becomes more 
and more saturated as it cools during the night. We see the 
result of this phenomenon in the formation of dew, which is 
the result of the condensation of water from the air. This 
condensation takes place at night because the air is more 
nearly saturated at that time than during the day. The increase 
in the saturation during the night has a great retarding effect 
on the rate of evaporation. With the coming of daylight the 
rate of evaporation increases, owing to light, greater heat, and 
the fact that the air becomes less saturated with water as it 

