OHEMISTRY AND PHYSICS OF PLANTS. 71 
In aerial parts of plants the newly formed cells obtain all 
their water from the adjacent cells. 
133. Evaporation of Water.—In the aerial parts of plants 
the evaporation of water from their surfaces is a far more 
powerful disturbing cause than either of the two preceding. 
Whenever a cell is exposed to dry air at ordinary tempera- 
tures a portion of its water passes off by evaporation; this 
immediately disturbs the equilibrium of water throughout 
the tissue, and the more rapid or the longer continued the 
evaporation the-greater the disturbance. 
134, Evaporation from living cells or tissues is depen- 
dent upon a number of conditions, some of which are en- 
tirely exterior, while others are connected with the struc- 
ture of the plant itself. Among the former, the most 
important is the condition of the air as to the amount of 
moisture which it contains. In air saturated with moisture 
no evaporation can take place; but whenever the amount 
of moisture falls below the point of saturation, if the other 
conditions are favorable, evaporation takes place. 
135. The temperature of the air (and, as a consequence, 
that of the plant also) has some effect upon the rapidity of 
evaporation. It appears that there is an increase in the 
amount of water given off as the temperature rises; this 
may be due, however, to the fact that with such increase 
of the temperature of the air there is generally a considera- 
ble decrease in its moisture. The direct influence of light 
upon evaporation is also somewhat doubtful. While there 
can be no doubt that plants generally lose more water in 
the light than in darkness, it appears to be due to the in- 
creased heat and dryness which are common accompani- 
ments of the increase of light. 
136. In enumerating the internal conditions, one general 
