THE WATER ZZV THE PLANT. 169 



ing -water, but this can only be the case with the external 

 cells ; the deep-lying ones must obtain their supply from the 

 cells which surround them. In aerial parts of plants the 

 newly formed cells obtain all their water from the adjacent 

 cells. 



2 20.— 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 olf 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. 



Evaporation (called also transpiration and exhalation) 

 from living cells or tissues is dependent upon a number of 

 conditions, some of which are entirely exterior, while others 

 are connected with the structure 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 satu- 

 rated 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. The temperature of the air (and, as a conse- 

 quence, 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 ofE 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 considerable 

 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 may be questioned whether this is not 



* Many experiments, at first sight, seem to sliow that plants evapo- 

 rate water in air saturated with moisture ; but Knop has found 

 (" Versuchs-Stationen," Vol. VI., p. 255) that, under similar conditions, 

 moist pieces of paper or wood also evaporate water, thus showing that 

 the air, instead of being saturated, lacked somewhat of being so, 



