CHEMISTBT 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 eraporation 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; thib 

 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 



