CBEMISTBT AND PIITSICS OF PLANTS. 73 



■which, ■when the breathing-pores are open, expand and 

 contract ■with every change of temperature or atmospheric 

 pressure, and thus permit the escape of considerable 

 amounts of water; -when, on the other hand, the breathing- 

 pores are closed, little or no escape of moisture is possible. 



140. The opening and closing of the breathing-pores 

 appear to depend upon the amount of light; they open 

 more widely the greater the amount of light, and close 

 almost completely in darkness. The amount of moisture 

 on the surface of the epidermis appears also to affect some- 

 what the opening and closing of the breathing-pores; when 

 the epidermis is very dry they are generally closed, and 

 vice versa. 



141. The Amount of Evaporation. — The conditions con- 

 trolling evaporation are thus seen to be many and various. 

 They never, or but very rarely, act singly, two or more of 

 them usually acting together with varying intensity, so 

 that the problem of the amount of evaporation taking place 

 at any particular time is a complex and difficult one. All 

 the observations yet made, and ■which have necessarily 

 been upon a very small scale, indicate that the rate of evap- 

 oration is actually very slow. 



142. A given area of leaf-surface will evaporate much 

 less -water than an equal area of water-surface. The amount 

 of the former has been estimated at from one seventeenth to 

 one third of the latter, varying of course in different plants. 

 A grape-leaf has been found to evaporate in twelve hours 

 of daylight an amount of water equal to a film covering 

 the leaf only .13 mm. (.005 in.) deep; a cabbage-leaf for 

 the same time .31 mm. (.012 in.); an apple-leaf .25 mm. 

 (.01 in.). An oak-tree was found to have evaporated in 

 one season, during the time it was covered with foliage, an 



