102 BOTANY. 



little or no escape of moisture is possible. The fact that 

 the breathing-pores open and close, and that they are open 

 when the conditions of the air favor less evaporation, and 

 closed under opposite conditions, indicates that their func- 

 tion in respect to evaporation is to prevent or check it. 



179. 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 evapora- 

 tion is relatively very slow. 



180. A given area of leaf -surf ace 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 cov- 

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

 the same time, .31 mm. (.013 in.); an apple-leaf, .35 

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

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

 an amount of water equal to a layer 33 mm. (about 1^ in.) 

 deep over all its leaf-surface. When we remember that 

 the usual evaporation from a water-surface for the same 

 period is from 500 tt) 600 or more milimeters (20 to 35 in.) 

 we must conclude that leaves, instead of being organs for 

 increasing evaporation, are able to successfully resist evap- 

 oration. 



