THE WATER m THE PLANT. 171 



number, size, and condition {i.e., whether open or closed) 

 of the stomata. As previously described (paragraph 130, p. 

 99), the stomata are placed over intercellular spaces, which 

 are in communication with the intercellular passages of the 

 plant. These spaces and passages are filled with moist air 

 and gases, which, when the stomata 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 stomata are closed, 

 little or no escape of moisture is possible. The openmg and 

 closing of the stomata 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 

 aflEect somewhat the opening and closing of the stomata ; 

 W^hen the epidermis is very dry the stomata are generally 

 closed, and vice versa. 



223.— 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 evaporation is 

 actually very slow. Thus Hales long ago found that the 

 amount of water evaporated from a vine in twelve hours of 

 daylight equalled a film only .13 mm. (.005 in.) thick, and 

 having an extent as great as that of the evaporating surface ; 

 the amount from a cabbage in the same time equalled a film 

 .31 mm. (.012 in.) thick ; from an apple tree, .25 mm. (.01 

 in. ) thick ; from a sunflower in a day and a night, equal to 

 a -film .15 mm. (.006 in.) thick.* Miiller found the rate of 

 evaporation from the leaves of Hmmanthus puniceus to be 

 only one seventeenth as rapid as that from an equal area of 

 water during the same time. Sachs found the evaporation 



* " Statical Essays : Vegetable Statics," by Stephen Hales. 1737. 

 Fourth edition. 1769. p. 31. 



