CHEMISTRY AND PHYSICS OF PLANTS. 13 
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 .18 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 
