1920] FREEMAN— EVAPORATION AND TRANSPIRATION 209 



by no means always the case. There are other factors, for instance, 

 such as the amount of light reaching the plant on more or less 

 cloudy days, the age of the plants, and moisture content of the 

 soil, all of which certainly affect the condition of the stomata and 

 hence the evaporating efficiency of the leaves. These would, 

 moreover, affect both types approximately alike. 



Another disturbing factor, which would not equally affect the 

 plants, is the area of foliage of each inclosed in the evaporating 

 chamber. If such an amount of foliage is inclosed in one cylinder 

 as to give its outgoing air a markedly higher dewpoint than that 

 of the other cylinder, the evaporating rate per unit surface will be 

 cut down by the greater relative humidity in the cylinder. The 

 water demand upon a given area of the leaves will be reduced, and 

 they will respond by opening wider their stomata. This will give 

 the leaves of this cylinder a greater standard dewpoint than will 

 be found for the leaves in the cylinder having the less surface 

 exposed. In practical work, therefore, it is necessary either to 

 inclose in each cylinder an approximately equal area of leaves 

 (which is extremely difficult), or to so vary the wind movement as 

 to have the dewpoint of the air, coming off each cylinder, approxi- 

 mately equal. This can be easily accomplished in the apparatus 

 used by the writer, by pinching down the air supply tube of one 

 or the other cylinders with a screw clamp. 



Summary 



by 



of a porous cup atmometer inclosed in a glass cylinder of one 



which 



tion formula is offered which may take any of the following forms: 



t-h , x /-/, 



(a) y = z(t-t 1 ); (b) y = —^)n', ( c ) V 



c(w)*+ 



a 



In these formulae y - rise in the dewpoint of the air caused by 

 the loss of water to it of a given evaporating surface; ^tempera- 

 ture of the air; /, = dewpoint of the outside air; z = constant used 

 when the area and wind movement remain constant; n= exponent 

 of w, used when the area remains constant; c = constant coefficient 



