2io BOTAXICAL GAZETTE [September 



of w, used either when the area is constant (when a does not 

 appear in the formula) or when a varies and hence appears in the 

 formula; a = area of the evaporating surface which is always 

 expressed as the ratio of the surface exposed to that exposed 

 when c and k were determined; when c and k are determined, the 

 area then used is taken as unity and all other areas expressed in 

 terms of it; k = constant used in the exponent of w to adjust the 

 area unit to the wind movement unit. 



2. These formulae appear to be general in type and capable 

 of use in any situation where y is measurable. It is possible that 

 with some modification these formulae may also be of use when y 

 cannot be measured, but where it is possible to measure directly 

 the actual evaporation per unit area. 



3. Under temperature changes only, alfalfa leaves appear to 

 act as physical evaporating surfaces. 



4. Changes in the dewpoint of the air result in profound changes 

 in evaporating efficiency of leaf surfaces. This is probably a result 

 of the opening and closing of the stomata. 



5. It is possible to make use of these formulae in comparing 

 the evaporating efficiency of different species of plants and inter- 

 preting results which would otherwise appear hopelessly confusing. 



6. Distinct pure races of alfalfa exhibit measurable differences 

 in the rate of evaporation per unit area of their leaves. Such 

 differences may be of economic value in semi-arid or irrigated 

 regions where production depends principally upon the efficiency 

 of the use of the available water supply. 



societe sultanienne d'agriculture 



Cairo, Egypt 







