CLIMATIC CONDITIONS OF THE UNITED STATES. 363 



vegetation and maximum wind-velocity, but such attempts would 

 be well-nigh useless at present. 



G. SUNLIGHT AS A CONDITION INFLUENCING WATER-LOSS FROM PLANTS. 



(TABLE 21, PLATE 69.) 



As has been remarked, the influence exerted by light conditions upon 

 plant growth is very complex, comprising several kinds of influences 

 that are themselves quite different. With regard to general plant 

 growth, which mainly interests us in the present studies, it is obvious 

 that the most important light-relations are those of transpiration and 

 photosynthesis. 



The second of these relations is quite out of reach at present, for 

 before the hght conditions of different climatic regions may be com- 

 pared with reference to the possibihty of plant photosynthesis, the 

 light-supply must be measured especially in terms of those ranges of 

 wave-lengths that are known to influence the photosynthetic process; 

 it is clear enough that the total light-supply is not in itself the condition 

 to be studied in this connection, and spectrophotometry has not yet 

 progressed far enough to furnish the instruments and methods here 

 required. When it becomes possible to measure and record, at each 

 chmatological station, the daily supply of solar radiant energy of cer- 

 tain relatively small ranges of wave-lengths, then, and then only, will 

 the photosynthetic powers of different regions become capable of being 

 satisfactorily compared, but this seems unlikely to occur in the very 

 near future. 



The light-relation of transpiration is much more easily approach- 

 able than is that of photosynthesis, but the chmatological records 

 so far available are not suitable for even the relatively simple com- 

 parisons required in this connection. Here the chmatological con- 

 dition to be studied is the power of the surroundings to accelerate the 

 rate of water-loss from plants through the action of impinging solar 

 radiant energy. Since no two plants are to be regarded as exactly 

 alike, and since the internal conditions of the light-absorbing surface are 

 as influential in determining the light effect upon evaporation as are 

 the external light conditions, it becomes necessary here (as in the case 

 of the evaporating power of the air) to adopt some standard light- 

 absorbing surface and to measure the light-effect upon that surface. 

 The measurement of sunshine has usually been accomplished by 

 methods that depend upon the increase in temperature occasioned by 

 the exposure of a certain blackened surface for a specified time period. 

 Thus, the black-bulb thermometer has been employed by various 

 authors in the comparison of the total intensity of sunshine energy 

 received at different stations or for different days, etc., at the same 

 station. The instruments so far described have not been generally 

 satisfactory, being diflicult of adequate operation and of calibration. 



