290 ENVIRONMENTAL CONDITIONS. 



climatology, and it is practically unsatisfactory on account of the 

 inadequacy of the information in this regard which is now available. 



(3) Absorbed Radiation. 



Reverting again to the conditions controlling water-loss from plants, 

 we have said that there are, generally, two of these — the evaporating 

 power of the air and the intensity of absorbed radiant energy. The 

 first of these has been discussed in sufficient detail for present purposes 

 and the second remains to be considered. By far the greater portion 

 of the radiant energy intercepted by plant surfaces comes directly 

 from the sun; other sources of radiant energy appear to be practically 

 negligible under most natural conditions. It is therefore absorbed 

 sunshine (light and heat) that needs attention at this point. 



The intensity of absorbed solar radiation is determined by three 

 conditions — the intensity of the impinging rays, the angle at which 

 they meet the exposed surfaces, and the absorbing power of the sur- 

 faces. The last is an internal condition, effective within the plant, 

 with hke transpiring power, water-absorbing power, etc. With this, as 

 other internal conditions, practically nothing of a quantitative nature 

 has yet been attempted.^ 



The angle at which the impinging rays meet plant surfaces varies 

 with the time of day, with the season, and with the shape and position 

 of the plant; but since ordinary plants offer absorbing surfaces to solar 

 radiation at all possible angles, it is only in special studies of special 

 species (as of "compass plants," for example) that this matter may 

 require attention. We may ignore the angle of incidence in our present 

 discussion.^ 



The intensity of the impinging radiation is obviously the feature 

 dealt with in climatology as sunshine intensity. For the measurement 

 of this, various methods have been devised from time to time (such 

 as the black-bulb thermometer, the bolometer, the pyrheUometer, the 

 Hicks solar radio-integrater and several forms of photographic actinom- 

 eters), but no data are available for a quantitative cHmatological 

 study of this condition. It appears probable that the radio- atmometer^ 

 may furnish adequate information for ecological purposes, when its 

 value in this connection has become appreciated. 



A very distant approach toward the measurement of sunshine 

 intensity, and the only systematic attempt in this direction thus far 



* See, in this regard, Livingston 1911, a. 



^ Briggs and Shantz have argued that only the vertical component of solar radiation is to be 

 considered as effective upon plants. It seems to us that this question requires experimental 

 investigation before its detailed discussion may be attempted. We may add here the remark 

 that the surfaces of most plant leaves occupy almost all conceivable angles with the vertical, so 

 that the exposure of the plant as a whole must approach being equivalent to that of a sphere 

 or of a vertical cylinder with spherical top. For the opposite argument, see L. J. Briggs and 

 H. C. Shantz, Hourly transpiration rate on clear days as determined by cyclic environmental 

 factors, U. S. Dept. Agric, Jour. Agiic. Res. 5: 583-649, 1916. 



'Livingston, B. E., A radio-atmometer for measuring light intensity, Plant World 14: 96-99, 

 1911.— Idem 1911, b; Idem 1915, a; Idem 1916, b. 



