No. 511] PRESENT PROBLEMS IN PLANT ECOLOGY 427 



tioned upon the angle made with the sun's rays. The 

 law of the sine may, therefore, not he neglected. C2) 

 "Within the atmosphere we have the phenomenon of dif- 

 fuse light from the sky. At the earth's surface it In- 

 comes a factor of no small importance, very likely sur- 

 passing for vegetation in general that of the direct rays 

 of the sun. It is, of course, derived from direct light, 

 but for it the law of the sine is far from valid. In gen- 

 eral, it is subject to less variation. It 'is probably 

 affected by many factors not easily observed, presence 

 of dust, vapor, etc., and its numerical computation her- 

 ders on the impossible. A table showing light, intensity 

 for any given day or hour, might possibly, if all fac- 

 tors were taken into account, show the value of direct 

 light, but it is open to question, if this is what the 

 botanist chiefly wants to know. (3) The length of the 

 path of a ray of light through the earth's atmosphere, 

 and, therefore, the fraction of it which is absorbed, in- 

 creases with decreasing elevation of the sun. Quite 

 apart from diminution due to decreasing angle of inci- 

 dence, the light wanes as the sun approaches the horizon. 

 This fact is sometimes clearly seen; sometimes appar- 

 ently overlooked. (4) Absorption in the lower, denser, 

 dust, water-vapor and carbon dioxide bearing layers of 

 the atmosphere is relatively far greater than in the upper 

 and rarer ones. This last holds good, especially for the 

 shorter rays. This third consideration comes to the 

 foreground in a study of altitude and vegetation. 



Here then are several varying factors; some of them 

 difficult to calculate, and none of which may be neglected. 

 Moreover, as will be shown below, there is reason for 

 doubting whether any theoretical calculations are valid, 

 even for clear days. 



