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THE AMERICAN NATURALIST [Vol. XLIII 



4. Light.— Since a certain proportion of light is ab- 

 sorbed during passage through the atmosphere and par- 

 ticularly by the layers next the earth's surface, it seems 

 plain that light becomes more intense with increasing 

 altitude. If so, its relation to vegetation is a matter of 

 much interest. 



Notwithstanding that the question has received con- 

 siderable attention, our knowledge of it is still in a rather 

 unsatisfactory condition as will appear from the follow- 

 ing: 



Bonnier, Schimper, Schroeter and indeed most of those 

 who have written upon the subject, express the belief 

 that the more intense light is a factor of importance in 

 connection with Alpine vegetation. They give reasons 

 indicating that a greater intensity exists, and noting the 

 reduced leaves and shoots and prominent flowers of 

 alpine plants, state that the former is the cause— in part 

 at least— of the latter. The conclusion, however, has 

 not been put to the test of discriminating experiment. 



Any further discussion of the subject brings in a gen- 

 eral consideration of light as an ecological factor. I 

 trust a brief digression may be permissible. 



Since light is a form of solar energy, efforts have nat- 

 urally been made to compute its intensity from astro- 

 nomical data. Attempts have been made in several 

 quarters to calculate light intensity for any day and 

 hour of sunshine for the year at a given latitude. They 

 do not, however, seem to have been happy in escaping fun- 

 damental error. For, in making calculations from the 

 sun's altitude, there is not one varying factor, but several 

 which must be reckoned with: (1) The amount of 

 radiant energy falling upon a horizontal surface varies 

 with the sine of the sun's altitude. This law is per- 

 ceived by every one. It has sometimes been neglected, 

 in the thought apparently, that since leaves stand at all 

 angles, it is unnecessary to reckon what a horizontal sur- 

 face may receive. Yet the direct light available for the 

 total vegetation of a hectare or any other area is func- 



