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THE AMERICAN NATURALIST [Vol. XLIII 



the world show astonishing irregularities. In Buiten- 

 zorg, the light diminished rapidly between eleven and 

 twelve o'clock under apparently clear sky. Similar data 

 were recorded for Cairo, Egypt. The maximum inten- 

 sity obtained anywhere in the world was not in the trop- 

 ics, but in Yellowstone Park. Though some of his re- 

 sults seem rather incredible, it must be owned that his 

 methods appear ample and critical— far beyond any- 

 thing else undertaken by a botanist. He has emphasized 

 the distinction between direct zmd diffuse light. He 

 finds the proportion of the former to increase greatly 

 with altitude, and sees in this a factor of much impor- 

 tance for vegetation. His results in regards to altitude 

 and light are not so full as could be wished, but he at- 

 tributes the maximum observed in the Yellowstone to 

 altitude, and, in general, assigns a high importance to 

 light as a factor in alpine climate. 



On the other hand, Clements states that in Colorado 

 the variation of light with altitude amounts to a very 

 small percentage and concludes that such differences as 

 exist are too small to receive serious consideration in a 

 study of mountain vegetation. The chief thing, there- 

 fore, that appears certain is that the whole question is 

 in an unsatisfactory condition. In such a situation, one 

 or two points must be kept clearly in view. 



First, data as to light intensity must be obtained by 

 methods which will pass muster with physicists. A cer- 

 tain body of such data are available. Time is lacking 

 to take up the question here, but the work of Cayley, 

 Violle, Langley and others might be cited to show that a 

 considerable increase in light intensity with altitude does 

 exist. 



In the second place, it must be borne in mind that the 

 relation of the plant to light is a complex one. In some 

 of the work above quoted there seems to be the whole- 

 sale error of assuming that the importance of light for 

 the plant is to be judged by observation upon its rela- 

 tion to the process of photosynthesis. It seems super- 



