64 



The Plant World. 



various parts of the world show astonishing irregularities. In 

 Buitenzorg the light diminished rapidly between 11 and 12 

 o'clock on a clear day. Similar data were recorded for Cairo, 

 Egypt. The greatest intensity found anywhere in the world 

 was not in the tropics, but in the Yellowstone. Though some 

 of his results seem hard to credit without further confirmation, 

 it must be owned that his methods seem much more ample and 

 critical than does similar work by other botanists. He attributes 

 much importance to the intenser light of higher altitudes. On 

 the other hand, Clements states that in Colorado the increase of 

 light intensity with increasing altitude is too small to be con- 

 sidered in studies of vegetation. 



Further study of the whole question of light as an ecological 

 factor is much to be desired. In so doing, data as to intensity 

 must be obtained by methods which will pass muster with 

 physicists. A certain body of fairly reliable data is now in 

 existence in the work of Cayley, Yiolle, Langley and others. 

 This goes to show that light is considerably more intense at high 

 altitudes, particularly in the more refrangible end of the spectrum. 



In giving attention to this matter it seemed to me desirable 

 to learn something of the responses made by plants to variations 

 in quality of light. Several series of experiments have accord- 

 ingly been carried out, in which both the plants experimented 

 upon and the controls were exposed to the same amount of 

 daylight (deemed sufficient for normal development), but the 

 subjects of the experiment received in addition a certain amount 

 of blue-violet light. The results so far, while scarcely conclusive, 

 suggest that plants do respond to such a variation in quality of 

 light. Internodes have been shorter and leaves more hairy in 

 the bluer light. 



It has also been largely held that evaporation increases 

 with altitude on account of diminished pressure and increasing 

 air movement. Such a fact would be of great importance for 

 vegetation. The more xerophytic forms of alpine plants har- 

 monize well with the belief in its existence. Schimper gives 

 prominence to the idea, Schroeter likewise, and the latter 

 writes vividly of the drying of skin experienced by alpinists, 

 and refers to a kind of dry cured meat prepared in some valleys 

 of the Alps, the process being made possible, it is said, by the 

 increased evaporation occurring at high altitudes. With a view 



