No. 511] PRESENT PROBLEMS IN PLANT ECOLOGY 429 



fluous to emphasize that light is not only a source of 

 energy, but a factor which now stimulates, now inhibits 

 various activities of the plant in a profound degree. 



It is the highly refrangible rays which suffer greatest 

 absorption by the atmosphere. The light at the surface 

 would, therefore, differ in qualitij from that at higher 

 altitudes. In studying the question, it appeared to me 

 desirable to learn whether any responses by plants could 

 be found to alterations in quality— alterations, neverthe- 

 less, in which all kinds of rays should still be present. 

 I therefore endeavored to set up experiments which 

 should merely add a proportion of certain rays to an 

 already sufficient daylight illumination. The results, 

 while far from being as full and conclusive as could be 

 wished, seem to indicate pretty clearly that plants do 

 respond to such a variation in quality. Internodes were 

 observably shorter and leaves more hairv under the bluer 

 light. 



Perhaps the whole question can be summed up in say- 

 ing that the relation of vegetation to variations in light 

 due to altitude are poorly understood; that more data 

 from trained physicists would be welcome, but that the 

 experiments still remain to be made which would enable 

 us to interpret such data with confidence. 



5. Evaporation.-Smce pressure becomes less, wind 

 velocity increases, and insolation becomes greater with 

 increasing altitude, it has seemed necessary to conclude 

 that the evaporation rate increases. Toward the sum- 

 mits of many mountains, e. g., the White Mountains and 

 Adirondacks, the decisive relation of wind to forest 

 vegetation can hardly be doubted. In these cases there 

 is good ground for assigning evaporation as the cause 

 of timber line with all that it involves. The death of 

 buds and twigs is probably chiefly due to drying in cold 

 weather. 



Most of the leading writers agree without question 



