40 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



and other plant parts of which we obtain only a partial measure- 

 ment through ordinary air temperatures. That this is an important 

 condition affecting distribution of every species is evidenced by the 

 fact that with increase of both altitude and latitude, or, in short, 

 with decrease of air temperature, a given plant seems to require 

 more light for its development. This evidence is not in itself con- 

 clusive, because, on a given site, more light obtained by wider 

 spacing will usually mean more moisture, which may often be the 

 controlling factor. Again, in a given locality, the species which 

 thrive best in air of low temperature always seem more tolerant of 

 shade. 



Perhaps it is best to analyze the situation at the outset according 

 to physical principles and logic rather than on the basis of ques- 

 tionable evidence. The latter has been mentioned to forewarn the 

 student of some of the pitfalls of poorly conceived observational 

 methods. 



The radiant energy available to the plant may consist of an infinite 

 variety of rays or wave lengths, from the most subdued heat to the 

 ultra-violet light. The effect of each of these wave lengths is 

 entirely dependent upon the nature of the absorbants in the plant. 

 Thus the organic material of the cell walls and the water within the 

 cells are capable of absorbing readily the red and infra-red or 

 " heat " rays of the solar spectrum. The chloroplasts show an 

 ability to absorb visible rays, the proportionate absorption of the 

 various wave lengths varying in different plants. Of the absorp- 

 tion of ultra-violet light by leaves practically nothing is known as 

 yet on account of the difficulties of observation in this end of the 

 spectrum. We may, however, safely assume a considerable absorp- 

 tion of these invisible rays. 



There is practically no question that each of the chemical elements 

 found in the plastids (or, for that matter, anywhere in the leaf 

 cells) absorbs the kind of rays which it would absorb under any 

 other condition. Thus the " selective absorption " by different 

 plants may be mainly the resultant of different amounts and pro- 

 portions of such of the elements as iron, sodium, and potassium. 



All rays which are absorbed are heating, and all may assist in 

 bringing about chemical reactions, of which the first in importance 

 to the plant is the union of H2O and CO* to form carbohydrates. 

 The function of the chlorophyll and of the chloroplasts is to con- 

 centrate sufficient energy at a given point to effect this difficult com- 

 bination. The kinds of rays which are essential to photosynthesis, 

 therefore, are the kinds which the substances in the chloroplasts are 

 capable of absorbing ; and, as has been said, the substances may vary 

 according to the kind of plant and according to the solutes which 

 the soil is capable of supplying. 



