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. 
