72 BOTANY: PRINCIPLES AND PROBLEMS 



the light which it reflects. We know, for example, that a green 

 substance like chlorophjdl absorbs in general those wave-lengths 

 which are not green and reflects those which are green or greenish- 

 yellow. We can determine more accurately, however, the kind of 

 light which is absorbed by a substance, if we break up into a 

 spectrum the light which has passed through that substance. 

 Such a spectrum displays perfectly dark regions, or absorption 

 hands, in those portions which correspond to the particular kinds 

 or wave-lengths of light which the substance has absorbed. The 

 absorption spectrum of chlorophyll (Fig. 37) shows a narrow, 

 sharp, black band in the orange-red and a wider, less definite 

 one in the blue, thus indicating that it is chiefly these two kinds 

 of light which chlorophyll absorbs, and suggesting that the red 

 and blue rays in sunlight, and no others, furnish the energy used 

 in the process of photosynthesis. Chlorophyll possesses the 

 remarkable power of utilizing energy from this source in the 

 manufacture of food, an ability that is unique in the organic 

 world. 



The intensity as well as the character of the light affects the 

 rate at which photosynthesis proceeds. The process begins at 

 illuminations of very low intensity, reaches its maximum at 

 about that of bright diffuse daylight, and decreases again in 

 light which is so bright as to injure protoplasm. Photosynthesis 

 may be readily accomplished in artificial light of the proper 

 intensity and wave-lengths. 



Given a supply of the necessary raw materials, a sufficient 

 temperature, the presence of chlorophyll, and light of proper 

 character and intensity, photosynthesis may go on anywhere in 

 the plant. Although these conditions are preeminently fulfilled 

 in the mesophyll of the leaves, they may also be present to a 

 lesser extent in petioles, stipules, calyx-lobes, and other organs, 

 thus insuring a utiHzation of these regions to produce a small 

 supplementary food supply. 



Products. — ^Let us now turn from a consideration of the neces- 

 sary conditions for photosynthesis to a study of its products. 

 The details of the process whereby carbon dioxide unites with 

 water are not yet known, but the formation of formaldehyde 

 (CH2O) is perhaps one of the prehminary steps. The first 

 product which can be recognized, however, and a substance which 

 is therefore of unique interest, is glucose or grape sugar, C6H12O6, 

 formed according to the following equation: 



