NUTRITION, STORAGE, AND RESPIRATION 



99 



absorption bands, of which the most noteworthy are two well-marked 

 bands close together in the red, and three broader bands, which in stron^^ 

 solutions obliterate the greater part of the violet end of the spectrum. 

 From this we learn that by chlorophyll, which is necessary for Photo- 

 Synthesis, certain red and violet rays of light are actually stopped and 

 absorbed. It is also known that light which has passed through 

 green leaves is less effective in Photo-Synthesis than an equal intensity 

 of normal light. These facts indicate that the absorbed rays are 

 intimately connected unth the synthetic process. 



Fig. 71A. 

 Part of leaf of Narcissus in transverse section. Two of the cells of the mesophyll 

 are drawn in in detail : that to the right as seen in surface view from without ; that to 

 the left in optical section. The chloroplasts are shown black. ( x 300.) F. O. B. 



Carbon dioxide is present in atmospheric air constantly, but in very 

 small proportion. The latest estimates rate it normally at about 

 3 parts in 10,000. But as its molecules are mobile among those of 

 the other atmospheric gases, it follows that if molecules of the gas be 

 absorbed at any point, the place of those absorbed will be immediately 

 taken by others from a distance. Thus the small proportion in which 

 carbon dioxide is present is no disadvantage. The case of submerged 

 green plants is much the same as that of sub-aerial plants. Their 

 supply of carbon dioxide is presented in solution in the water which 

 bathes their surfaces. Here again the supply is mobile and subject 

 to diffusion in the water. Thus, whether in the atmosphere or in 

 solution in water, even where the proportion of COo is small, it is 

 readily available for the purposes of the plant. 



