280 



VEGETABLE PHYSIOLOGY 



a solution of chlorophyll is seen to be robbed of rays in 

 these regions, and hence to present the appearance of a 

 band of the different colours crossed by several dark bands 

 (fig. 130). The greater part of the energy so obtained in 

 the cells which contain the chloroplasts is at once expended, 

 partly in constructing carbohydrate food materials and 

 partly in evaporating the water of transpiration. The latter 

 process is much the more expensive ; recent observations 

 have made it probable that 98 per cent, of the radiant 



400 



i n m TV 



FIG. 130. ABSORPTION SPECTRA OF CHLOROPHYLL AND 

 XANTHOPHYLL. (After Kraus.) 



energy actually absorbed during bright sunshine is at once 

 devoted to this purpose. 



When we speak of radiant energy we must remember 

 that the rays of the visible spectrum do not supply all the 

 energy which the plant obtains. It has been suggested by 

 several botanists with considerable plausibility that the 

 ultra-violet or chemical rays can be absorbed and utilised 

 by the protoplasm without the intervention of any pigment 

 such as chlorophyll. There is some evidence pointing to 

 this power in the cells of the higher plants. Certain 

 bacteria also construct organic material from simple com- 

 pounds of nitrogen and carbon dioxide, though it is not 

 probable that they utilise radiant energy directly. 



