The Intake of Carbon 215 



sunlight. When, however, it is asked how economic or 

 efficient is this world-distributed apparatus with respect 

 to the energy received, one experiences at first a keen dis- 

 appointment to ascertain that the highest estimates indi- 

 cate an effectiveness of only 3 per cent, and, according to 

 other estimates, it may be as low as .5 per cent. Still, 

 the amount of light absorbed by the leaf is considerable, 

 and it is important to note the result of this. 



In diffuse light the leaf may absorb 95 per cent falling 

 upon it, while in direct light only about one half is ab- 

 sorbed, or reflected. In either case much of this absorp- 

 tion is due to the chlorophyll bodies which have a capacity 

 of from 10 to 20 times or more the amount effective in 

 actual photosynthesis. The surplus energy absorbed is 

 in part operative in raising the temperature of the leaf, 

 which, according to Blackman, may be in direct sunlight 

 from 10 to 15 C. higher than that of the surrounding air. 

 This surplus, of course, induces a more intensive evapora- 

 tion. Perhaps if we knew more of the physical and chem- 

 ical changes involved in food-making, this efficiency would 

 be unchallenged. 



n6a. Light, intensity and quality. The relation of 

 food manufacture to the intensity and quality of light is 

 most complex. Under favorable conditions of tempera- 

 ture the working capacity of many plants is proportional 

 to the increase in light intensity, at least up to the point 

 where the available CO 2 is not a limiting factor. Never- 

 theless, experiments made from another standpoint indi- 

 cate that with respect to photosynthesis under field condi- 

 tions there are shade-loving plants plants which seem 

 to be thoroughly attuned to a maximum capacity for 



