NUTRITION 55 



utilizable light, and that flat organs hke leaves cannot 

 profitably consist of many layers of chlorophyll-containing 

 cells. Examination of plant parts and organs shows that 

 chlorophyll is developed in greatest amount in the outer- 

 most layers of the parts exposed to light, i. e. in the outer- 

 most of the cortical parenchyma cells and in the uppermost 

 of the mesophyll layers of the leaf. Ferns and other shade 

 plants develop chloroplastids in the epidermal cells, but in 

 plants growing in situations abundantly lighted, it is not 

 necessary to dispose the chlorophyll in the cells of the ex- 

 posed surfaces. 



The effectiveness of chlorophyll as an absorber of needed 

 energy is proved by the excessively small quantity which is 

 adequate to the needs of the plant. According to the fre- 

 quently quoted estimates made by Tschirch, * there are be- 

 tween 0.2 and 1.00 of a gram per square meter of leaf- 

 surface, or, in Eicinns, about 0.1^ of the chloroplastids is 

 chlorophyll pigment, and in leaves 2-i% of the ash-free dry- 

 substance, 1.75-3.50% of the total dry-substance, is chloro- 

 phyll. As might be expected from the small amount of 

 chlorophyll, the amount of energy absorbed by the leaf 

 bears only a small proportion to the total amount falling 

 upon it as sunshine on a bright day. Brownf estimates 

 that 600,000 calories per square meter fall on a sunflower 

 leaf per hour on a bright August day in England — certainly 

 more than this in many other parts of the world. Sun- 

 flower leaves under these conditions form 0.8 gram of 

 carbohydrate per square meter per hour, which requires 

 3,200 calories or 0.5% of the available energy. In diffuse 

 daylight the percentage of energy absorbed and used is 

 more in proportion to the amount received. It would seem, 

 then, that the chlorophyll apparatus is so adjusted as to 

 make the most of the amount of light daily available rather 

 than being most efficient only when the light is unusually 

 strong. 



The relative values, as sources of energy, of the rays com- 

 posing sunlight are indicated by the proportions of the 



* Tschirch, A. Angewandte Pflanzenanatomie, p. 57 et seq., 1889. 



t Brown, H. T. The fixation of carbon by plants. Nature, Sept., 1899. 



