36 PLANT PHYSIOLOGY 



efficiently when the radiant energy is distributed over a longer 

 period of time at a lower intensity than when the opposite condi- 

 tion holds. The ripening and growth of plants can hence be speeded 

 up enormously by using artificial light and increasing the working 

 day of the plant. For this reason orange growers of central Cal- 

 ifornia where the summer days are very long can market their 

 fruit a month earlier than their competitors 400 miles farther 

 south where the days are shorter. 



Efficiency of the Leaf. — Brown found that a sunflower leaf 

 received on a sunny day 600,000 gram-calories of heat energy 

 per sq. meter per hour. During the same time the same area 

 produced 0.8 g. of carbohydrates which requires 3,200 gram- 

 calories for its manufacture. Thus the leaf was able to store up 

 in the manufactured food only 0.5% of the solar energy which 

 fell upon it. Considered as a mechanism for transforming solar 

 energy into the chemical energy of organic compounds, it is a 

 machine of very low efficiency. However, it is not so bad as it 

 sounds. Of the light which falls upon the leaf 30-60% is reflected 

 and transmitted and only 40-70% is absorbed by the leaf. If 

 we use the higher figure of 70% actually absorbed by the leaf, 

 of this a large fraction, or 60-65% of the total light falling 

 on the leaf, is absorbed by the chlorophyll-free part of the leaf, 

 leaving only about 5-10% of the total light falling upon the leaf 

 to be absorbed by the chlorophyll-containing cells, while the 

 chlorophyll itself probably gets no more than 3-4% of the total 

 light. This may be summarized as follows: Of the total light 

 falling on the leaf, 



30% is reflected and transmitted. 



70% is absorbed by the leaf. 



60% is absorbed by the chlorophyll-free part of the leaf, — 

 epidermis, cell walls, water, etc. 



10% is absorbed by the chlorophyll-containing cells. 



3.5% is absorbed by the chlorophyll apparatus and used in 

 photosynthesis. 



A green leaf compared with a white leaf of the same plant ab- 

 sorbs about 4% more light which agrees well with the above figure 

 of 3.5%. Inasmuch as the chlorophyll actually absorbs only 

 3.5% we must now multiply our efficiency figure by 100/3.5 

 which shows the chlorophyll apparatus to be 15% efficient rather 

 than 0.5%. 



