62 COBN CHOPS 



chapter on physiology. This has been well expressed 

 by Abbe, as follows : ' — 



" The growth of a plant and the ripening of the fruit 

 is accomplished by a series of molecular changes in which 

 the atmosphere, the water, and the soil, but especially 

 the sun, play important parts. In this process a vital 

 principle is figuratively said to exist within the seed or 

 plant and to guide the action of the soil, the water, and 

 the air into such new chemical combinations as will 

 build up the leaf, the woody fiber, the starch, the pollen, 

 the flower, the fruit, and seed. . . . No plant life, not 

 even the lowest vegetable organism, is perfected, except 

 through the influence of the radiation from the sun. . . . 

 The radiation from any artificial light, especially the most 

 powerful electric light, will accomplish results similar to 

 sunlight; therefore it is not necessary to think that life, 

 or the vital principle, is peculiar to or emanates from the 

 sun, but on the contrary that living cells utilize the radia- 

 tions or molecular vibrations so far as possible to build 

 up the plant." 



49. The intensity of sunlight. — The intensity of the 

 sunlight received on the earth's surface is modified by the 

 altitude of the sun, which determines the total hours of 

 sunshine duration, by the atmosphere, and by the clouds. 



At high noon on a perfectly clear day, if there were no 

 atmosphere, the earth's surface would receive the full 

 effect of the sun's rays. When the sun is at zenith the 

 atmosphere absorbs about 12.5 per cent of the sun's 

 energy, so the efiiciency may be expressed as .875, assum- 

 ing the full effect to be 1. However, the sun is only at 

 zenith for a moment, therefore, as it approaches the hori- 



1 Abbe, Cleveland. Relations between Climates and Crops. U. S. 

 Weather Bureau, 1905 : 15. 



