FOOD 



237 



dependence of life upon its environments as do the facts 

 which this diagram represents. If you can form a clear 

 picture in your mind of what is caUed the nutritive cycle 

 (which is what this diagram portrays), then you have 

 grasped some of the greatest and most fundamental prin- 

 ciples of life. 



Note first the absolutely inorganic nature of the materials 

 with which the process of food construction is begun; the 



AIR 



CARBON DIOXIDE 



ENERGY FROM 

 SUNLIGHT STORED UP 



CARBON DIOXIDE 



ENERGY SET FREB 

 HEAT 



GREEN PUNTS 



ANIMALS 



AND PUNTS 



WITHOUT CHLOROPHYLL' 



WATER 



SOIL 



NITF 

 SALTS AND 

 OTHER SALTS 



AND OTHER SALTS 



FIG. 88. The nutritive cycle. 



materials which the protoplasm of green leaves uses in 

 the process, and by means of which this "living stuff" 

 maintains and renews itself. These materials (carbon 

 dioxide and water) existed on earth long before life began, 

 and they exist on other worlds than ours. 



But let us note at once that more than these raw ma- 

 terials, and the protoplasm which acts upon them, are 

 necessary to the process. There must be light. It is 

 from sunlight that green leaves absorb that radiant energy 



