CHAPTER XXI 

 SAPROPHYTISM AND SYMBIOSIS 



300. All Food Organic. — In more ways than one it is 

 true that life is dependent upon antecedent life. An 

 illustration of this principle is seen in the case of nutri- 

 tion, for no Kving thing, neither plant nor animal, can 

 utilize directly as food the unaltered inorganic elements 

 and compounds derived from the air and soil. This was 

 clearly shown in Chapter VII. While plants are com- 

 monly said to obtain their food from the air and soil, 

 strictly speaking this is not true. Carbon, oxygen, 

 hydrogen, phosphates, nitrates, sulphates, et cetera are 

 no more plant food than are flour, water, sugar, and salt 

 bread. Just as the elements composing bread would, 

 if eaten separately, make a very unsavory and poor diet, 

 so the inorganic elements and compounds, as such, would 

 not be able at all to nourish plants. They must first be 

 broken down (if compounds), and then recombined into 

 the organic compounds of carbohydrates, proteins, and 

 fats. 



301. — ^Necessity for Chlorophyll. — As we have seen in 

 Chapter VII, this recombination of inorganic chemical 

 elements into organic compounds is the function of 

 chlorophyll. Thus it is that green plants are absolutely 

 essential to all life, and as we learned in Chapter XIX, 

 this explains why all non-green plants are found only in 

 intimate association, either with living green plants or 

 with the organic remains or products of other living 

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