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 living 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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