48 Elementary Biology. 



carbonic acid, and oxy-salts; the source of the hydrogen 

 is water, which is decomposed in the chlorophyll -bearing 

 cells in presence of sunlight ; sulphur is absorbed in the 

 form of soluble salts ot sulphuric acid ; the other elements 

 also are absorbed in the form of various salts from the soil 

 or other medium in which the plant lives. The various 

 sources of these different substances and the changes which 

 they undergo in the phnt organism will be dealt with more 

 fully afterwards (p. 188). Reference is made to the subject 

 at this point chiefly to show what are the main conditions 

 of life in regard to food-supply. 



In section iv it was pointed out that in consequence of 

 the relationship of the plant world to carbonic acid and cf 

 the animal world to oxygen, a balance of gases was main- 

 tained in the atmosphere. Having now reviewed generally the 

 condition of environment necessary for the maintenance 

 of life, we shall be able to understand more readily the 

 importance of the subject of gaseous balance in the air. 



SECTION VI. THE BALANCE OF NATURE. 



In explaining what is meant by the balance of nature it 

 will be necessary to repeat certain statements already made, 

 with the view of explaining the relationship in- which they 

 stand to each other. 



We have seen that animals and plants or parts of plants not 

 possessed of chlorophyll require organic compounds for their 

 sustenance, as well as salts and water, the organic compounds 

 being obtained directly or indirectly from the vegetal world. 

 They remove oxygen from the atmosphere, and return 

 carbonic acid with the evolution of energy ; i.e. the solar 

 energy originally stored in the organic compound manifests 

 itself in the various phenomena of life, ultimately becoming 

 dissipated as heat into space, the products of disintegration 

 being at the same time returned to the inorganic world. 



On the other hand we have seen that plants possessed of 



