CHAPTER III 

 THE MATERIAL BASIS OF THE BODY 



SECTION I 



THE ELEMENTARY CONSTITUENTS OF 

 PROTOPLASM 



THE material basis of which living organisms are built up is derived from 

 the surrounding medium, and the elements which compose the framework 

 of the body must therefore be identical with those found in the earth's crust. 

 Not all the elements are so utilised in the formation of living matter. Every 

 living organism without exception contains the following elements : carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, potassium, 

 sodium, calcium, magnesium, and iron. In addition to these twelve elements 

 others are found in certain organisms, sometimes to a large extent, but it is 

 not known how far they are necessary to the proper development of these 

 organisms, and it is certain that they do not form an integral constituent 

 of all organisms. Of these elements we may mention especially silicon, 

 iodine, fluorine, bromine, aluminium, manganese, and copper. Dealing with 

 the first class, which includes those essential to all forms of life, we find that 

 their relative proportions in living organisms have little or no relation to 

 their proportions in the environment of the organisms. Their presence, 

 however, in the latter is a necessary condition of life. In the case of plants 

 which have a fixed habitat and cannot move in search of food, the growth 

 of the plant is limited by the amount of the necessary element which is 

 present in smallest quantities in the surrounding medium. This is what is 

 meant by the agriculturist's ' Law of the Minimum. ' Of the elements derived 

 from the earth's crust, those present in the smallest amounts in most soils 

 are potassium, nitrogen, and phosphorus. The growth of a crop in any given 

 soil is determined by the amount of that one of these three substances which 

 is present in smallest quantities, and the aim of agriculture is to supply to 

 every soil the ingredient thus present in minimal amount. 



Carbon forms the greater part by weight of the solid constituents of 

 living protoplasm. The proximate constituents of living organisms are 

 practically all carbon compounds, so that organic chemistry, which was 

 originally the chemistry of substances produced by the agency of living 

 organisms, has come to be synonymous with the chemistry of carbon com- 



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