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 forma- 

 tion of living matter. Every living organism without exception 

 contains the following elements : carbon, hydrogen, oxygen, nitrogen, 

 sulphur, phosphorus, chlorine, potassium, sodium, calcium, magne- 

 sium, 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 orga- 

 nisms, 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, i.e. those which are 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 sur- 

 rounding 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 which is 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, 



39 



