THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 39 



with bases in the earth's crust, and life, without any source of carbon, must 

 become extinct. 



Hydrogen exists almost exclusively in the form of water. In this form 

 it is taken up by plants and animals, with the exception of a small proportion 

 absorbed in the form of ammonia. In this form too it is discharged by living 

 organisms. Oxygen is the only element which, in all the higher organisms at 

 any rate, is taken up in the free state. It forms one-fifth of the atmosphere 

 and, as the oxides of the various metals, a considerable fraction of the earth's 

 crust. It takes a position apart from the other focd-stuffs in that its 

 presence is the essential condition for the utilisation of their potential energy. 

 In the living cells it combines with the oxidisable compounds formed by the 

 agency of the living protoplasm, with the production of carbon dioxide and 

 water, and the evolution of energy. This process is spoken of as respira- 

 tion. 



Like the three elements we have already considered, nitrogen is also 

 derived directly or indirectly from the surrounding atmosphere. In conse- 

 quence of its feeble combining power for other elements and the instability of 

 its compounds, very little nitrogen is to be found in the combined state in the 

 earth's crust, whereas it constitutes four-fifths of the atmospheric gases. 

 It can be taken up by most plants only in the form of ammonia, nitrites, 

 or nitrates. To animals these compounds are useless, and their only 

 source of nitrogen is the protein which has been built up by the agency 

 of the plant cell. Since nitrogen in the free state is useless to nearly all 

 living organisms, the existence of life must depend on the amount of com- 

 bined nitrogen which is available. In view oj: the small tendency presented 

 by this element to enter into combination, it becomes interesting to inquire 

 into the source of the combined nitrogen which is the common capital of the 

 living kingdom. There are certain cosmic factors which result in the pro- 

 duction of combined nitrogen. The passage of electric sparks or of the 

 silent discharge through moist air leads to the production of ammonium 

 nitrite. 



N 2 + 2H 2 = NH 4 N0 2 . 



Every thunderstorm therefore will result in the production of small quan- 

 tities of ammonium nitrite, which will be washed down with the rain and 

 serve as a source of combined nitrogen to the soil. Every decaying vegetable 

 or animal tissue serves as a source of ammonia, so that from various causes 

 the soil may contain nitrogen in the form of ammonia or of ammonium nitrite. 

 These forms .of combined nitrogen are not however suitable for all classes of 

 plants. Most moulds can assimilate ammonia as ammonium carbonate or 

 as amino-acids or amines, provided that they are supplied at the same 

 time with sugar, the oxidation of which will serve them as a source of energy. 

 Some moulds, many of the higher plants, and especially the Graminese, 

 which include the food-producing cereals, require their nitrogen in the 

 condition of nitrates. It is necessary therefore that the ammonia or 

 nitrites in the soil shall be converted into this highly oxidised form. This 



