NATURE OF PLANTS 6i 



flowers and fruit like those of the pea. Minute plants called 

 bacteria gain access to the roots of these plants through the root 

 hairs and cause wart-like enlargements, tubercles, on the roots 

 (Fig. 38). These bacteria have the power of combining the 

 nitrogen of the air with other elements, termed the fixation of 

 nitrogen, and so forming a nitrogen-bearing compound that can 

 be absorbed by the plant. Nitrogen is one of the very essential 

 elements required in the construction of the living substance of 

 the plant. Strangely enough, though the atmosphere contains 

 over 70 pel cent, of this gas, the plant has no way of utilizing it 

 directly but only in compounds, such as ammonia and nitrates, 

 etc. Compounds cf this kind exist in very meager quantities 

 upon the earth. The only sources of supply of any consequence 

 are the decaying animal and vegetable life and manures; the 

 guano deposits, now p.actically consumed ; and the saltpeter beds 

 of Peru and Chile, which will be exhausted by 1925. Conse- 

 quently widespread alarm has arisen lest famines ultimately result 

 through lack of these nitrogen compounds, and the situation be- 

 comes the more serious since from one half to two thirds of the 

 nitrogen compounds placed on the soils are lost annually in vari- 

 ous ways, especially by the leaching out of these substances by 

 rains and drainage waters, see page 47. Naturally of late years 

 much attention has been directed to devices for uniting the nitro- 

 gen of the air with other elements in order to find a substitute for 

 the rapidly disappearing nitrogenous compounds. Working upon 

 the fact that nitrogen compounds are formed in the atmosphere 

 by electrical discharges, several countries are now manufacturing 

 by electrical processes large quantities of nitric acid. A much 

 more serviceable source of nitrogen compounds for fertilizing pur- 

 poses has been discovered in the production of calcium cyanamide. 

 Calcium carbide (CaC2), which forms acetylene gas when 

 placed in water, is produced in electric furnaces by heating at a 

 high temperature coke, sawdust and burnt lime. When heated 

 red hot, CaC2 unites with nitrogen, forming calcium cyanamide 

 (CaCN2) and pure carbon. CaCN2 slowly gives off ammonia 

 and consequently when mixed with the soil it becomes an ex- 

 cellent fertilizer, being quite as valuable (weight for weight of 



