NATURE OF PLANTS 



57 



woody plants, as the beech drop, orobranches, broom rape, etc. 

 One of the most remarkable features of roots and one of the 

 most important in the economy of the earth is found in that large 

 family of bean or leguminous plants that are characterized by 

 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 



FIG. 38. Tubercles formed on roots of lupine by nitrogen fixing bacteria. 



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 per cent, of this gas, the plant has no way of utilizing it 

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

 Compounds of 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 practically consumed ; and the saltpeter beds of Peru 

 and Chile, which will be exhausted by 1925. Consequently wide- 

 spread alarm has arisen lest famines ultimately result through 

 lack of these nitrogen compounds, and the situation becomes the 



