NATURE OF PLANTS 65 



in their growth. If this plant happens to contain foods suitable 

 to the dodder then a second stimulus is aroused which causes root- 

 like branches to form. These organs penetrate the branches upon 

 which the dodder is growing and absorb foods from it. Many 

 plants live as parasites on the roots of a variety of herbaceous and 

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



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



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

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

 family of bean or leguminous plants that are characterized by 

 flowers and fruit like those of the pea. Minute plants, 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 ab- 

 sorbed by the plant. To give an idea of the importance and 

 work of these bacteria, it is estimated that an acre of clover 

 yielding two tons contains 100 Ibs. of nitrogen 74 Ibs. of which 

 has been derived from the air. The fixation of this amount of 

 nitrogen by the bacteria in the roots of the clover would nearly 

 suffice for the production of 50 bushels of corn and 30 bushels 

 of wheat. Nitrogen is one of the very essential elements re- 

 5 



