6o 



ROOT FUNCTIONS 



other trees where they germinate, the roots penetrating the 

 branches and withdrawing the necessary foods. The mistletoe 

 withdraws largely crude materials from the branch because it 

 has pale green leaves and is therefore capable of manufacturing 

 some food on its own account. Our common dodder (Cuscuta) 

 is entirely dependent upon other plants and illustrates the results 

 that follow from the disuse of any function. It has' lost its chloro- 

 plasts, the leaves -are reduced to inconspicuous scales and the stem 

 resembles a coarse yellow string while the root has disappeared 



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



entirely. The seeds produce a small thread-like shoot which soon 

 perishes unless it comes in contact with a plant upon which it 

 can live. These thread-like plants are sensitive to touch and so 

 coil about the branches of any plant that they may chance to hit 

 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. 

 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 



