194 



Science of Plant Life 



Roots in relation to bacteria and fungi. The roots of many 

 plants have bacteria or fungi growing about them or inside 



them. The best-known crop 

 plants belonging to this group 

 are the clover, cowpea, and 

 alfalfa; their roots develop 

 small nodules in which cer- 

 tain kinds of bacteria change 

 nitrogen of the air into nitro- 

 gen compounds which may 

 be used by the plants. More 

 information about these bac- 

 teria wdll be found in a later 

 chapter (page 258). 



Many of our trees and 

 shrubs have fungi surround- 

 ing their roots. The beech 

 tree, for example, flourishes 

 only when it grows under 

 such conditions. The difficulty in transplanting azaleas, 

 laurels, and rhododendrons from the woods to our lawns lies 

 largely in supplying conditions favorable to the fungi that 

 invest the roots. It is easy to supply the proper shade and 

 water conditions for the shrubs,' but it is difficult to furnish 

 soil conditions favorable to the life of the fungi. The trans- 

 planting of these shrubs is therefore most frequently success- 

 ful when they are planted in large bodies of soil brought with 

 them frorn their natural habitat. Just how the fungi aid the 

 plant is not understood ; that they are essential is very clear. 

 Commercial uses of roots. The fleshy roots of the sweet 

 potato, yam, turnip, carrot, beet, celeriac, and salsify are 



Fig. 118. Roots of soy bean, showing nodules 

 containing nitrogen-fixing bacteria. 



