THE SOIL AND THE PLANT 



33 



This passage of liquids from one side of a membrane to 

 another is called osmosis, and has many uses in the plant 

 kingdom. We say a root takes nourishment by osmosis. 



SECTION X ROOT TUBERCLES 



Tubercle is a big word, but you ought to know how to 

 pronounce it and what is meant by root tubercles. 



We are going to tell you what a root tubercle is and 

 something about its importance to agriculture. When you 

 have learned this, we are 

 sure you will want to 

 examine some plants for 

 yourself in order that you 

 may see just what tuber- 

 cles look like on a real 

 root. 



Root tubercles do not 

 form on all plants that 

 farmers grow. They are 

 formed only on those 

 kinds that botanists call 

 legumes. The clovers, 

 cowpeas, vetches, and 

 alfalfa are all legumes. 

 The tubercles are little 

 knotty, wartlike growths FlG - 2 3- TUBERCLES ON THE ROOTS 



, . , OF A COWPEA 



on the roots of the plants 



just named. These tubercles are caused by bacteria, or 



germs, as they are sometimes called. 



