ROOTS AND THEIR WORK 



95 



it rate 5 x. 



plants mentioned are found little swellings or tubercles; in the 

 tubercles exist millions of tiny plants called bacteria, which 

 take out nitrogen from the at- 

 mosphere and fix it so that it 

 can be used by the plant ; that 

 is, they form nitrates for the 

 plants to use. These bacteria, 

 alone of all the living plants, 

 have the power to take the free 

 nitrogen from the air and make 

 it over into a form that can be 

 used by the roots. 



This fact is made use of by 

 careful farmers who wish to 

 make as much as possible from 

 a given area of ground in a given 

 time. Such plants as are hosts 

 for the nitrogen-fixing bacteria 

 are planted early in the season. 

 Later these plants are plowed in 

 and a second crop is planted. 

 The latter grows quickly and lux- 

 uriantly because of the nitrates 

 left in the soil by the bacteria 

 which lived with the first crop. 

 For this reason, clover is often grown on land in which it is pro- 

 posed to plant corn, the nitrogen left in the soil thus giving nour- 

 ishment to the young corn plants. The annual yield of the average 

 farm may be greatly increased by this means. 



Forms of Roots and their Relation to the Life of the Plant. — 

 Roots assume various forms. The form or position of the root is 

 usually dependent on the needs of the plant, the roots acting to 

 help it succeed in certain localities. 



Food Storage. — The use to the plant of the food stored in the 

 taproot may be understood if we think of the life history^ of the 

 parsnip. Such a plant produces no seed until near the end of 

 the second year of its existence. After forming seeds it dies. The 



Tubercles on clover roots. 



