ROOTS AND THEIR WORK 95 



Rotation of Crops. The facts mentioned above are 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 luxuriantly 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 

 nourishment to the young corn plants. This is known as rotation 

 of crops. The annual yield of the average farm may be greatly 

 increased by this means. 



Soil Exhaustion may be Prevented. Besides the rotation of 

 crops, other methods are used by the farmer to prevent the exhaus- 

 tion of raw food material from the soil. One method known as 

 fallowing is to allow the soil to remain idle until bacteria and oxida- 

 tion have renewed the chemical materials used by the plants. 

 This is an expensive method, if land is dear. The most common 

 method of enriching soil is by means of fertilizers, material rich in 

 plant food. Manure is most frequently used, but many artificial 

 fertilizers, most of which contain nitrogen, are used, because they 

 can be more easily transported and sold. Such are ground bone, 

 guano (bird manure), nitrate of potash, and many others. These 

 contain as well other important raw food materials for plants, 

 especially potash and phosphoric acid. Both of these substances 

 are made soluble so as to be taken into the roots by the action 

 of the carbon dioxide in the soil. 



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 de- 

 pendent on the needs of the plant, the roots acting to help it succeed in 

 certain localities. 



Food Storage in Roots and its Economic Importance. The use to the 

 plant of the food stored in the taproot may be understood if we take up 

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

 the end of the second year of its existence, its growth the first summer 

 forming the root we use as food. After forming seeds the plant dies. 



bers that they cannot do their work effectively. If then the soil is heated artificially 

 or treated with antiseptics so as to kill the protozoa, the bacteria which escape 

 multiply so rapidly as to make the land much richer than before. 



