8 ELEMENTS OF BOTANY. 



soil, never elongates by intercellular growth, but only increases its length 

 by additions to its growing extremity. Its increase in diameter is accom- 

 plished in the same manner as that of the stem. 



Without entering too minutely into the structure of 

 the growing extremity of the root, it is necessary to ob- 

 serve that this is made up of a mass of cells which mul- 

 tiply rapidly and are constantly building up tissue (Fig. 14) 

 in a forward direction. They are, moreover, endowed with 

 the power of rapid absorption, and are, in fact, the innu- 

 merable mouths which feed the growing plant. In order to 

 increase the absorbing surface of the roots, the younger 

 ones are provided with elongated cells, commonly called 

 growing eltrTmfty root-hairs. Though of microscopic size, these are pro- 

 of a root, with root- duced in such infinite numbers as to be of immense ser- 

 hairs. Magnified. vice> ^ h owever> they are only required during the pe- 

 riod of active growth, they are not evident in autumn after the season's 

 work is accomplished. 



FUNCTIONS OF ROOTS. 



The mechanical functions of the roots in supporting the stem have 

 already been alluded to. We have now to consider their other important 

 offices. 



As animals feed upon plants, so plants feed upon minerals. And as 

 they are not provided with organs for the mastication of their food, they 

 must necessarily receive it in a state of subdivision suited to their needs 

 and powers of absorption. In this form they find it in the damp earth 

 which their roots penetrate. Every rootlet is a seeker for food and every 

 growing cell is hungry. Water, with gases and mineral salts in solution, 

 is greedily sucked up and carried toward the sunlight for elaboration. 



Roots are, moreover, endowed, to some extent, with the power of se- 

 lecting the proper nourishment for the plant which they feed, and they 

 will take this or nothing. The most careless farmer has learned that he 

 cannot successfully raise the same crop on a field year after year without 

 fertilizers. The explanation is simple. Suppose corn be planted year 

 after year in the same ground. The roots of each succeeding crop find 

 less and less nourishment, until finally partial or complete starvation re- 

 sults. And this occurs, too, while there may be still plenty of food fit for 

 other plants. 



Hence the rotation of crops, a principle at the foundation of successful 

 farming, depends upon the selective powers of roots. 



But all plants are not fixed in the soil and do not draw their nourish- 

 ment from it. Many aquatic plants float in the water ; these find their 

 proper food in that element. Others are parasitic upon the stems or roots 



