ROOTS 



67 



of leaves and steins, but the epidermal cells of the older roots are 

 cutinized to a greater or less degree. 



Growth. — The most active cell growth and cell division is 

 at the tip of the root just within or back of the root-cap. Back 

 of this point for a distance of 2 tO' 4 mm. the cell divisions are 

 less rapid, but the cell elongations are very pronounced. It is 

 the zone of rapid elongation 

 (Fig. 47). The growth of 

 the root tip is accompanied 

 by the addition of cells to the 

 inner surface of the root-cap, 

 thus compeoisating for those 

 on the outer surface, which 

 are gradually destroyed as 

 tbe tip advances througk the 

 soil. In the older roots a corh 

 cambium and a bark covering 

 are formed. This is very 

 common in the perennial 

 s b r u b s and trees. These 

 old roots have po-actically lost the power of absorption. 



Branching of Roots. — The new root branches originate quite 

 differently from the stem brancbes (page 54). They begin 

 in the cells just outside the xylem, pusb through the cortex, 

 gradually absorbing the older tissues and finally come to 

 the surface (Fig. 48). This is known as the endogenous method 

 of branchijig. 



Absorption. — Tbe root-hairs are formed with the growth and 

 elongation of the roots and are always rich in protoplasm. In 

 brief, they are ideal structures for tbe taking in of water and 



Fig. 47. — Germinating beans showing region of 

 elongation. 



