Ch. V, 2] 



STRUCTURE OF ROOTS 



217 



The most highly developed roots, those of our exogenous 

 trees, show three distinct though intergradient parts, — 

 viz. the slender white tips, the elongated fibers, and the 

 thick woody parts. 



The tips, best seen in material grown for the purpose in 

 moist air or moss, show really four parts (Fig. 159). First, 



Fio. 160. — The root-hair zones and growth zones in some common roots ; 

 X \. From the left, Pea, Radish, Corn, Lupine, and, below, Oats. The 

 seeds were germinated in moss behind sloping glass plates. 



the actual end of the root consists of a root cap, formed 

 from behind by the very delicate growth tissue, to which 

 it acts as a protection in the advance of the root through 

 the soil. Second, just behind the root cap lies a yel- 

 lowish spot, which marks the growing point, the place 



