STEMS, ROOTS, AND LEAVES 



111 



The pericyde, which is the layer of cells just beneath the 

 endodermis, is very important in roots, since it is the place of 

 origin of cork and of lateral roots. When older roots begin to 

 form cork, it does not arise from the epidermis or from the cor- 

 tex, as in stems, but from the pericycle. The cells of the peri- 

 cycle therefore act as a cork cambium and thus originate cork 

 layers within the root by cell 

 division and cell differentia- 

 tion into cork. When one or 

 more cork layers are formed, 

 the cortex and the epidermis 

 dry up and peel off, leaving 

 the cork as the outer brown 

 bark of the root. 



Lateral roots also arise 

 from the pericycle and bore 

 their way through the cortex 

 and epidermis. They thus 

 differ in their origin from 

 the leaves and branches, 

 which arise by the division 

 and differentiation of cells 

 in the epidermis and outer 

 cortex. 



In young roots the xylem 

 and phloem have the ar- 

 rangement which is repre- 

 sented in the figure. The 

 xylem occupies the center of the root, thus excluding a definite 

 central pith. The phloem alternates with the radiating arms of 

 the xylem and so gives rise to what is often termed the radial 

 arrangement of phloem and xylem. 



The cambium arises outside of the xylem and inside of the 

 phloem masses. Its general method of developing tissues by cell 

 division is identical with that of the cambium of stems, already 

 described. The ultimate structure of old roots is therefore very 

 similar to that of stems, except that the center of a mature root 



FIG. 59. Root system of a plant of red 

 clover ( Trifolium pratense) 



Note the tendency of the main roots to pene- 

 trate to great depths in the soil 



