ROOT. 



79 



the root. While the angular 'diver- 

 gence is thus quite regular, the longi- 

 tudinal intervals at which the branches 

 will be formed, which determines 

 their distribution along the length of 

 the root, are unequal (fig. 92). 



When secondary roots arise from 



j 



the shoot, they have a fixed relation 

 to the leaves, or they are formed 

 upon the buds produced in the axils 

 of the leaves, or they may arise at 

 indefinite points along the internodes. 

 In the first case, roots may be pro- 

 duced either opposite a leaf, or in 

 pairs, right and left of the base of the 

 leaf. 



95. Origin. --The origin of root- 

 branches and of secondary roots is 

 rarely exogenous ; that is, the root is 

 not commonly produced by the divi- 

 sion of cells which lie upon the sur- 

 face of a member. In the great 

 majority of cases the origin of the 

 roots is endogenous ; that is, the for- 

 mation of the root is begun by the 

 division of cells lying in the interior 

 of the member producing it. In most 

 cases these divisions begin very near 

 to the surface of the stele, either just 

 without it, in the endodermis, or just 

 within it, in the pericycle. Soon a 

 growing point is formed (fig. 93). 

 The rootlet is thus in its early stage 

 completely hidden, being buried 



FIG. 92. Seedling pea, showing 

 three vertical ranks of branch- 

 es along the main root. These 

 are numbered i, 2, 3. Natural 

 size. After Frank. 



