DIFFERENTIATION OF TISSUES 



295 



bundle structure of the root appears only in young and 

 active roots. In older ones certain secondary changes take 

 place which obscure the structure and result in a resem- 

 blance to the stem. 



The origin of branches in roots is also peculiar. In 

 stems branches originate at the surface, involving epi- 

 dermis, cortex, and vascular bundles, such an origin being 

 called exogenous ("produced outside 7 '); but in roots 

 branches originate on the vascular cylinder, burrow through 

 the cortex, and emerge at the surface (Fig. 277). If the 

 cortex be stripped off from a root with branches, the 

 branches are left attached to the woody axis, and the cor- 

 tex is found pierced with holes made by the burrowing 

 branches. Such an origin is called endogenous, meaning 

 " produced within." 



To sum up the peculiarities of the root, it may be said 

 to develop a root-cap, to have a solid vascular cylinder in 

 which the xylem and phloem are arranged to form a bundle 

 of the radiate type, and to branch endogenously. 



FIG. 2~8. A section through the leaf of lily, showing upper epidermis (ue), lower epi- 

 dermis (le) with its stomata (st). mesophyll (dotted cells) composed of the palisade 

 region (/>) and the spongy region <'/?/?) with air spaces among the cells, and two 

 veins (v) cut across. From "Plant Relations.'" 



