DIFFERENTIATION OF TISSUES 



295 



in young and active roots. In older ones certain secondary 

 changes take place which obscure the structure and result 

 in a resemblance to the stem. 



The origin of branches in 

 roots is also peculiar. In stems 

 branches originate at the sur- 

 face, involving epidermis, cor- 

 tex, and vascular bundles, such 

 an origin being called exogenous 

 (" produced outside ") ; 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 cortex is found 

 pierced with holes made by the burrowing branches. Such 

 an origin is called endogenous, meaning " produced within.'' 



Fig. 277. Endogenous origin of 

 root branches, showing them (a) 

 arising from the central axis (/> 

 and breaking through the cortex 

 (r).— After Vines. 



< 4 



St <3t 



Fig. :J78. A section through the leaf of lily, showing upper epidermis (««), lower epi- 

 dermis Ue) with its stomata (»*0. mesophyll (dotted cells) composed of the palisade 

 region (p) and the spongy region 0/>> with air spaces among the cells, and two 

 veins (r) cut across.— From " Plant Relations." 



