STRUCTURE 



291 



into the elongating region, increase actively in length 

 by the formation and enlargement of the vacuole, and 



FIG. 47. Diagrams illustrating the structure of the root, a, in 

 longitudinal section. A, apical region ; E, elongating region ; R, 

 root-hair region; y.c.. root cap ; p.m., primary meristem ; v.c., vascular 

 cylinder; px., protoxylem; c, cortex; >./., piliferous layer; r.h.,root 

 hairs, b, part of vascular cylinder (the conjunctive tissue ot 

 the cylinder is supposed to be transparent, the endodermis opaque). 

 C, the same with endodermis removed; end., endodermis; per., 

 pericycle ; ph., phloem ; px., protoxylem ; mx., metaxylem. d, Dia- 

 gram of cross-section of cylinder through origin of branch root ; 

 cor 6t end b , x b , p.m b , v.c b , cortex, endodermis, xylem, primary 

 meristem and root cap of branch root. On the left the primary 

 meristem of the opposite branch is just formed in the pericycle. 

 The root in b d is tetrarch, i.e. with 4 xylem and 4 phloem strands. 



gradually become differentiated into the various tissues. 

 The force of elongation of the cells in this region exerts 



