STRUCTURE 



291 



into the elongating region, increase actively in length 

 by the formation and enlargement of the vacuole, and 



a 



m 



p 



r.c 



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

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

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

 cylinder; px., protoxylem; c, cortex; p.l., piliferous layer ; r.h.,Toot 

 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 ; 

 cort, end/,, Xg,, p.mt, r.Ct, 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 



