SPERMATOPHYTES 



247 



bundles are formed (as in Rheum). It is interesting to note that these 

 dicotyledons with broken-up cylinders belong in the region of the Archi- 

 chlamydeae from which the monocotyledons are believed to have 

 arisen. 



Root 



The structure of roots is relatively uniform throughout vascular 

 plants, so that a general description may apply to all groups. 



Elongation. — The growing point of the root is not at the surface of 

 its tip, but just beneath, be- 

 ing covered by a tissue of 

 protective cells called the 

 root cap (fig. 552) . As a con- 

 sequence, the group of meri- 

 stematic cells within the root 

 tip forms four generative 

 regions: (i) dermatogen, (2) 

 periblem, and (3) plerome, 

 as in the stem; to these is 

 added (4) alyptrogen which 

 forms the root cap, the lat- 

 ter renewed from beneath as 

 it wears off outside (fig. 552). 



Root hairs. — Behind the 

 root cap the root hairs ap- 

 pear, which are produced by 

 the epidermal cells, and are 

 really enormous extensions 

 of the surface of epidermal 

 cells (figs. 553, SS4). Root 

 hairs are relatively short 

 lived, but new ones are 

 formed constantly as the 

 root elongates. 



The vascular system. — 

 The vascular anatomy of 

 the root is of the same general 

 type throughout vascular 

 plants. Thevascularcylinder 



Fig. 552. — Longitudinal section of root tip oi 

 spiderwort (Tradescatitia), showing root cap (c), 

 dermatogen (e), periblem (p), and plerome (J>1). — 

 After Coulter. 



