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, 554). 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. The vascular cylinder 
FIG. 552. Longitudinal section of root tip of 
spiderwort (Tradescantia), showing root cap (c), 
dermatogen (e), periblem (/>)> and plerome (^/). 
After COULTER. 
