144 PHARMACEUTICAL BOTANY 
phloem while in Angiosperms sieve tubes, companion cells, 
phloem parenchyma, bast fibers, secretion cells and stone cells 
may occur in this region. 
PriMARY PHLOEM (PROTOPHLOEM) is the phloem formed by 
the plerome and occurs directly beneath the pericycle. 
Fic. 82.—A Polyarch Radial Bundle. Cross-section through a portion of a 
root of Acorus calamus, a monocotyledon. A, Cortical parenchyma; B, endodermis; 
C, pericycle; E, phloem; F, xylem. At Y, Y, are large tracheal tubes, which were 
- formed last, the narrow tubes near the periphery of the xylem being formed first. 
At the center of the root, within the circle of the polyarch, radial vascular bundle, 
occur thin-walled, parenchymatous pith cells. (From Sayre after Frank.) 
SECONDARY PHLOEM (METAPHLOEM) is phloem formed by 
the cambium. 
LeaF AND BrancH TRaces.—These are prolongations of the 
vascular bundles from the stele of the stem which extend into 
leaves and branches. (See Fig. 84.) 
THE STELE 
The STELE represents the central cylinder of a stem or root 
and includes all of the tissues interior to the cortex. 
