Page 105 - Pharmacognosy 2 PG303

Pharmacognosy-2 (PG303) Level 2 Clinical Pharmacy-Pharm D

2- Vascular tissues: It is formed of a number of vascular bundles (VB) whose their types can
be determined according to relatives position of xylem and phloem as follow:
a- Collateral VB: when xylem and phloem patches are lying on the same radius. The VB
is described as open when cambium exists between xylem and phloem (as in Dicots)
or closed on the absence of cambium (as in Monocots).
b- Bicollateral VB: when xylem occurs between two patches of phloem on the same
radius. In this type of the outer phloem is separated from xylem by cambium, whereas
the inner one is separated from protoxylem by ordinary parenchyma which may
become meristematic as in Cucurbitaceae.
c- Concentric VB: xylem is concentrated in the center and surrounded by phloem
(vasicentric) e.g., Male fern or vice versa (Amphivasal).
d- Radial VB: xylem and phloem patches are on different radii (as in roots).

A- Phloem:
It is mostly of the primary type in herbaceous stems but become accompanied with
secondary elements in woody stems. Sometimes, it becomes absorbed and disappears totally and
occasionally replaced by fibers. Generally, it is composed of sieve tubes which may be
accompanied by companion cells and phloem parenchyma. It may contain phloem fibers, secretory
structures and idioblasts.

B- Cambium:
Its existence is a marker of open VBs. It is formed of several layers of brick-like, thin-
walled, non- vasculated cells with dense protoplasm. It may be either intrafascicular (in- between
xylem and phloem of each VB) or interfascicular (crossing M.R. and forming continuous ring).

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