Page 106 - Pharmacognosy 2 PG303

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

C- Xylem:
It is generally lignified, mostly composed of primary elements in herbaceous stems which
contain vessels with various thickened walls and have a relatively large diameter than other
elements of wood but may also show a little number of secondary elements. In Dicotyledons,
vessels are arranged in more or less radial rows while in Monocots, xylem vessels are few and
forming y- shaped. The elements of xylem include vessels of various sizes and types of thickenings
(small spiral and annular for protoxylem, and large reticulate and pitted for metaxylem), wood
fibers, wood parenchyma and tracheids. In stems, xylem is always endarch (i.e. with protoxylem
directed towards the center).

3- Pith (Medulla):
It is the central ground tissue of the stem. It is of parenchymatous nature in young stems,
but usually lignified formed of thin- walled pitted cells e.g., Lobelia or thickened cells in old stems.
The cells may contain starch (as in Senna)., mucilage (as in Sassafras), tannin or calcium oxalate
(e.g. clustes in Cannabis and Senna, microsphenoid as in Belladonna). The pith may show the
following structures:
- Sclerenchymatous cells isolated or in- groups.
- Secretary elements as in Umbelliferae.
- Perimedullary phloem: small phloem patches at the periphery of the pith. Its existence is a
marker of certain families e.g. Solanaceae, Euphorbiaceae and Apocynaceae.

Secondary thickening:
1- In Monocots:
The stem increases in diameter due to the expansion of the permanent primary elements
and/ or increase in number of vascular bundles.

2- In Dicots:
The activation of some primary medullary ray cells leads to the formation of interfascicular
cambium which produces new secondary xylem inward and secondary phloem outward in the form
of continuous ring of vascular tissues. These secondary vascular tissues show newly formed
parenchymatous strands running to varying distances into the phloem and xylem which is called
the secondary medullary rays. Due to the increase in diameter of the stem by the formation of the
secondary vascular elements, the outer protective layers subject to rupture, a process which leads
to the formation of another secondary cambium (phellogen) in the cortex with the subsequent
formation of periderm including the cork and phelloderm. The remaining epidermis in herbs means
no or very little secondary thickening occurred.

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