Page 160 - Pharmacognosy 2 PG303

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

year corm; p. c, present year corm); V, Stem tuber of potato; Vl,
rhizome of Conna; VII, bulb, of Allium (d. st. ; discoid stem; t. b,
terminal bud; a. b, axillary bud); VIII, sucker of Mentha.

In many pteridophytes, the subterranean stem has a dictyostele showing
transverse section, several separate meristeles, each consisting of a concentric
vascular strand surrounded by a distinct endodermis as in filix mas.
Sclerenchymatous hypodermis is of common occurrence but usually not lignified.
The tegumentary tissue is either consisting of few regular layers of cork cells, but
with no rhytidoma as in Liquorice, or in some cases, of irregularly arranged
suberised cells derived from the outer layers of the cortex forming the metaderm
as in White and Black Hellebore. Sometimes the outer covering is formed of
the epidermis as in Couch grass, Colchicum corm, Filix mas (cork around root
scars only), Podophyllum (cork of one layer beneath the epidermis).

Secretory structures are frequently present e.g. secretory ducts in arnica,
secretion cells in Ginger, laticiferous vessels in Dandelion rhizome etc.

Subterranean Roots
The root is that portion of the plant axis which in seedlings, grows vertically

downwards into the soil away from light. It does not show nodes and interodes
and does not bear leaves or buds. Its growing point is covered by a special tissue
called' root-cap or calyptra. The root bears only one kind of lateral appendages i.e.
branches which are similar in general characters and in structure to the main root.
The origin of branches is described as endogenous, because the growing point
arises in the outermost layer of the stele, in the pericycle, and the branch bores
its way through the cortical tissues by the secretion of enzymes. The branching of
the root is usually lateral, very rarely dichotomous as in Lycopodinae. The lateral
branches are, as a rule, thinner and grow less strongly than the parent root.

A root is usually constructed of four zones. These are the growing point,
the zone of elongation, the zone of absorption and the zone of lateral branches.
The growing point is usually about one millimeter long and is covered by the root
cap which is usually darker in color and slimy, helping the root to penetrate between
the soil particles. The zone of elongation behind the growing point, is smooth and
free of any outgrowths, and in this zone the increase of length of the root takes
places. The zone of absorption (the zone of root hairs), which are selender,
unicellular, un-branched prolongation of the cells of the superficial layer. The zone
of the lateral branches occupies the majority of the root. There are several types
of roots.

The most important of these are:
1. Primary roots: these are developing into tap roots as in senega,

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