Page 162 - Pharmacognosy 2 PG303

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

First, the cambium is originated in the parenchyma on the inside of the
phloem groups then in' the conducting parenchyma and lastly in the pericycle
outside the protoxylem; appearing when becoming continuous, as stellate or wavy
ring. The cambium gives to the inner side below the phloem more xylary tissue than
it gives phloem tissue to the outside, thus pushing the primary phloem outwards
and in many cases the primary xylem inwards, thus in later stage the cambium
becoming circular in form. Opposite each protoxylem group, the cambium usually
produces parenchyma only forming the primary medullary ray. With the
formation of many secondary tissues, the primary phloem is crushed and
becomes hardly distinguished or even totally absorbed or occasionally
transformed into sclerenchyma. At the same time a phellogen is developed in the
pericyclic region giving cork to the outside and large amount of phelloderm to the
inside, thus cutting off all the external tissues. As a result of this, the endoderims,
primary cortex and piliferous layer are thrown off and are absent in old roots. The
primary medullary rays usually become narrower except in certain herbaceous
plants where the primary rays remain very wide, dividing the secondary xylem to
separate distinct wedges.

Many variations from the normal type of structure may exist in subterranean
organs as follows:

In subterranean stems e.g. in Rhubarb abnormal bundles are produced in
the pith in the form of radiating structures called star spots consisting of a central
dark spot of collapsed perimedually phloem surrounded by an abnormally
developed cambium which gives phloem to the inside and xylem to the outside
with radiating slightly curved dark brown medullary rays.

Anomalous structure in subterranean Organs
In Roots e.g. In Aconite, the cambium retains the stellate form and only

little secondary xylem, but very wide phloem,consisting mostly of parenchyma
with little amount of sieve tissue are produced. It also shows wide medullary rays
and well developed parenchymatous pith. The tegumentary tissue in. Aconite is
not formed of cork cells, but of irregularly arranged suberised cells derived from
the outer layers of the cortex forming the metaderm. In Senega are present
irregularly wedged wood due to the formation of exceptionally wide one rarely
two or more parenchymatous medullary rays, while the secondary phloem is
abnormally developed at one place producing an external ridge, the keel. In
Belladonna and Gentian, the xylem is well developed'and constituting the main
part of the root, but is formed mostly of parenchyma with scattered vessels in
groups, as well as with several abnormally developed islands of interxyllary
phloem. In Jalap, tertiary cambia in the form of circles, curves or concentric rings,
producing only parenchyma on both sides, are devloped in the wide

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