Biology Today — December 2017

Epidermis

Single outermost layer with thick cuticle

Cortex

Several layered

parenchymatous cells

Hard bast

The distinct sieve tubes and associated

cells of primary phloem are capped by a

patch of sclerenchyma called hard bast. Cork cambium

Arises in primary phloem and cut off cork layers

Bar of sclerenchyma on outside.

Secondary phloem Cambium

Consists of bands of sieve tubes, companion

cells and parenchyma alternating with

tangential bars of sclerenchyma. Primary ray

Broad, multiseriate, separating the vascular bundles

Pith

(Large and parenchymatous)

Fig.: Secondary growth in stem of Vitis vinifera

Primary xylem

Secondary ray

Distinct and extend upto

secondary phloem.

Secondary xylem

Consists of a series of wide vessels

and some amount of parenchyma.

Cork

ANOMALOUS SECONDARY GROWTH

The term anomalous secondary growth is used to indicate the forms of cambial activity that deviate from commonly occurring
secondary growth. The anomalies may be in growth in thickness, resulting in peculiar structures of xylem and phloem. Anomaly
may arise on account of abnormal activity of normally positioned cambium or normal activity of abnormally positioned cambium
anomalous growth varies considerably in different plants.

Anomalous Secondary Growth in Dicots

Anomalous growth in dicot root

Secondary growth begins with the formation of primary cambium. It is formed from the parenchyma cells between the
xylem and phloem groups except opposite the two protoxylem groups where it arises from pericycle. This cambium produces a
ring of closely arranged collateral vascular bundles. It ceases to function. The second cambial ring now develops from phloem
parenchyma outside the first cambial ring. It forms secondary collateral vascular bundles. These are separated by radially formed
secondary parenchyma. This cambial ring also ceases to function. The third ring of cambium is now produced from the pericycle.
At this stage, pericycle divides and becomes many layered from which cambial rings develop successively. As a result, ring of
vascular bundles alternating with storage parenchyma are formed. All the cambia are normal in behaviour producing xylem and
phloem in usual way but the outer cambia are progressively less active than the inner cambia. Consequently thinner rings appear
in outward direction.

Periderm

First layer of

secondary xylem

Growth layers

Xylem

Fig.: T.S. of root of Beta vulgaris (Diagrammatic)

Fig.: Detailed structure of a part of T.S. of root of Beta vulgaris

Storage parenchyma

Secondary xylem

Secondary

phloem

Cambium