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