SECONDARY GROWTH
The growth of the roots and stems in length with the
help of apical meristem is called the primary growth.
Apart from primary growth, most dicot plants exhibit
the increase in girth. This increase is called secondary
growth.
Secondary Growth in Dicot Stem
In a dicot plant, secondary growth in stem occurs both in
the stele and in the cortex. The dicot stem, in its primary
state of growth contains narrow layers of intrafascicular
cambium in between the xylem and phloem.
The tissues involved in the secondary growth are the
two lateral meristems, i.e., vascular cambium and cork
cambium.
1. Vascular Cambium
The meristematic layer that is responsible for cutting off
vascular tissues such as xylem and phloem is called vascular
cambium. It is present in a patch of a single layer in young stem
which later on develops into a complete ring.
Formation of Cambium Ring
The parenchyma cells of the primary medullary rays adjacent to
the intrafascicular cambium undergo dedifferentiation and give
rise to interfascicular cambium. This joins the intrafascicular
cambium of either side to form a complete ring of meristem
called the cambium ring.
Activity of Cambium Ring
The cambium ring becomes active and begins to form new cells
both towards and inner and the outer sides. The cambium ring
is made up of two types of cells, ray initials and fusiform initials.
The cells added to the inner side of cambium ring by the
division of the fusiform initials gradually become the elements
of the secondary xylem. While, the cells added to the outer side
of the cambium become elements of the secondary phloem
While, the cells added by the division of ray initials to the inside
well as outside become elements of the secondary medullary rays.
The cambium is generally more active on the inner side than the
outer. As a result, the amount of secondary xylem produced is
more than secondary phloem and soon forms a compact mass.
The primary and secondary phloem get gradually crushed due to
the continued formation and accumulation of secondary xylem.
The primary xylem however, remains more or less intact, in or
around the centre.
At some places, the cambium forms a narrow band of
parenchyma, which passes through the secondary xylem and the
secondary phloem in the radial direction. These are secondary
med’ ” Iry rays.
Springwood and Autumnwood
The activity of cambium is under the control of many
physiological and environmental factors. In temperate regions,
the climatic conditions are variable through the year.
In springs, cambium is very active and produces a large number
of xylary elements having vessels with wider cavities. The wood
formed in this season is called spring wood or early wood.
In autumn, the cambium is less active and forms few xylary
elements that have narrow vessles. Thus, the wood formed is
called autumnwood or latewood.
2. Cork Cambium
The stem continues to increase in girth due to the
activity of vascular cambium. Due to this, the outer
cortical and epidermis layers are broken down and needs
to be replaced to provide new protective cell layers.
Therefore, another meristematic tissue called cork
cambium or phellogen develops usually in the cortex
region.
Phellogen is a couple of layers thick. It is made of
narrow, thin-walled and nearly rectangular cells.
Phellogen cuts off cells on both sides. The outer cells
differentiate into cork or phellem while the inner cells
differentiate into secondary cortex or phelloderm.
The cork is impermeable to water due to suberin
deposition in the cell wall. The cells of secondary cortex
are parenchymatous. The phellogen, phellem and
phelloderm are collectively known as periderm.
Bark
Bark is a non-technical term used to describe all tissues
exterior to the vascular cambium, therefore including
secondary phloem. The bark refers to a number of tissues,
i.e., periderm and secondary phloem.
The bark that is formed early in the season is called early
or soft bark. Towards the end of the season, late or hard
bark is formed.
Lenticels
At certain regions of stem, the phellogen cuts off closely
arranged parenchymatous cells on the outer side instead
of cork cells.
These parenchymatous cells soon rupture the epidermis,
forming a lens-shaped openings called lenticels. The
lenticels are mostly found in woody trees.
Function
The lenticels permit the exchange of gases between the
outer atmosphere and the internal tissue of the stem.
Secondary Growth in Roots
The secondary growth in the root is the thickness
due to the formation of secondary tissues by lateral
meristems. With the exception of some annuals,
most of the dicots and gymnosperms show
secondary growth in their roots. Monocot roots
do not undergo any secondary growth. It occurs
by the reappearance of two types of secondary
tissues, i.e., the secondary vascular tissues called
cambium and periderm. These tissues are formed
by meristems are vascular cambium and cork
cambium, respectively.
i. Formation of Vascular Cambium
The conjunctive parenchyma cells, on the lateral
sides of the phloem bundles as well as pericycle
cells lying outside the protoxylem end becomes
brick-shaped and meristematic. These develop
into a wavy band of vascular cambium. The
vascular cambium of the root is a secon
meristem. It continues to form secondary xylem
on the inner side and secondary phloem on the
outer side.
Secondary phloem consists of sieve tubes,
companion cells, phloem parenchyma and phloem
fibres. The secondary xylem contains elements like
vessels, xylem parenchyma and xylem fibres.
Activity of Vascular Cambium
The vascular cambium derived from the pericycle
gives rise to only ray initial cells. The formation of
these ray cells is slower, than the formation of
secondary vascular tissues Due to this , the
depressed parts of vascular cambium move
outwardly and ultimately the cambium becomes
circular.
ii. Formation of Cork Cambium
The pericycle layer, after a few divisions becomes converted into a
secondary meristem called cork cambium or phellogen. The cells of
phellogen divide both towards the outside as well as inside.