Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
When a meristematic tissue cambium is present inside a vascular bundle, the bundle is said to be
(a) conjoint
(b) open
c) closed
(d) collateral
Answer:(b) open
cambium
👉The meristem that occurs in the mature regions of roots and shoots of
many plants, particularly those that produce woody axis and appear
later than primary meristem is called the secondary or lateral meristem.
They are cylindrical meristems. Fascicular vascular cambium,
interfascicular cambium and cork-cambium are examples of lateral
meristems. These are responsible for producing the secondary tissues.
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👉The Vascular Tissue System
The vascular system consists of complex tissues,
the phloem and the xylem.The xylem and
phloem together constitute vascular bundles
(Figure 6.5). In dicotyledonous stems, cambium
is present between phloem and xylem. Such
vascular bundles because of the presence of
cambium possess the ability to form secondary
xylem and phloem tissues, and hence are called
open vascular bundles. In the monocotyledons,
the vascular bundles have no cambium present
in them. Hence, since they do not form secondary
tissues they are referred to as closed. When
xylem and phloem within a vascular bundle are
arranged in an alternate manner along the
different radii, the arrangement is called radial
such as in roots. In conjoint type of vascular
bundles, the xylem and phloem are jointly
situated along the same radius of vascular
bundles. Such vascular bundles are common
in stems and leaves. The conjoint vascular
bundles usually have the phloem located only
on the outer side of xylem.
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👉Dicotyledonous Root
Look at Figure 6.6 (a), it shows the transverse
section of the sunflower root. The internal tissue
organisation is as follows:
The outermost layer is epiblema. Many of
the cells of epiblema protrude in the form of
unicellular root hairs. The cortex consists of
several layers of thin-walled parenchyma cells
with intercellular spaces. The innermost
layer of the cortex is called endodermis.
It comprises a single layer of barrel-shaped
cells without any intercellular spaces. The
tangential as well as radial walls of the
endodermal cells have a deposition of
water-impermeable, waxy material suberin
in the form of casparian strips. Next to
endodermis lies a few layers of thick-walled
parenchyomatous cells referred to as
pericycle. Initiation of lateral roots and
vascular cambium during the secondary
growth takes place in these cells. The pith
is small or inconspicuous. The
parenchymatous cells which lie between
the xylem and the phloem are called
conjuctive tissue. There are usually two
to four xylem and phloem patches. Later,
a cambium ring develops between the
xylem and phloem. All tissues on the
innerside of the endodermis such as
pericycle, vascular bundles and pith
constitute the stele.
👉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 dicotyledonous
plants exhibit an increase in girth. This
increase is called the secondary growth.
The tissues involved in secondary growth
are the two lateral meristems: vascular
cambium and cork cambium.
👉Vascular Cambium
The meristematic layer that is responsible
for cutting off vascular tissues – xylem and
pholem – is called vascular cambium. In
the young stem it is present in patches as
a single layer between the xylem and
phloem. Later it forms a complete ring.
👉Formation of cambial ring
In dicot stems, the cells of cambium present
between primary xylem and primary
phloem is the intrafascicular cambium.
The cells of medullary rays, adjoining these intrafascicular cambium become
meristematic and form the interfascicular cambium. Thus, a continuous
ring of cambium is formed.
👉Activity of the cambial ring
The cambial ring becomes active and begins to cut off new cells, both
towards the inner and the outer sides. The cells cut off towards pith,
mature into secondary xylem and the cells cut off towards periphery
mature into secondary phloem. The cambium is generally more active
on the inner side than on 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 phloems 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 directions. These are the secondary
medullary rays (Figure 6.9).
👉Spring wood and autumn wood
The activity of cambium is under the control of many physiological and
environmental factors. In temperate regions, the climatic conditions are
not uniform through the year. In the spring season, cambium is very
active and produces a large number of xylary elements having vessels
with wider cavities. The wood formed during this season is called spring
wood or early wood. In winter, the cambium is less active and forms
fewer xylary elements that have narrow vessels, and this wood is called
autumn wood or late wood.
The spring wood is lighter in colour and has a lower density whereas
the autumn wood is darker and has a higher density. The two kinds of
woods that appear as alternate concentric rings, constitute an annual ring.
Annual rings seen in a cut stem give an estimate of the age of the tree.
👉Cork Cambium
As the stem continues to increase in girth due to the activity of vascular
cambium, the outer cortical and epidermis layers get broken and need to
be replaced to provide new protective cell layers. Hence, sooner or later,
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 impervious to water due to suberin deposition in the cell wall.
The cells of secondary cortex are parenchymatous. Phellogen, phellem,
and phelloderm are collectively known as periderm. Due to activity of
the cork cambium, pressure builds up on the remaining layers peripheral
to phellogen and ultimately these
layers die and slough off. Bark is a
non-technical term that refers to all
tissues exterior to the vascular
cambium, therefore including
secondary phloem. Bark refers to a
number of tissue types, viz.,
periderm and secondary phloem.
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. Name the
various kinds of cell layers which
constitute the bark.
At certain regions, 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 lensshaped
openings called lenticels.
Lenticels permit the exchange of
gases between the outer atmosphere
and the internal tissue of the stem.
These occur in most woody trees
👉Secondary Growth in
Roots
In the dicot root, the vascular
cambium is completely secondary in
origin. It originates from the tissue
located just below the phloem
bundles, a portion of pericycle tissue,
above the protoxylem forming a
complete and continuous wavy ring,
which later becomes circular (Figure
6.11). Further events are similar to
those already described above for a
dicotyledon stem.
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