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Anatomy of Flowering Plants
The Meristematic Tissues


A tissue is a group of cells having a common origin and performing the same function
...
These buds are capable of forming branches and flowers
...




Following division in both primary and secondary meristems, the new cells so formed lose
the capacity to divide and become permanent tissues
...




Consists of simple tissues such as parenchyma, sclerenchyma, or collenchyma
...




Vascular bundles = Xylem + Phloem



There are two types of vascular bundles:



Xylem and phloem can be arranged in two different kinds of arrangement within a vascular
bundle
...
Such arrangement usually has phloem located outer to xylem
...


Pericycle

Initiation of lateral roots and vascular
cambium during the secondary
growth takes place in these cells
...
On maturity, a cambium ring develops between xylem and phloem
...

Anatomy of Stem
Dicot and Monocot Stem

Tissue
organization

Dicot stem

Monocot stem

Epidermis

Covered by cuticle

Same as dicot stem

Cortex
(divided into
three parts)

Contains few layers of
collenchymatous cells

Figure



Hypodermis



Cortical layer



Endodermis

Pericycle

Contains parenchymatous cells
with conspicuous intercellular
spaces

Contains few layers of sclerenchymatous cells
Rich in starch

Rich in starch

On the inside of endodermis and
above the phloem in the form of
semi-lunar patches of
sclerenchyma

Same as dicot stem

Vascular
bundle

These are arranged in a ring
...
They are
conjoint, open, and have endarch
protoxylem
...

Phloem parenchyma is absent and water containing
cavities are present
...

Bulliform cells (modified epidermal cells) are present
along the vein which absorb water and get turgid
...

Lateral meristems consist of:



Vascular cambium



Cork cambium
Vascular Cambium



Meristematic layer responsible for vascular tissue formation



In the young stem, it is present in patches between the xylem and the phloem; later, it
forms a ring



Formation of the cambial ring:



Intrafascicular cambium − cells of the cambium present between primary xylem and
primary phloem in a dicot stem



Cells of medullary rays adjoining the intrafascicular cambium also become meristematic,
and form interfascicular cambium thereafter
...




Activity of the cambial ring:



Cambial ring cuts off cells both on the inner and the outer sides; generally active on the
inner side



Cells towards the pith form the secondary xylem, and those towards the periphery form the
secondary phloem
...




This may lead to the crushing of phloem at some places due to continuous accumulation of
the secondary xylem
...
These pass through the secondary xylem and phloem in a radial fashion, and so they

are known as secondary medullary rays
...

Secondary Growth in Roots


In a dicot root, the vascular cambium is completely secondary
...




Formed in a wavy manner, but later on becomes circular



Further events are similar to the secondary growth in a dicot stem

Products of Secondary Growth
Spring Wood and Autumn Wood


The activity of cambium is under the control of many physiological and environmental
factors
...
This wood is called spring wood or early wood
...




In winters, xylem is less active and forms fewer xylary elements with narrow vessels
...
It is darker and has higher density
...
, are deposited on it



Makes the plant durable and resistant to attack by microbes and pests



Non-conducting



Sapwood:



Constitutes the peripheral region of the secondary xylem



Lighter in colour



Conducting
Cork Cambium (Phellogen)



As secondary growth starts, the outer cortical and the epidermis layer break down
...




Made of narrow, thin-walled, rectangular cells, and is a couple of layers thick



Leads to the formation of cells on both sides:



Outer cells − Differentiate into the cork (Phellem)



Inner cells − Differentiate into the secondary cortex (Phelloderm)



Phellogen + Phellem + Phelloderm = Periderm



Phellogen exerts pressure on its peripheral layers, and due to this, these layers die
...
This makes it impervious to water
...

These openings permit the exchange of gases

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