Anatomy of Flowering Plants
A tissue is a group of cells having a common origin and usually performing a
common function. Based on cell’s capability to divide, tissues are classified
into two main groups which are as follows:
1. Meristematic and
2. Permanent tissues.
Meristematic Tissues: Cells in the meristematic tissue are capable of
dividing. Meristematic tissues are found in those regions which need to grow
continuously. For example, root tips and stem tips contain meristematic tissues.
Meristematic tissues are of following types:
(a). Primary Meristems and
(b) Secondary Meristems
1. Primary Meristems: Primary meristems appear early in the life of a
plant and are responsible for the formation of primary plant body. Primary
growth involves development of new parts of a plant and growth in length of a
particular part. Primary meristems are of two types:
(a). Apical Meristem: They are found in root tips and stem tips. In stems
during the formation of leaves and elongation of stem, some cells are left
behind from shoot tip and constitute the axillary bud. Such buds are capable of
forming a branch or a flower.
(b). Intercalary Meristem: They are found between mature tissues. They
occur in grasses and regenerate parts removed by the grazing herbivores.
After divisions of cells in both primary and as well as secondary meristems, the
newly formed cells become structurally and functionally specialised and lose the
ability to divide. Such cells are termed permanent or mature cells and
constitute the permanent tissues. During the formation of the primary plant
body, specific regions of the apical meristem produce dermal tissues, ground
tissues and vascular tissues. Permanent tissues are of two types:
1. Simple Tissues and
2. Complex Tissues
1. Simple Tissues: A simple tissue is made of only one type of cells.
Simple tissues are of following types:
Parenchyma: Parenchyma forms the major component within organs. The cells
of the parenchyma are generally isodiametric. They may be spherical, oval,
round, polygonal or elongated in shape. Their walls are thin and made up of
cellulose. They may either be closely packed or have small intercellular spaces.
The parenchyma performs various functions like photosynthesis, storage,
Collenchyma: The collenchyma occurs in layers below the epidermis in
dicotyledonous plants. It is found either as a homogeneous layer or in patches.
It consists of cells which are much thickened at the corners due to a deposition
of cellulose, hemicellulose and pectin. Collenchymatous cells may be oval,
spherical or polygonal and often contain chloroplasts. These cells assimilate
food when they contain chloroplasts. Intercellular spaces are absent. They
provide mechanical support to the growing parts of the plant such as young stem
and petiole of a leaf.
Sclerenchyma: Sclerenchyma consists of long, narrow cells with thick and
lignified cell walls having a few or numerous pits. They are usually dead and
without protoplasts. On the basis of variation in form, structure, origin and
development, sclerenchyma may be either fibres or sclereids. The fibres are
thick-walled, elongated and pointed cells, generally occuring in groups, in
various parts of the plant. The sclereids are spherical, oval or cylindrical,
highly thickened dead cells with very narrow cavities (lumen). These are
commonly found in the fruit walls of nuts; pulp of fruits like guava, pear and
sapota; seed coats of legumes and leaves of tea. Sclerenchyma provides
mechanical support to organs.
2. Complex Tissues: The complex tissues are made of more than one type of
cells and these work together as a unit. They are of two types:
(a). Xylem and
Xylem: Xylem is composed of four different kinds of elements, namely,
tracheids, vessels, xylem fibres and xylem parenchyma.
• Tracheids are elongated or tube like cells with thick and lignified
walls and tapering ends. These are dead and are without protoplasm. The inner
layers of the cell walls have thickenings which vary in form. In flowering
plants, tracheids and vessels are the main water transporting elements.
• Vessel is a long cylindrical tube-like structure made up of many cells
called vessel members, each with lignified walls and a large central cavity. The
vessel cells are also devoid of protoplasm. Vessel members are interconnected
through perforations in their common walls. The presence of vessels is a
characteristic feature of angiosperms.
• Xylem fibres have highly thickened walls and obliterated central
lumens. These may either be septate or aseptate.
• Xylem parenchyma cells are living and thin-walled, and their cell walls
are made up of cellulose. They store food materials in the form of starch or
fat, and other substances like tannins. The radial conduction of water takes
place by the ray parenchymatous cells.
• Primary xylem is of two types – protoxylem and metaxylem. The first
formed primary xylem elements are called protoxylem and the later formed primary
xylem is called metaxylem
• In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies
towards the periphery of the organ. This type of primary xylem is called
• In roots, the protoxylem lies towards periphery and metaxylem lies towards the
centre. Such arrangement of primary xylem is called exarch.
• Xylem transports mineral and water from roots to stems and leaves. Xylem also
gives mechanical support to the plant.
Phloem: transports food materials, usually from leaves to other parts of
the plant. Phloem in angiosperms is composed of sieve tube elements, companion
cells, phloem parenchyma and phloem fibres. Gymnosperms have albuminous cells
and sieve cells. They lack sieve tubes and companion cells.
• Sieve tube elements are also long, tube-like structures, arranged
longitudinally and are associated with the companion cells. Their end walls are
perforated in a sieve-like manner to form the sieve plates. A mature sieve
element possesses a peripheral cytoplasm and a large vacuole but lacks a
nucleus. The functions of sieve tubes are controlled by the nucleus of companion
• The companion cells are specialised parenchymatous cells, which are
closely associated with sieve tube elements. The sieve tube elements and
companion cells are connected by pit fields present between their common
longitudinal walls. The companion cells help in maintaining the pressure
gradient in the sieve tubes.
• Phloem parenchyma is made up of elongated, tapering cylindrical cells
which have dense cytoplasm and nucleus. The cell wall is composed of cellulose
and has pits through which plasmodesmatal connections exist between the cells.
The phloem parenchyma stores food material and other substances like resins,
latex and mucilage. Phloem parenchyma is absent in most of the monocotyledons.
• Phloem fibres (bast fibres) are made up of sclerenchymatous cells.
These are generally absent in the primary phloem but are found in the secondary
phloem. These are much elongated, unbranched and have pointed, needle like
apices. The cell wall of phloem fibres is quite thick. At maturity, these fibres
lose their protoplasm and become dead. Phloem fibres of jute, flax and hemp are
• The first formed primary phloem consists of narrow sieve tubes and is referred
to as protophloem and the later formed phloem has bigger sieve tubes and is
referred to as metaphloem.
THE TISSUE SYSTEM
On the basis of their structure and location, there are three types of tissue
1. Epidermal Tissue System,
2. Ground or Fundamental Tissue System and
3. Vascular or Conducting Tissue System
1. Epidermal Tissue System: The epidermal tissue system forms the
outer-most covering of the whole plant body and comprises epidermal cells,
stomata and the epidermal appendages – the trichomes and hairs.
The epidermis is the outermost layer of the primary plant body. It is made up of
elongated, compactly arranged cells, which form a continuous layer. Epidermis is
usually singlelayered. Epidermal cells are parenchymatous with a small amount of
cytoplasm lining the cell wall and a large vacuole. The outside of the epidermis
is often covered with a waxy thick layer called the cuticle which prevents the
loss of water. Cuticle is absent in roots.
Stomata are structures present in the epidermis of leaves. Stomata regulate the
process of transpiration and gaseous exchange. Each stoma is composed of two
beanshaped cells known as guard cells. In grasses, the guard cells are dumbbell
shaped. The outer walls of guard cells (away from the stomatal pore) are thin
and the inner walls (towards the stomatal pore) are highly thickened. The guard
cells possess chloroplasts and regulate the opening and closing of stomata.
Sometimes, a few epidermal cells, in the vicinity of the guard cells become
specialised in their shape and size and are known as subsidiary cells. The
stomatal aperture, guard cells and the surrounding subsidiary cells are together
called stomatal apparatus.
The cells of epidermis bear a number of hairs. The root hairs are unicellular
elongations of the epidermal cells and help absorb water and minerals from the
soil. On the stem the epidermal hairs are called trichomes. The trichomes in the
shoot system are usually multicellular. They may be branched or unbranched and
soft or stiff. They may even be secretory. The trichomes help in preventing
water loss due to transpiration.
2. The Ground Tissue System
All tissues except epidermis and vascular bundles constitute the ground tissue.
It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma.
Parenchymatous cells are usually present in cortex, pericycle, pith and
medullary rays, in the primary stems and roots. In leaves, the ground tissue
consists of thin-walled chloroplast containing cells and is called mesophyll.
3. The Vascular Tissue System
The vascular system consists of complex tissues, the phloem and the xylem.The
xylem and phloem together constitute vascular bundles.
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 on different radii, the arrangement is called radial such as in roots. In
conjoint type of vascular bundles, the xylem and phloem are situated at 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.
ANATOMY OF DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS
1. Dicotyledonous Root
The outermost layer is epidermis. Many of the epidermal cells 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 waterimpermeable, 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.
2. Monocotyledonous Root
The anatomy of the monocot root is similar to the dicot root in many respects.
It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith. As
compared to the dicot root which have fewer xylem bundles, there are usually
more than six (polyarch) xylem bundles in the monocot root. Pith is large and
well developed. Monocotyledonous roots do not undergo any secondary growth.
3. Dicotyledonous Stem
The transverse section of a typical young dicotyledonous stem shows that the
epidermis is the outermost protective layer of the stem. Covered with a thin
layer of cuticle, it may bear trichomes and a few stomata. The cells arranged in
multiple layers between epidermis and pericycle constitute the cortex. It
consists of three sub-zones. The outer hypodermis, consists of a few layers of
collenchymatous cells just below the epidermis, which provide mechanical
strength to the young stem. Cortical layers below hypodermis consist of rounded
thin walled parenchymatous cells with conspicuous intercellular spaces. The
innermost layer of the cortex is called the endodermis. The cells of the
endodermis are rich in starch grains and the layer is also referred to as the
starch sheath. Pericycle is present on the inner side of the endodermis and
above the phloem in the form of semi-lunar patches of sclerenchyma. In between
the vascular bundles there are a few layers of radially placed parenchymatous
cells, which constitute medullary rays. A large number of vascular bundles are
arranged in a ring; the ‘ring’ arrangement of vascular bundles is a
characteristic of dicot stem. Each vascular bundle is conjoint, open, and with
endarch protoxylem. A large number of rounded, parenchymatous cells with large
intercellular spaces which occupy the central portion of the stem constitute the
4. Monocotyledonous Stem
The monocot stem has a sclerenchymatous hypodermis, a large number of scattered
vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a
large, conspicuous parenchymatous ground tissue. Vascular bundles are conjoint
and closed. Peripheral vascular bundles are generally smaller than the centrally
located ones. The phloem parenchyma is absent, and water-containing cavities are
present within the vascular bundles.
5. Dorsiventral (Dicotyledonous) Leaf
The vertical section of a dorsiventral leaf through the lamina shows three main
parts, namely, epidermis, mesophyll and vascular system. The epidermis which
covers both the upper surface (adaxial epidermis) and lower surface (abaxial
epidermis) of the leaf has a conspicuous cuticle. The abaxial epidermis
generally bears more stomata than the adaxial epidermis. The latter may even
lack stomata. The tissue between the upper and the lower epidermis is called the
mesophyll. Mesophyll, which possesses chloroplasts and carry out photosynthesis,
is made up of parenchyma. It has two types of cells – the palisade parenchyma
and the spongy parenchyma. The adaxially placed palisade parenchyma is made up
of elongated cells, which are arranged vertically and parallel to each other.
The oval or round and loosely arranged spongy parenchyma is situated below the
palisade cells and extends to the lower epidermis.
There are numerous large spaces and air cavities between these cells. Vascular
system includes vascular bundles, which can be seen in the veins and the midrib.
The size of the vascular bundles is dependent on the size of the veins. The
veins vary in thickness in the reticulate venation of the dicot leaves. The
vascular bundles are surrounded by a layer of thick walled bundle sheath cells.
6. Isobilateral (Monocotyledonous) Leaf
The anatomy of isobilateral leaf is similar to that of the dorsiventral leaf in
many ways. It shows the following characteristic differences. In an isobilateral
leaf, the stomata are present on both the surfaces of the epidermis; and the
mesophyll is not differentiated into palisade and spongy parenchyma.
In grasses, certain adaxial epidermal cells along the veins modify themselves
into large, empty, colourless cells. These are called bulliform cells. When the
bulliform cells in the leaves have absorbed water and are turgid, the leaf
surface is exposed. When they are flaccid due to water stress, they make the
leaves curl inwards to minimise water loss. The parallel venation in monocot
leaves is reflected in the near similar sizes of vascular bundles (except in
main veins) as seen in vertical sections of the leaves.
4. 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.
(a). 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.
In dicot stems, the cells of cambium present between primary xylem and primary
phloem is the intrafascicular cambium. The cells of medullary cells, 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
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
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.
Heartwood and sapwood: In old trees, the greater part of secondary xylem
is dark brown due to deposition of organic compounds like tannins, resins, oils,
gums, aromatic substances and essential oils in the central or innermost layers
of the stem. These substances make it hard, durable and resistant to the attacks
of microorganisms and insects. This region comprises dead elements with highly
lignified walls and is called heartwood. The heartwood does not conduct water
but it gives mechanical support to the stem. The peripheral region of the
secondary xylem, is lighter in colour and is known as the sapwood. It is
involved in the conduction of water and minerals from root to leaf.
(b). 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
Lenticels permit the exchange of gases between the outer atmosphere and the
internal tissue of the stem. These occur in most woody trees.
3. 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. Further events are similar to those already
described above for a dicotyledon stem.
Secondary growth also occurs in stems and roots of gymnosperms. However,
secondary growth does not occur in monocotyledons.