ANATOMY OF FLOWERING PLANTS

Anatomy of every plant changes as per the habitat and so does the morphology. Anatomy of plants is all about studying internal structures of plants. Plants in desert areas have succulent leaves while aquatic plants have floating leaves with stomata on the upper side similarly trees in forest have strong, wooden bark and crops in fields have much delicate.

TYPES OF TISSUE SYSTEM

Tissue system is a tissue or a group of tissues derived from a portion of meristem which performs a similar function in the plant body irrespective of its position. On the basis of their location and structure, Sach (1875) recognised three systems in plants: (1) Epidermal tissue system (2) Ground tissue system (3) Vascular tissue system

Epidermal Tissue System

The word epiderm itself suggests the meaning, i.e. 'epi' means outer and 'derm' means skin/covering/ layer. The epidermal tissue system acts as the outer covering of the whole plant body. It is made up of epidermal cells, stomata and epidermal outgrowths such as trichomes.

Epidermal cells are made up of parenchymatous tissue and have large vacuoles. These cells are covered by a thick waxy layer known as cuticle. Thickness of cuticle is inversely proportional to transpiration i.e. more is the thickness of the cuticle, less is the transpiration rate and vice versa.

Important - Cuticle is absent in submerged hydrophytes and roots of plants.

Stomata are structures found mostly on the surface of leaves and young stems. It is made up of stomatal pore and is guarded by guard cells which are surrounded by subsidiary cells. The outside walls of guard cells that are away from the stomatal pore are thin and the inner walls are thicker that is towards the stomatal pore. The guard cells have chloroplasts and control the opening and closing of stomata. Guard cells found in dicots are bean-shaped and those found in monocots like grass are dumb-bell shaped. Occasionally, a few epidermal cells near the guard cells become specialised in shape and size and are referred to as subsidiary cells. About 85 to 90% of transpiration in plants occurs through stomata. These also help in exchange of gases. The stomatal aperture, guard cells and the surrounding subsidiary cells together are called as stomatal apparatus.

Important

The stomata that open during day are photoactive stomata and stomata that open at night are scotoactive stomata.

Epidermal outgrowth consists of small finger-like hairs present on the surface of roots and stems. The hairs present on the surface of roots are unicellular and help in absorption of water and minerals from soil. The hairs present on the surface of stems are multicellular, and secretory in nature, they are known as trichomes which prevent loss of water during transpiration.

Ground Tissue System

Ground tissue system makes up the majority of the plant's body and extends from below the epidermis to the centre. All excluding epidermis and vascular tissue, comes under ground tissue system. It consists of hypodermis, cortex, pericycle, medullary rays and pith. There is no differentiation in the ground tissue system of monocotyledonous stems, where vascular bundles are scattered.

Important

Ground tissue system is dominated by parenchyma tissue.

Ground tissue system of leaves is called mesophyll. Mesophyll is made up of two types of photosynthetic cells. palisade and spongy.

Vascular Tissue System

The complex permanent tissues such as xylem and phloem form the vascular tissue system. Vascular tissue consists of two types of vascular bundles which are as follows:

(1) Radial vascular bundles

(2) Conjoint vascular bundles

Radial vascular bundles are found in roots of plants in which xylem and phloem are arranged alternately at different radii.

Conjoint vascular bundles are mainly found in the stem and leaves of plants in which xylem and phloem are arranged at the same radius joining each other. There are two types of conjoint vascular bundles, namely:

(1) Collateral vascular bundles

(2) Concentric vascular bundles

In collateral vascular bundles, xylem and phloem are arranged laterally, where phloem is located at the outer side of bundles.

In dicots and gymnosperms, vascular bundles contain cambium in between phloem and xylem which is responsible for secondary growth of tissues in plants; such types of vascular bundles are called conjoint open vascular bundles.

In monocots, cambium is absent in between phloem and xylem; such types of vascular bundles are called conjoint closed vascular bundles.

In concentric vascular bundles, xylem and phloem are arranged in circles or concentric rings. There are two types of concentric vascular bundles as follows:

(1) Amphicribral or Hadrocentric vascular bundles: This type of vascular bundle is found in pteridophytes like fern, in which xylem is placed at the centre and surrounded by phloem in circles.

(2) Amphivasal or Leptocentric vascular bundles: Vascular bundles, in which phloem is present at the centre surrounded by xylem in concentric circles, are known as Amphivasal vascular bundles.

ANATOMY OF PLANTS

DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS

Dicotyledonous Root

The different types of cells and tissues of root arise from root apical meristem which is present at the sub-terminal position and remains protected by root cap. Roots are much simpler than stems in their internal structure. They possess the following tissues:

(1) Epiblema or Piliferous layer: It is the outermost layer of the root It is made up of thin-walled. living and flattened parenchymatous cells. Some cells give rise to tubular outgrowth called Root hairs. These hairs are generally smaller than other epiblema cells and unicellular in nature. Due to root hairs, the epiblema is also called piliferous layer. These are responsible for absorption of water and minerals from the soil.

(2) Cortex: It is located just below the epiblema. It is made up of many layers of thin-walled parenchymatous cells. These cells enclose intercellular space for the diffusion of gasses. The cells of cortex store food and they also conduct water from the epiblema to inner parts.

(3) Endodermis: It is the innermost layer of the cortex It is made up of a single layer of barrel -shaped cells which do not possess intercellular space. These cells possess a band of thickening that runs along their radial and tangential walls. This band of thickening is called Casparian strip which is made up of suberin and lignin and is impermeable for substance. All the tissues on the inner side of endodermis constitute a stele. It consists of pericycle, vascular bundles and pith.

(4) Pericycle: Endodermis is followed by one or a few layers of pericycle. The cells of pericycle are thin walled parenchymatous. It plays an important role as it forms a part of vascular cambium and cork cambium also develops from it during secondary growth. All lateral roots arise from the pericycle. Pericycle is absent in the roots of some aquatic plants.

(5) Vascular bundles: Inner to pericycle there are few (2-6) alternately arranged bundles of xylem and phloem. The xylem and phloem strands are arranged on different radii. Between the xylem and phloem thin-walled parenchymatous conjunctive tissues are present Protoxylem is located in contact with the pericycle while metaxylem is present towards the centre of the root. The metaxylem elements of different xylem bundles are separate from each other so a pith is present in the centre of the root In between two adjacent xylem bundles, there is one phloem bundle. They are separated from each other by one or more layers of small thin-walled cells called conjunctive tissue. Later on, these become meristematic to form vascular cambium.

(6) Pith: It is mostly absent and if present, made up of parenchyma cells which lack intercellular spaces. The main function of these cells is to store food and waste materials.

Important

Stele is the central part of the root or stem containing the tissues derived from the procambium. It is present on the inner side of the endodermis of a dicotyledonous root such as the pericycle, vascular bundles and pith.

Monocotyledonous Root

There is no difference between the young and old roots of monocotyledonous plants as they lack secondary growth. The monocot roots consist of the following tissues:

(1) Epiblema or Piliferous layer: It is single-layered and made up of thin-walled cells. Some of them give rise to root hairs which are tubular and take part in absorption of water and minerals. Cuticle is absent in both epiblema and root hairs. Because they absorb water and mineral salts.

(2) Cortex: It is a wide region and made up of thin-walled parenchyma cells that enclose intercellular spaces for exchange of gases. It stores food. In older roots, the cortex becomes thick-walled and suberised. This is called exodermis which acts as a protective layer and to some extent absorbtive in function.

(3) Endodermis: It is the inner layer of cortex which is made up of a single layer and consists of barrel-shaped thick-walled cells which do not possess intercellular spaces. The young endodermal cells possess an internal strip of suberin and lignin which are generally known as Casparian strips. The cells of endodermis, lying opposite to protoxylem, are thin-walled and are called passage cells which help in conduction of fluids.

(4) Pericycle: It is found below the endodermis. It is single-layered and has thin-walled cells. In monocots, the pericycle does not form cambium. It only produces lateral roots. The pericycle is composed of thin-walled parenchymatous cells in the young root. But later on, it becomes thick-walled in many monocots.

(5) Vascular bundle: It is in the form of alternate and radial xylem and phloem bundle. The vascular bundles are arranged in the form of a ring around a central pith. The xylem bundles are exarch, ie protoxylem lies outward while metaxylem faces inward. The number of xylem bundles is more than six Hence, it is called polyarch condition. Phloem bundles alternate with xylem bundles and is separated from each other by conjunctive tissue.

(6) Pith: It is large in monocots and made up of parenchymatous cells having intercellular spaces and food Is stored in pith cells.

DICOT ROOT	MONOCOT ROOT
The epiblema, the cortex and even the endodermis are peeled off and replaced by cork.	Cork is not formed. The cortex and the endodermis persist Only the epiblema is peeled off.
Endodermis is less thickened and Casparian strips are more prominent.	Casparian strips are visible only in the younger root. The endodermal cells later become highly thickened.
Pericycle produces lateral roots, cork cambium and part of the vascular cambium.	Pericycle produces lateral roots only.
The number of xylem and phloem bundles varies from 2-5 or sometimes 80.	Xylem and phloem bundles are numerous and are 8 or more in number.
Xylem vessels are generally angular.	Xylem vessels are oval and round in shape.
Conjunctive tissue is parenchymatous.	Conjunctive tissue may be parenchymatous or sclerenchymatous.
Conjunctive parenchyma forms the cambium.	Conjunctive parenchyma does not produce cambium.
Pith is either absent or very small.	A well-developed pith is present in the centre of the root.

Dicotyledonous Stem

Dicotyledonous stem shows the following structure from outside to inside:

(1) Epidermis: It is the outermost layer of the stem, made up of compactly arranged parenchymatous cells. The outer walls of cells possess cuticles whereas the inner walls of the epidermal cells are thin. The epidermis contains minute pores called stomata in the young stem which help in exchange of gases. Epidermal cells have no chloroplast. The tissue between the epidermis and pericycle is called Cortex.

(2) Cortex: The epidermis is followed by a few to several layered thick cells made up of thin-walled parenchymatous cells and having intercellular spaces. Cortex below the hypodermis is made up of thin-walled parenchymatous cells with conspicuous intercellular spaces. Its main function is the storage of food.

(3) Endodermis: It is the innermost layer of the cortex made up of barrel-shaped cells which do not enclose intercellular spaces. It is a wavy layer of one cell in thickness. It contains starch grains as a food reserve and lacks Casparian strips. Due to the presence of starch grains in these cells, this is called Starch sheath.

(4) Pericycle: It is a layer which is found between the endodermis and vascular bundle. It is made up of sclerenchymatous cells as well as parenchymatous cells. It has alternating patches of thick-walled and thin-walled cells. It lies outside of the vascular bundle and thick-walled patches associated with the phloem part of the vascular bundle and thin-walled patches usually occur above the medullary rays. The sclerenchymatous cells of the pericycle provide mechanical strength while the parenchymatous stores food. As the bundle caps are associated with phloem part of vascular bundles, the sclerenchymatous pericycle is also called hard blast.

(5) Vascular bundles: The vascular strand is in the form of a eustele or a ring of vascular bundle present around the central pith and inner to the pericycle. Each vascular bundle is conjoint i.e., xylem and phloem both are present in the same radius. The phloem to the outside, xylem towards the inner side and strip of cambium in between the two so it is open. The xylem is endarch le, the protoxylem lies towards the pith and metaxylem towards periphery.

(6) Pith: It forms the centre of the stem and is made up of rounded parenchymatous cells which enclose intercellular spaces. These cells store food.

Monocotyledonous Stem

A monocot stem lacks secondary growth. Therefore, possesses only the primary permanent tissues. The various tissues, unlike a dicot stem, are not arranged in concentric rings. The solid stem can be seen in plants like Zea mays, Asparagus or fistular stem can be seen in grass (they have a central cavity). Monocotyledonous stems possess the following tissues from periphery to centre:

(1) Epidermis: It is the outermost single-layered protective coverings made up of living parenchymatous cells. The outer walls of epidermal cells possess deposition of silica and cutin. It is highly cutinized to prevent water loss through evaporation. Silica provides stiffness. Hairs are generally absent. At places, the epidermis possesses stomata for gaseous exchange.

(2) Hypodermis: It is 2-3 layered thick and generally lies below the epidermis. It is made up of sclerenchymatous cells which have lignified cell walls. It provides mechanical strength to stem.

(3) Ground tissue: This layer does not show distinction into cortex, endodermis, pericycle, pith and pith rays. It is made up of parenchymatous cells. The ground tissue stores food. Some of the outer cells may also synthesise food due to the presence of chloroplast (chlorenchymatous cells).

(4) Vascular bundles: The vascular bundles are numerous lies scattered throughout the ground tissue. The vascular bundles are rounded in which phloem lies towards the outside and xylem on the inner side. Cambium is absent so the vascular bundles are conjoint and closed. Xylem is the endarch, i.e. protoxylem towards the centre and metaxylem towards the periphery. Phloem consists of sieve tubes and companion cells. The phloem parenchyma is absent.

DICOT STEM	MONOCOT STEM
Stomata have kidney-shaped guard cells.	Stomata usually possess dumb-bell shaped guard cells.
Stomata usually possess dumb-bell shaped guard cells.	The hypodermis is formed of non-green sclerenchyma fibres.
The ground tissue is differentiated into cortex, endodermis. pericycle, pith, etc.	The ground tissue is a mass of similar cells
The stem is always solid.	The stem is generally hollow in centre.
The vascular bundles are arranged in ring around the pitch.	The vascular bundles are scattered throughout the ground tissue.
Medullary rays occur in between vascular bundles for radial conduction.	Medullary rays are absent.
The vascular bundles are wedge-shaped in outline.	They are oval or round in outline.
The vascular bundles are open due to the presence of cambium in between phloem and xylem.	The vascular bundles are closed.
The stem shows secondary growth due to the formation of secondary vascular tissue and periderm.	Secondary growth is usually absent.

Dorsiventral (Dicotyledonous) Leaf

Most of the dicotyledonous leaves are dorsiventral Dorsoventral (Bifacial): The leaves are commonly horizontal in orientation with distinct upper and lower surfaces. The upper surface is also called inner, adaxial or ventral surface. The lower surface Is correspondingly called outer, adaxial or dorsal surface Mesophyll is distinct as a palisade or spongy tissue with a palisade usually restricted to the upper side.

They show three distinct ports:

(1) Epidermis: The leaf contains an upper surface called adaxial and a lower surface called abaxial They are made up of a single layer of parenchymatous cells which generally lack chloroplast. The outer walls are cutinised which prevents excessive transpiration. The abaxial epidermis contains more stomata as compared to adaxial epidermis. The main function of epidermis is to provide protection of internal tissue and exchange of gases.

(2) Mesophyll: The tissue lying between the adaxial and abaxial epidermis which contains vein, chlorenchymatous. It is made up of parenchymatous cells containing a large number of chloroplasts and having intercellular spaces. Mesophyll differentiated into two parts Palisade parenchyma and Spongy parenchyma. The palisade lies below the upper epidermis and consists of elongated columnar cells which are arranged vertically and parallel to each other and have narrow intercellular spaces for exchange of gases. These are the main sites of photosynthesis because they are rich in discoid chloroplast. The Spongy parenchyma lies between the lower epidermis and palisade parenchyma and are oval and rounded. They are loosely arranged and have large intercellular spaces; they contain less chloroplast than in palisades.

(3) Vascular system: It contains a large number of vascular bundles. It lies at the boundary between the palisade and the spongy region and is seen in veins and midrib. Vascular bundles of varying sizes depending upon the venation. The vascular bundle is surrounded by sheaths of compactly arranged parenchyma cells called bundle sheaths. The vascular bundles are conjoint and collateral Xylem lies towards the upper side and phloem towards the lower side. The main function of vascular bundle is conduction.

Important

Dorsiventral leaves orient themselves at an angle to the main axis and perpendicular to the direction of sunlight Most dicots have dorsiventral leaves that are net-veined. including bushes and most trees.

Isobilateral (Monocotyledonous) Leaf

The isobilateral monocot leaves usually do not show a distinction between petiole and lamina. The leaf base is commonly sheathing, that is, covering the stem partially or completely. The venation is parallel Both surfaces are equally green. Most of the monocotyledonous leaves are isobilateral. The internal structure also does not show much differentiation between the upper and lower side. They also possess three main parts:

(1) Epidermis: The epidermis consists of compactly arranged parenchymatous cells that are cutinised. The stomata are present on both the upper side and lower sides of the epidermis. The upper epidermis contains groups of larger thin-walled cells over the region of vein called bulliform or motor cells. These are highly vacuolated and colourless. In water deficiency conditions, the bulliform cells lose water and become flaccid as a result the leaf gets rolled up to reduce the surface for exposure. These cells play an important role in the unrolling of leaves. The main functions of epidermis are protection and gaseous exchange.

(2) Mesophyll: It is present in between the two layers of epidermis. They are isobilateral enclose intercellular space and contain a large number of chloroplasts which help in photosynthesis. Mesophylls of monocot leaves are not differentiated into palisade and spongy tissues.

(3) Vascular system: The monocot leaves possess parallel venation. Vascular bundle is surrounded by a single sheath of compactly arranged parenchymatous cells called bundle sheath. The vascular bundles are conjoint. Xylem lies towards the upper side and phloem lies towards the lower side. Its main function is transporting materials.

DICOT (Dorsiventral) LEAF	MONOCOT (Isobilateral) LEAF
The upper surface is dark green while the lower surface is light green.	The two surfaces are equally green.
Silica is not normally deposited on the epidermal cells.	Silica deposition occurs on the walls of epidermal cells.
Stomata are absent or less abundant on the upper side.	The stomata are equally distributed on the two sides.
The stomata have kidney-shaped guard cells.	The stomata have dumb-bell shaped guard cells.
The veins do not run parallel Instead. they form reticulation.	The veins run parallel to one another.
Mesophyll is differentiated into two parts, upper palisade and lower spongy.	Mesophyll is undifferentiated.
Protoxylem is indistinguishable.	Larger vascular bundles may show distinction into protoxylem and metaxylem.
Bundle sheath is generally a single Layer and is formed of colourless cells.	Bundle sheath may be single or double-layered. The cell generally possess chloroplast
Hypodermis of the midrib region is collenchymatous.	Hypodermis of the midrib region is sclerenchymatous.