Class 11th Biology : CHAPTER 6 ANATOMY OF FLOWERING PLANTS

 

CHAPTER 6

ANATOMY OF FLOWERING PLANTS

Syllabus: Anatomy and functions of tissue systems in dicots and monocots.

 

Anatomy

1. Study of internal structure of plants is called anatomy.

2. Within angiosperms, the monocots and dicots are also seen to be anatomically different. Internal structures also show adaptations to diverse environments.

GROWTH OF A PLANT

1.     Here Primary meristem undergoes differentiation and became primary permanent tissue. For example:- Parenchyma, Collenchyma, Sclerenchyma, Primary xylem and phloem and Primary medullary rays. This happens in both monocots and dicots.

Medullary rays (also known as primary medullary rays or pith rays) are radial sheets or ribbons extending from the central pith of a plant stem or root outwards to the cortex. They are composed of parenchyma cells and are found mainly in vascular plants like dicots. Their primary functions include: 1.     Transport of Nutrients and Water: o    Medullary rays facilitate the lateral transport of water, minerals, and nutrients between the xylem and phloem. This horizontal movement supplements the vertical movement of substances through the plant. 2.     Storage: o    The cells within the medullary rays also store food (such as starch), waste products, and other nutrients that can be utilized by the plant when needed. 3.     Structural Support: o    Medullary rays contribute to the mechanical support of the plant, helping maintain the integrity and stability of the stem or root.

 

 

2.     Primary permanent tissue sometimes dedifferentiate in case of dicot plants and functions as secondary meristem and forms vascular cambium and cork cambium.

3.     Cambium means meristematic tissue which divides continuously.

Cork Cambium (Phellogen): Function: Produces the outer protective layers of the plant, specifically cork (phellem) and secondary cortex (phelloderm). Vascular Cambium: Function: Produces the vascular tissues necessary for the plant’s growth in girth—namely, secondary xylem (wood) and secondary phloem. Periderm: It replaces epidermis during secondary growth

 

4.     Now secondary meristem undergoes redifferentiation and become secondary permanent tissue. For example cork, sec. cortex, sec. xylem and sec. phloem.

 

6.1 THE TISSUE SYSTEM

1. Different types of tissues based on the types of cells present.

2. On the basis of their structure and location, there are three types of tissue systems.

3. These are as follows :

a) Epidermal tissue system

b) The ground or fundamental tissue system

c) The vascular or conducting tissue system.

6.1.1 Epidermal Tissue System

1. The epidermal tissue system forms the outer-most covering of the whole plant body and comprises of :

a) epidermal cells

b) stomata

c) epidermal appendages – the trichomes and hairs.

 

 

 

a) Epidermal cells

          i.                   The epidermis is the outermost layer of the primary plant body.

        ii.                   It is made up of elongated, compactly arranged cells, which form a continuous layer.

      iii.                   Epidermis is usually single layered.

      iv.                   Epidermal cells are parenchymatous with a small amount of cytoplasm lining the cell wall and a large vacuole.

        v.                   The outside of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water.

      vi.                   Cuticle is absent in roots.

 

 

 

 

 

 

 

b) Stomata

i) These structures present in the epidermis of leaves.

ii) Stomata regulate the process of transpiration and gaseous exchange.

iii) Each stoma (plural-stomata) is composed of two bean shaped cells known as guard cells which enclose stomatal pore.

iv) In grasses, the guard cells are dumb-bell shaped.

v) The outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened.

vi) The guard cells possess chloroplasts and regulate the opening and closing of stomata.

vii)  Sometimes, a few epidermal cells, in the vicinity (near) of the guard cells become specialised in their shape and size and are known as subsidiary cells.

viii) The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus (Figure 6.1).

c) Epidermal appendages – the trichomes and hairs.

i) The cells of epidermis bear a number of hairs.

ii) The root hairs are unicellular elongations of the epidermal cells and help absorb water and minerals from the soil.

iii) On the stem the epidermal hairs are called trichomes.

iv) The trichomes in the shoot system are usually multicellular.

v) They may be branched or unbranched and soft or stiff.

vi) They may even be secretory.

vii) The trichomes help in preventing water loss due to transpiration.

(Trichomes can create a microclimate around the leaf surface by trapping a layer of still air. This layer acts as a barrier against air movement and helps to reduce the rate of transpiration.)

 

6.1.2 The Ground Tissue System

1.     All tissues except epidermis and vascular bundles constitute the ground tissue.

2.     It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma.

3.     Parenchymatous cells are usually present in cortex, pericycle, pith and medullary rays, in the primary stems and roots.

A.    Cortex is the area below epidermis to the pericycle.

i).     The area below epidermis is known as hypodermis. It is present in stem only.

ii).   Below hypodermis is general cortex.

iii). Below general cortex endodermis is present (only in roots). Endodermis is the Innermost layer of the cortex, just outside the pericycle.

 

B.     Pericycle

The pericycle is a layer of cells found in plant roots, located just inside the endodermis and surrounding the vascular tissue (xylem and phloem). It plays a crucial role in the growth and development of roots. The pericycle is responsible for initiating the growth of lateral roots (side roots) by dividing and forming new cells. In some plants, it also helps in secondary growth by contributing to the thickening of roots.

 

C.     Pith

 

The pith is the central part of the stem or root in some vascular plants, especially dicots. It consists of soft, spongy parenchyma cells that store nutrients and water. The pith is surrounded by the vascular tissue (xylem and phloem) and can either be hollow or solid, depending on the plant species.

 

D.    Medullary rays

Medullary rays (also known as primary medullary rays or pith rays) are radial sheets or ribbons extending from the central pith of a plant stem or root outwards to the cortex. They are composed of parenchyma cells and are found mainly in vascular plants like dicots. Their primary functions include:

1.     Transport of Nutrients and Water:

o    Medullary rays facilitate the lateral transport of water, minerals, and nutrients between the xylem and phloem. This horizontal movement supplements the vertical movement of substances through the plant.

2.     Storage:

o    The cells within the medullary rays also store food (such as starch), waste products, and other nutrients that can be utilized by the plant when needed.

3.     Structural Support:

o    Medullary rays contribute to the mechanical support of the plant, helping maintain the integrity and stability of the stem or root.

 

E.     Stele

i).     Everything inside endodermis is collectively called as stele.

 

4.     In leaves, the ground tissue consists of thin-walled chloroplast containing cells and is called mesophyll.

6.1.3 The Vascular Tissue System

1. The vascular system consists of complex tissues, the phloem and the xylem.

2. The xylem and phloem together constitute vascular bundles (Figure 6.2).

 

In dicotyledonous stems

i) In dicotyledonous stems, cambium is present between phloem and xylem.

(In dicotyledonous stems, the term "cambium" refers to a type of meristematic tissue, specifically the vascular cambium. The vascular cambium is a thin layer of actively dividing cells located between the phloem and xylem in the stem. It is responsible for the secondary growth of the stem, which leads to an increase in girth or thickness.)

ii) 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

i) In the monocotyledons, the vascular bundles have no cambium present in them.

ii) Hence, since they do not form secondary tissues they are referred to as closed.

3. Depending upon the arrangement of vascular bundles, the arrangement is of two types:

a) Radial                                b) Conjoint

a) Radial

 

i) 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 of both monocotyledons and dicotyledons plants.

 

b) Conjoint

i) In conjoint type of vascular bundles, the xylem and phloem are jointly situated along the same radius of vascular bundles.

ii) Such vascular bundles are common in stems and leaves.

iii) The conjoint vascular bundles usually have the phloem located only on the outer side of xylem.

6.2 ANATOMY OF DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS

6.2.1 Dicotyledonous Root Look at Figure 6.3 (a),

1. It shows the transverse section of the sunflower root.

2. The internal tissue organisation is as follows:

a) Epiblema

i) The outermost layer is epiblema.

(The epidermis is the outermost layer of cells that covers the whole plant. In the region of the root this layer is called the epiblema.)

ii) Many of the cells of epiblema protrude in the form of unicellular root hairs.

b) Cortex

i) The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces.

c) Endodermis

i) The innermost layer of the cortex is called endodermis.

ii) It comprises a single layer of barrel shaped (cylindrical and elongated) cells without any intercellular spaces.

iii) 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.

 

 

d) Pericycle

i) Next to endodermis lies a few layers of thick-walled parenchyomatous cells referred to as pericycle.

ii) Initiation of lateral roots (exogenous roots as initiates from inside), vascular cambium and cork cambium during the secondary growth takes place in these cells. Only 1/3^rd part of vascular cambium forms from pericycle.

(Primary Growth: Primary growth refers to the increase in length or height of the plant body. It occurs at the apical meristems located at the tips of roots and shoots, resulting in the elongation of these structures.

Secondary Growth: Secondary growth refers to the increase in girth or thickness of the plant body. It occurs at the lateral meristems, specifically the vascular cambium and the cork cambium, leading to the radial expansion of stems and roots.)

e) The pith

i) It is small or inconspicuous (not clearly visible).

ii) The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue. 2/3^rd part of vascular cambium forms from conjuctive tissue.

iii) There are usually two to four xylem and phloem patches.

iv) Later, a cambium ring develops between the xylem and phloem.

v) All tissues on the innerside of the endodermis such as pericycle, vascular bundles and pith constitute the stele.

 

6.2.2 Monocotyledonous Root

1. The anatomy of the monocot root is similar to the dicot root in many respects (Figure 6.3 b).

2. It has epidermis, cortex, endodermis, pericycle, vascular bundles and pith.

3. As compared to the dicot root which have fewer xylem bundles, there are usually more than six (polyarch) xylem bundles in the monocot root.

4. Pith is large and well developed.

5. Monocotyledonous roots do not undergo any secondary growth so vascular cambium and cork cambium are absent.

6.2.3 Dicotyledonous Stem

1. The transverse section of a typical young dicotyledonous stem shows that the epidermis is the outermost protective layer of the stem (Figure 6.4 a).

2. Covered with a thin layer of cuticle, it may bear trichomes and a few stomata.

3. The cells arranged in multiple layers between epidermis and pericycle constitute the cortex.

4. It consists of three sub-zones.

i) The outer hypodermis, consists of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem.

ii) Cortical layers below hypodermis consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces.

iii) The innermost layer of the cortex is called the endodermis. But it is not a true endodermis.

5.     The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath.

6.     Pericycle is present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.

7.     In between the vascular bundles there are a few layers of radially placed parenchymatous cells, which constitute medullary rays. These medullary or pith rays are essential for the radial conduction of the water, minerals and other organic substances. They transport the substances from center to periphery.

8.     Later on these medullary rays dedifferentiate and forms interfascicular cambium. It also helps in the secondary growth.

9.     A large number of vascular bundles are arranged in a ring ; the ‘ring’ arrangement of vascular bundles is a characteristic of dicot stem.

10.Each vascular bundle is conjoint, open, and with endarch protoxylem.

10. A large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith.

 

 

6.2.4 Monocotyledonous Stem

1.     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 (clear) parenchymatous ground tissue (Figure 6.4 b).

2.     Vascular bundles are conjoint and closed.

3.     Peripheral vascular bundles are generally smaller than the centrally located ones.

4.     Protoxylem lacunae are gaps or regions within the protoxylem tissue where there is no continuous lignification or secondary cell wall deposition.

·  Xylem vessel cells develop secondary walls to withstand water pressure and support the plant. ·  In the protoxylem, due to early formation and tissue stretching, the secondary wall may not form completely in some places — those places are called protoxylem lacunae.

 

5.     The phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles.

6.2.5 Dorsiventral (Dicotyledonous) Leaf

1.     Dorsiventral leaf means that both sides are different in appearance and structure.

2.     The vertical section of a dorsiventral leaf through the lamina shows three main parts, namely:

a) Epidermis                         b) Mesophyll                       c) Vascular system

 

a) Epidermis

i) The epidermis which covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf has a conspicuous (noticeable) cuticle.

ii) The abaxial epidermis generally bears more stomata than the adaxial epidermis.

iii) The latter may even lack stomata.

b) Mesophyll

i) The tissue between the upper and the lower epidermis is called the mesophyll.

ii) Mesophyll, which possesses chloroplasts and carry out photosynthesis, is made up of parenchyma.

iii) It has two types of cells – the palisade parenchyma and the spongy parenchyma.

·        The palisade parenchyma

i) The adaxially placed palisade parenchyma is made up of elongated cells, which are arranged vertically and parallel to each other.

·        The spongy parenchyma

i) The oval or round and loosely arranged spongy parenchyma is situated below the palisade cells and extends to the lower epidermis.

ii) There are numerous large spaces and air cavities between these cells.

The palisade parenchyma is specialized for efficient photosynthesis, with vertically arranged cells rich in chloroplasts, while the spongy parenchyma provides a pathway for gas exchange, with loosely arranged cells and larger intercellular spaces. Together, these two types of parenchyma tissues contribute to the overall functioning of leaves in capturing sunlight, performing photosynthesis, and exchanging gases with the atmosphere.

 

c) Vascular system

i) Vascular system includes vascular bundles, which can be seen in the veins and the midrib.

ii) The size of the vascular bundles are dependent on the size of the veins.

iii) The veins vary in thickness in the reticulate venation of the dicot leaves.

iv) The vascular bundles are surrounded by a layer of thick walled bundle sheath cells.

 

6.2.6 Isobilateral (Monocotyledonous) Leaf

1.     Isobilateral leaf means that both the sides look similar in appearance, equally green.

2.     The anatomy of isobilateral leaf is similar to that of the dorsiventral leaf in many ways.

3.     It shows the following characteristic differences.

4.     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.

 

4. In grasses

i) Certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells. These are called bulliform cells.

ii) When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed.

iii) When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.

iv) 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.

The vascular bundles appear to be relatively similar in size, apart from the main veins. This similarity in size indicates that the smaller veins and their associated vascular bundles are relatively uniform in their distribution and composition across the leaf.

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