Anatomy of Flowering Plants Class 11 Biology Chapter 6 Notes

Anatomy of Flowering Plants Class 11 Biology Chapter 6 Notes

Epidermal Tissue System

1. Location: The outermost covering of the entire plant body.
2. Components:
   • Epidermal Cells: Elongated, compactly arranged cells forming a continuous single-layered outermost layer of the plant.
   • Cuticle: A waxy thick layer on the outer surface of the epidermis that prevents water loss (absent in roots).
   • Stomata: Structures found in the epidermis of leaves, regulating transpiration and gaseous exchange.
   • Guard Cells: Two bean-shaped cells surrounding the stomatal pore, controlling stomatal opening and closing.
   • Subsidiary Cells: Specialized epidermal cells near guard cells, part of the stomatal apparatus.
   • Root Hairs: Unicellular elongations of epidermal cells on roots, aiding in water and mineral absorption.
   • Trichomes: Epidermal hairs on stems, which can be multicellular, branched or unbranched, and soft or stiff, often contributing to water loss prevention.
   • Secretory Trichomes: Trichomes that may release substances.
3. Cell Characteristics:
   • Epidermal cells are parenchymatous, with a small amount of cytoplasm lining the cell wall and a large vacuole.
4. Function:
   • Epidermis: Provides a protective outer covering for the plant body.
   • Cuticle: Prevents water loss through transpiration.
   • Stomata: Regulate transpiration and gas exchange.
   • Guard Cells: Control stomatal opening and closing.
   • Subsidiary Cells: Assist in stomatal regulation.
   • Root Hairs: Aid in water and mineral absorption from the soil.
   • Trichomes: Help prevent water loss due to transpiration and may have other functions.
   • Secretory Trichomes: Release substances for various purposes.

The epidermal tissue system is crucial for plant protection, gas exchange, and water management, with its various components and adaptations supporting these essential functions.

Ground Tissue System

1. Composition: All tissues in the plant body except the epidermis and vascular bundles make up the ground tissue system.
2. Tissue Types:
   • Parenchyma: Found in cortex, pericycle, pith, and medullary rays of primary stems and roots.
   • Collenchyma: Part of the ground tissue system.
   • Sclerenchyma: Part of the ground tissue system.
3. Distribution:
   • Parenchymatous Cells: Typically located in the cortex, pericycle, pith, and medullary rays of primary stems and roots.
   • Leaves: Ground tissue in leaves consists of thin-walled chloroplast-containing cells and is known as mesophyll.

The ground tissue system is responsible for various functions, including support, storage, and photosynthesis. It comprises different cell types with specific roles in different parts of the plant.

Vascular Tissue System

1. Composition: The vascular tissue system comprises two complex tissues, namely the phloem and the xylem.
2. Vascular Bundles: In plants, the xylem and phloem are found together in structures called vascular bundles.
3. Dicotyledonous Stems:
   • Cambium: Cambium tissue is present between the phloem and xylem in dicotyledonous stems.
   • Open Vascular Bundles: Vascular bundles in dicots with cambium can develop secondary xylem and phloem tissues, making them open vascular bundles.
4. Monocotyledons:
   • Closed Vascular Bundles: Monocots lack cambium in their vascular bundles, making them closed, and they cannot form secondary tissues.
5. Arrangement within Vascular Bundles:
   • Radial Arrangement: Xylem and phloem are alternately arranged along different radii in vascular bundles, common in roots.
   • Conjoint Vascular Bundles: Xylem and phloem are situated on the same radius of vascular bundles, often found in stems and leaves.
   • Phloem Location: In conjoint vascular bundles, the phloem is typically located on the outer side of the xylem.

The vascular tissue system is responsible for the transport of water, nutrients, and organic compounds throughout the plant. The presence or absence of cambium and the arrangement of xylem and phloem within vascular bundles can vary between different types of plants.

Dicotyledonous Root

1. Epiblema: The outermost layer of the root, also known as the epidermis. Many epiblema cells protrude as unicellular root hairs, which aid in water and mineral absorption.
2. Cortex:
   • Comprises several layers of thin-walled parenchyma cells with intercellular spaces.
   • Responsible for storage and provides mechanical support.
3. Endodermis:
   • The innermost layer of the cortex.
   • Consists of a single layer of barrel-shaped cells without intercellular spaces.
   • Deposits a water-impermeable waxy material called suberin in the form of Casparian strips on both tangential and radial cell walls.
   • Acts as a selective barrier controlling the movement of water and nutrients into the vascular tissue.
4. Pericycle:
   • Lies just next to the endodermis.
   • Comprises a few layers of thick-walled parenchymatous cells.
   • Initiates the formation of lateral roots and vascular cambium during secondary growth.
5. Pith:
   • Small or inconspicuous in dicotyledonous roots.
6. Conjunctive Tissue:
   • Parenchymatous cells located between the xylem and phloem.
   • Serves to connect and support the vascular tissue.
7. Vascular Bundles:
   • Typically two to four patches of xylem and phloem, with conjunctive tissue in between.
   • Cambium ring develops later between xylem and phloem, allowing for secondary growth.
8. Stele:
   • Comprises all the tissues on the inner side of the endodermis, including the pericycle, vascular bundles, and pith.

This internal tissue organization in dicotyledonous roots is essential for various functions, including water and nutrient transport, structural support, and the development of lateral roots and secondary growth. The presence of the Casparian strips in the endodermis helps in controlling the movement of substances into the vascular tissue.

Monocotyledonous Root

The anatomy of a monocotyledonous root shares similarities with the dicotyledonous root. However, there are some distinguishing features:

1. Epidermis: The outermost layer of the root, providing protection.
2. Cortex: Comprising parenchyma cells with intercellular spaces, responsible for storage and support.
3. Endodermis: The innermost layer of the cortex, characterized by the presence of Casparian strips to regulate water and nutrient movement.
4. Pericycle: Lies just beyond the endodermis and is involved in lateral root initiation.
5. Vascular Bundles:
   • In monocot roots, there are usually more than six xylem bundles (polyarch).
   • The arrangement and distribution of xylem and phloem bundles can vary.
6. Pith:
   • The pith in monocotyledonous roots is typically large and well-developed.
7. Secondary Growth: Unlike dicot roots, monocot roots do not undergo secondary growth. This means that they do not develop additional growth rings over time.

Monocotyledonous roots share many structural similarities with dicotyledonous roots, but they are characterized by a greater number of xylem bundles and a well-developed pith. Unlike dicot roots, monocot roots lack the capacity for secondary growth.

Dicotyledonous Stem

The transverse section of a typical young dicotyledonous stem reveals its internal anatomical features:

1. Epidermis:
   • Outermost protective layer of the stem.
   • Covered by a thin cuticle layer.
   • May have trichomes (hairs) and a few stomata.
2. Cortex:
   • Comprises multiple layers of cells between the epidermis and pericycle.
   • Consists of three subzones:
     • Outer Hypodermis: A few layers of collenchymatous cells just below the epidermis, providing mechanical strength to the young stem.
     • Cortical Layers: Rounded, thin-walled parenchymatous cells with noticeable intercellular spaces.
     • Endodermis (Starch Sheath): The innermost cortex layer with cells rich in starch grains.
3. Pericycle:
   • Located on the inner side of the endodermis.
   • Present above the phloem in the form of semi-lunar patches of sclerenchyma.
   • Involved in lateral root initiation.
4. Medullary Rays:
   • A few layers of radially placed parenchymatous cells situated between the vascular bundles.
5. Vascular Bundles:
   • Numerous vascular bundles are arranged in a ring, which is a characteristic feature of dicot stems.
   • Each vascular bundle is conjoint, open, and has endarch protoxylem (xylem tissue develops from the center outward).
6. Pith:
   • Central portion of the stem consists of numerous rounded, parenchymatous cells with large intercellular spaces.

The dicotyledonous stem exhibits a well-organized structure with specific layers and features, including collenchymatous cells for support, a starch sheath (endodermis), and a characteristic ring arrangement of vascular bundles with open conjoint structure. The pith occupies the central area of the stem.

Monocotyledonous Stem

The transverse section of a monocotyledonous stem reveals its distinct anatomical features:

1. Hypodermis:
   • Consists of sclerenchymatous cells, providing mechanical support to the stem.
   • Located just below the epidermis.
2. Vascular Bundles:
   • Numerous vascular bundles are scattered throughout the stem.
   • Each vascular bundle is surrounded by a sclerenchymatous bundle sheath, which provides protection and support to the vascular tissue.
   • Vascular bundles in monocots are conjoint (xylem and phloem are together) and closed (no cambium for secondary growth).
3. Ground Tissue:
   • Large, conspicuous parenchymatous ground tissue occupies the space between the vascular bundles.
   • The ground tissue primarily serves for storage and structural support.
4. Vascular Bundle Size:
   • Peripheral vascular bundles are generally smaller in size compared to the centrally located ones.
5. Phloem Parenchyma:
   • In monocot stems, phloem parenchyma is usually absent.
6. Water-Containing Cavities:
   • Water-containing cavities are present within the vascular bundles, contributing to water transport and storage.

Monocotyledonous stems have a unique structural organization with sclerenchymatous hypodermis, scattered and closed vascular bundles, a prominent parenchymatous ground tissue, and the absence of phloem parenchyma. The arrangement of vascular bundles and their associated features supports the stem’s functions in monocot plants.

Dorsiventral (Dicotyledonous) Leaf

The vertical section of a dorsiventral leaf, such as a typical dicot leaf, reveals the following key structural parts:

1. Epidermis:
   • Covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf.
   • Features a conspicuous cuticle, a waxy layer that reduces water loss.
   • The abaxial epidermis generally contains more stomata than the adaxial epidermis. In some cases, the adaxial epidermis may lack stomata.
2. Mesophyll:
   • The tissue sandwiched between the upper and lower epidermis.
   • Responsible for photosynthesis, as it contains chloroplasts.
   • Comprised of parenchyma cells, which have two distinct types:
     • Palisade Parenchyma: Located adaxially, these cells are elongated, arranged vertically, and parallel to each other. They are densely packed and serve as the primary site for photosynthesis.
     • Spongy Parenchyma: Found below the palisade cells and extending to the lower epidermis. These cells are oval or round and loosely arranged, with numerous air spaces and cavities between them.
3. Vascular System:
   • Includes vascular bundles, which are visible in the veins and midrib of the leaf.
   • The size of the vascular bundles depends on the size of the veins.
   • In dicot leaves with reticulate venation, the veins vary in thickness.
   • Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells, providing support and protection to the vascular tissue.

The dorsiventral leaf structure of dicots ensures efficient photosynthesis, as it allows for a larger surface area for capturing light and efficient gas exchange through stomata. The palisade parenchyma and spongy parenchyma work together to carry out photosynthesis, and the vascular system ensures the transport of water and nutrients throughout the leaf.

Isobilateral (Monocotyledonous) Leaf

The anatomy of an isobilateral leaf is similar to that of a dorsiventral leaf in many respects, but it exhibits some characteristic differences:

1. Stomata:
   • Stomata are present on both the upper (adaxial) and lower (abaxial) surfaces of the epidermis in isobilateral leaves. This is in contrast to dorsiventral leaves where stomata are primarily located on the abaxial surface.
2. Mesophyll:
   • Unlike dorsiventral leaves, the mesophyll of isobilateral leaves is not differentiated into palisade parenchyma and spongy parenchyma. Instead, the mesophyll is uniform in structure and function.

In some monocotyledonous leaves, especially in grasses, certain adaxial epidermal cells along the veins transform into large, empty, and colorless cells known as “bulliform cells.” These cells play a unique role in the leaf’s response to water availability. When bulliform cells absorb water and become turgid, the leaf’s surface is exposed, aiding in cooling and photosynthesis. However, when these cells become flaccid due to water stress, they cause the leaf to curl inwards, reducing the exposed surface area and minimizing water loss.

In isobilateral leaves with parallel venation, the sizes of vascular bundles are generally similar, except in the main veins. This consistent size of vascular bundles throughout the leaf is reflective of the parallel venation found in monocot leaves.