Chapter 2.2: Anatomy of Flowering Plants

1. Plant Tissues

• Definition: A plant tissue is a cohesive group of cells that are structurally and functionally related, sharing a common origin. The study of tissues, or histology, is fundamental to understanding the intricate anatomy of flowering plants.

• Types: Plant tissues are broadly categorized into two main types based on their developmental stage and capacity for cell division:

  • Meristematic Tissues: The Architects of Growth

    Meristematic tissues are the dynamic centers of plant growth, composed of undifferentiated, actively dividing cells. These tissues are responsible for the continuous development of the plant body.

    • Characteristics of Meristematic Tissue:

      • Cellular Composition: Composed of small, isodiametric, and thin-walled cells.
      • Cytoplasm: Dense and abundant, with a prominent nucleus.
      • Vacuoles: Typically small or absent, reflecting the high metabolic activity.
      • Intercellular Spaces: Absent, as the cells are compactly arranged.
      • Cell Division: Exhibit a high rate of mitotic division, leading to the formation of new cells.

    • Classification of Meristems:

      • Based on Origin:

        • Promeristem: The earliest and youngest meristematic tissue, found in the embryo.
        • Primary Meristem: Arises from the promeristem and is responsible for primary growth (e.g., apical meristem).
        • Secondary Meristem: Develops from permanent tissues and is responsible for secondary growth (e.g., vascular cambium, cork cambium).

      • Based on Location:

        • Apical Meristems: Located at the apices (tips) of roots and shoots. They are responsible for the primary growth of the plant, leading to an increase in length.
        • Intercalary Meristems: Found in the intercalary positions, between mature tissues. They are common in grasses and are responsible for the regeneration of parts removed by grazing herbivores.
        • Lateral Meristems: Located in the mature regions of roots and shoots, parallel to the longitudinal axis. They are responsible for secondary growth, leading to an increase in the girth or thickness of the plant. Examples include the vascular cambium and cork cambium.

  • Permanent Tissues: The Specialized Workforce

    Permanent tissues are derived from meristematic tissues and have undergone differentiation, losing their ability to divide. These tissues are specialized to perform specific functions in the plant body.

    • Simple Tissues: Composed of a single type of cell, forming a homogeneous group.

      • Parenchyma: The most common and versatile ground tissue.

        • Structure: Composed of isodiametric, thin-walled cells with large central vacuoles and prominent intercellular spaces.
        • Function: Performs a wide range of functions, including photosynthesis (in chlorenchyma), storage of food, water, and waste products, and secretion of substances like nectar and oils.
        • Location: Found throughout the plant body, in the cortex, pith, mesophyll of leaves, and as a component of vascular tissues.

      • Collenchyma: The supportive tissue of growing organs.

        • Structure: Composed of elongated cells with unevenly thickened primary walls, particularly at the corners, due to the deposition of cellulose, hemicellulose, and pectin.
        • Function: Provides mechanical support and flexibility to growing parts of the plant, such as young stems and petioles, allowing them to bend without breaking.
        • Location: Typically found in the hypodermis of dicot stems and in the petioles of leaves.

      • Sclerenchyma: The rigid and strengthening tissue.

        • Structure: Composed of long, narrow cells with thick, lignified secondary walls, which are often dead at maturity.
        • Function: Provides mechanical support and protection to the plant.
        • Types:
          • Fibres: Long, tapering cells that occur in bundles.
          • Sclereids: Short, irregular-shaped cells, also known as stone cells, found in the pulp of fruits like guava and pear.

    • Complex Tissues: Composed of multiple types of cells that work together as a unit.

      • Xylem: The primary water-conducting tissue.

        • Function: Responsible for the transport of water and minerals from the roots to the rest of the plant, and also provides mechanical support.
        • Components:
          • Tracheids: Elongated, tube-like cells with tapering ends and lignified walls, with pits for water conduction.
          • Vessels: Long, cylindrical structures made up of vessel members, connected through perforations in their common walls.
          • Xylem Fibres: Sclerenchymatous fibres associated with xylem.
          • Xylem Parenchyma: Parenchymatous cells that store food and assist in the short-distance transport of water.

      • Phloem: The primary food-conducting tissue.

        • Function: Transports sugars (produced during photosynthesis) from the leaves to other parts of the plant for storage or consumption.
        • Components:
          • Sieve Tube Elements: Long, tube-like structures, arranged longitudinally, with perforated end walls called sieve plates.
          • Companion Cells: Specialized parenchymatous cells that are closely associated with sieve tube elements and regulate their function.
          • Phloem Parenchyma: Parenchymatous cells that store food and other substances.
          • Phloem Fibres: Sclerenchymatous fibres that provide mechanical support.

2. The Tissue System: A Coordinated Framework

• Epidermal Tissue System: The Protective Outer Layer

  • Function: Forms the outermost covering of the plant body, providing protection against mechanical injury, water loss, and invasion by pathogens.
  • Components:
    • Epidermal Cells: The most abundant cells in the epidermis, forming a continuous, single layer.
    • Stomata: Specialized pores, typically found on the epidermis of leaves, that regulate gas exchange and transpiration. Each stoma is surrounded by two guard cells.
    • Epidermal Appendages:
      • Trichomes: Hairs or outgrowths from the epidermis, which can be unicellular or multicellular and serve various functions, including protection, secretion, and absorption.
      • Root Hairs: Unicellular extensions of epidermal cells in the root, responsible for the absorption of water and minerals from the soil.

• Ground Tissue System: The Bulk of the Plant Body

  • Function: Comprises all tissues except the epidermis and vascular bundles, and is responsible for a wide range of functions, including photosynthesis, storage, and support.
  • Components:
    • Parenchyma: Forms the bulk of the ground tissue, performing functions like photosynthesis and storage.
    • Collenchyma: Provides mechanical support to growing organs.
    • Sclerenchyma: Offers rigid support and protection.

• Vascular Tissue System: The Conductive Network

  • Function: Consists of the complex tissues, xylem and phloem, which are responsible for the transport of water, minerals, and food throughout the plant.
  • Vascular Bundles: The xylem and phloem are organized into discrete bundles, which can be of different types:
    • Radial: Xylem and phloem are arranged on different radii, as seen in roots.
    • Conjoint: Xylem and phloem are situated at the same radius, as seen in stems and leaves.
      • Collateral: Phloem is located on the outer side of the xylem.
      • Bicollateral: Phloem is present on both the outer and inner sides of the xylem.

3. Anatomy of Dicotyledonous and Monocotyledonous Plants

Dicot Root: A Detailed Cross-Section

• Epiblema (Epidermis): The outermost, single layer of thin-walled, parenchymatous cells. It is devoid of a cuticle and stomata, but bears numerous unicellular root hairs for water absorption.
• Cortex: A wide zone of parenchymatous cells that lies beneath the epiblema. It is responsible for the storage of food and the conduction of water from the root hairs to the inner tissues.
• Endodermis: The innermost layer of the cortex, composed of barrel-shaped cells. The radial and tangential walls of these cells are impregnated with suberin, forming the Casparian strips, which prevent the apoplastic movement of water.
• Stele: The central core of the root, consisting of all tissues internal to the endodermis.
  • Pericycle: A single layer of parenchymatous cells that lies just inside the endodermis. It is the site of origin of lateral roots and the vascular cambium.
  • Vascular Bundles: The vascular tissues are arranged in a radial manner, with alternating bundles of xylem and phloem. The number of xylem bundles typically ranges from two to six (diarch to hexarch). The protoxylem is located towards the periphery (exarch condition).
  • Pith: The central part of the stele, which is either absent or very small and composed of parenchymatous cells.

Monocot Root: A Comparative Analysis

The anatomy of a monocot root is largely similar to that of a dicot root, with a few notable exceptions:

• Vascular Bundles: The number of xylem bundles is typically more than six (polyarch).
• Pith: The pith is large, well-developed, and composed of parenchymatous cells.
• Secondary Growth: Monocot roots do not undergo secondary growth, as they lack a vascular cambium.

Dicot Stem: A Comprehensive Overview

• Epidermis: The outermost layer, composed of a single layer of compactly arranged, parenchymatous cells. It is covered by a waxy cuticle to prevent water loss.
• Cortex: The region lying below the epidermis, differentiated into three zones:
  • Hypodermis: A few layers of collenchymatous cells that provide mechanical support.
  • General Cortex: A zone of parenchymatous cells with intercellular spaces, involved in storage.
  • Endodermis: The innermost layer of the cortex, which is not as well-defined as in the root.
• Stele: The central part of the stem, consisting of the pericycle, vascular bundles, and pith.
  • Pericycle: A layer of sclerenchymatous cells that occurs in patches above the phloem (bundle cap).
  • Vascular Bundles: Arranged in a ring, and are conjoint, collateral, and open (i.e., with a strip of cambium between the xylem and phloem).
  • Pith: A large, central region of parenchymatous cells with intercellular spaces, involved in the storage of food.

Monocot Stem: A Distinctive Arrangement

• Epidermis: The outermost, single layer of parenchymatous cells, covered by a cuticle.
• Hypodermis: A thick, sclerenchymatous layer that provides mechanical support.
• Ground Tissue: A mass of parenchymatous tissue in which the vascular bundles are scattered.
• Vascular Bundles: Numerous, scattered throughout the ground tissue. They are conjoint, collateral, and closed (i.e., lacking a cambium). The vascular bundles are smaller towards the periphery and larger towards the center.
• Pith: The ground tissue is not differentiated into cortex and pith.

Dicot Leaf (Dorsiventral): A Study in Asymmetry

• Epidermis: The leaf is covered by an upper (adaxial) and a lower (abaxial) epidermis. The outer walls are cutinized. The lower epidermis usually has more stomata than the upper epidermis.
• Mesophyll: The ground tissue between the two epidermal layers, differentiated into:
  • Palisade Parenchyma: One or more layers of elongated, compactly arranged cells, rich in chloroplasts, located below the upper epidermis.
  • Spongy Parenchyma: A zone of irregularly shaped, loosely arranged cells with numerous air spaces, located below the palisade parenchyma.
• Vascular Bundles: Present in the veins and midrib, and are conjoint, collateral, and closed. They are surrounded by a bundle sheath of parenchymatous cells.

Monocot Leaf (Isobilateral): A Study in Symmetry

• Epidermis: Stomata are present on both the upper and lower epidermis.
• Mesophyll: The ground tissue is not differentiated into palisade and spongy parenchyma.
• Bulliform Cells: In some grasses, large, empty, and colorless cells, known as bulliform cells, are present in the upper epidermis. These cells help in the rolling and unrolling of the leaf in response to water availability.
• Vascular Bundles: Similar to those of a dicot leaf, but are more numerous and closely arranged.