Chapter Notes

The Tissue System

The study of the internal structure of plants is called anatomy. Just like in animals, the basic unit of a plant is the cell. These cells group together to form tissues, which in turn are organized into organs like roots, stems, and leaves. The internal structure of these organs can vary significantly, especially between the two major groups of flowering plants: monocots and dicots. Plant anatomy also reveals fascinating adaptations to different environments.

Based on their structure and location within the plant, tissues are grouped into three main tissue systems:

1. The Epidermal Tissue System
2. The Ground or Fundamental Tissue System
3. The Vascular or Conducting Tissue System

Epidermal Tissue System

The epidermal tissue system forms the complete outer covering of the plant's body. It acts like the plant's "skin" and includes three main components: epidermal cells, stomata, and epidermal appendages (trichomes and hairs).

• Epidermis: This is the outermost layer of the primary plant body.

  • It's usually a single layer of elongated, compactly arranged cells that form a continuous sheet.
  • These cells are parenchymatous, meaning they have a thin cell wall, with a large central vacuole and a thin lining of cytoplasm.
  • The outside of the epidermis is often coated with a waxy, thick layer called the cuticle, which is crucial for preventing water loss. The cuticle is absent in roots.

• Stomata: These are tiny pores found in the epidermis, especially on leaves.

  • Their main functions are to regulate transpiration (water loss) and allow for gaseous exchange (taking in CO₂ and releasing O₂).
  • Each stoma consists of a pore enclosed by two specialized cells called guard cells.
  • In most plants, guard cells are bean-shaped. In grasses, however, they are characteristically dumb-bell shaped.
  • The inner walls of the guard cells (facing the pore) are thick, while the outer walls are thin. This structural difference helps in the opening and closing of the pore.
  • Guard cells contain chloroplasts and actively control the size of the stomatal opening.
  • Sometimes, the epidermal cells surrounding the guard cells become specialized in shape and size. These are known as subsidiary cells.
  • The entire unit—comprising the stomatal pore, guard cells, and surrounding subsidiary cells—is called the stomatal apparatus.

• Epidermal Appendages: These are outgrowths from the epidermal cells.

  • Root Hairs: These are unicellular extensions of epidermal cells on the root. Their primary job is to absorb water and minerals from the soil.
  • Trichomes: These are the hairs found on the stem and other parts of the shoot system.
    • They are usually multicellular.
    • They can be branched or unbranched, soft or stiff, and some may even be secretory (releasing substances).
    • A key function of trichomes is to help prevent water loss from transpiration.

The Ground Tissue System

The ground tissue includes all the tissues in a plant except for the epidermis and the vascular bundles. It makes up the main "bulk" of the plant body.

• It is composed of simple tissues like parenchyma, collenchyma, and sclerenchyma.
• In primary stems and roots, ground tissue is found in areas like the cortex, pericycle, pith, and medullary rays.
• In leaves, the ground tissue is called mesophyll. It consists of thin-walled cells that contain chloroplasts and are the primary site of photosynthesis.

The Vascular Tissue System

The vascular tissue system is the plant's transportation network. It is made of complex tissues, primarily xylem and phloem, which are bundled together to form vascular bundles.

• Xylem: Transports water and minerals from the roots to the rest of the plant.
• Phloem: Transports food (sugars produced during photosynthesis) from the leaves to other parts of the plant.

The arrangement and type of vascular bundles differ between plant groups:

• Open vs. Closed Vascular Bundles:

  • Open Vascular Bundles: Found in dicot stems. A layer of cambium is present between the xylem and phloem. This cambium is a meristematic tissue that allows for the formation of secondary xylem and phloem, leading to secondary growth (increase in girth).
  • Closed Vascular Bundles: Found in monocots. There is no cambium between the xylem and phloem. As a result, these plants cannot form secondary tissues and do not undergo secondary growth.

• Arrangement of Vascular Bundles:

  • Radial Arrangement: Xylem and phloem are arranged in an alternating pattern on different radii (like spokes on a wheel). This arrangement is characteristic of roots.
  • Conjoint Arrangement: Xylem and phloem are situated together on the same radius. This is common in stems and leaves. In conjoint bundles, the phloem is typically located on the outer side of the xylem.

Anatomy of Dicotyledonous and Monocotyledonous Plants

To understand the internal organization of tissues, we can study transverse sections of roots, stems, and leaves from both dicots and monocots.

Dicotyledonous Root (e.g., Sunflower Root)

• Epiblema: The outermost layer, with some cells extending to form unicellular root hairs.
• Cortex: A large region made of several layers of thin-walled parenchyma cells with intercellular spaces.
• Endodermis: The innermost layer of the cortex. It is a single layer of tightly packed, barrel-shaped cells. Their radial and tangential walls have a waterproof, waxy deposit of suberin, forming the Casparian strips.
• Stele: All the tissues located inside the endodermis. It includes the pericycle, vascular bundles, and pith.
• Pericycle: A few layers of thick-walled parenchymatous cells just inside the endodermis. Lateral roots and vascular cambium (during secondary growth) originate from these cells.
• Vascular Bundles: There are usually two to four (diarch to tetrarch) patches of xylem and phloem arranged radially. The tissue between the xylem and phloem is called conjuctive tissue.
• Pith: The central core is very small or inconspicuous.

Monocotyledonous Root

The anatomy of a monocot root is quite similar to a dicot root, but with a few key differences:

• Vascular Bundles: It has many more xylem bundles, typically more than six. This condition is called polyarch.
• Pith: The central pith is large and well-developed.
• Secondary Growth: Monocot roots do not undergo secondary growth.

Dicotyledonous Stem

• Epidermis: The outermost protective layer, covered with a cuticle. It may have trichomes and a few stomata.
• Cortex: This region is divided into three sub-zones:
  1. Hypodermis: A few layers of collenchymatous cells just below the epidermis, providing mechanical strength.
  2. Cortical Layers: Rounded, thin-walled parenchymatous cells with intercellular spaces.
  3. Endodermis: The innermost layer of the cortex, also called the starch sheath because its cells are rich in starch grains.
• Pericycle: Found on the inner side of the endodermis, present as semi-lunar patches of sclerenchyma above the phloem.
• Vascular Bundles: A key feature is that the vascular bundles are arranged in a ring. Each bundle is conjoint, open (contains cambium), and has endarch protoxylem (protoxylem faces the center).
• Medullary Rays: Layers of radially placed parenchyma cells located between the vascular bundles.
• Pith: A large, central region composed of rounded, parenchymatous cells with large intercellular spaces.

Monocotyledonous Stem

• Hypodermis: It is sclerenchymatous, providing significant mechanical support.
• Vascular Bundles: They are scattered throughout the ground tissue, not arranged in a ring.
  • Each bundle is conjoint and closed (no cambium).
  • They are surrounded by a sclerenchymatous bundle sheath.
  • Peripheral (outer) bundles are generally smaller than the centrally located ones.
• Ground Tissue: The ground tissue is not differentiated into cortex, pericycle, and pith. It is a large, continuous mass of parenchymatous cells.
• Other Features: Phloem parenchyma is absent, and water-containing cavities are often present within the vascular bundles.

Dorsiventral (Dicotyledonous) Leaf

A vertical section of a dicot leaf shows three main parts: epidermis, mesophyll, and the vascular system.

• Epidermis: Covers both the upper (adaxial) and lower (abaxial) surfaces.
  • It has a conspicuous cuticle.
  • The lower (abaxial) epidermis typically has more stomata than the upper (adaxial) one, which may even lack them completely.
• Mesophyll: The tissue between the two epidermal layers. It contains chloroplasts and is the site of photosynthesis. It is differentiated into two types:
  1. Palisade Parenchyma: Located on the adaxial side, it consists of elongated cells arranged vertically and parallel to each other.
  2. Spongy Parenchyma: Situated below the palisade layer, it consists of oval or round, loosely arranged cells with numerous large air cavities.
• Vascular System: This includes the vascular bundles, which are visible as veins and the midrib.
  • The size of the bundles depends on the size of the vein, which reflects the reticulate venation of dicot leaves.
  • Bundles are surrounded by a layer of thick-walled bundle sheath cells.

Isobilateral (Monocotyledonous) Leaf

The anatomy is similar to a dicot leaf in many ways but has distinct differences:

• Stomata: They are present on both the upper (adaxial) and lower (abaxial) surfaces of the epidermis.
• Mesophyll: It is not differentiated into palisade and spongy parenchyma. The cells are more or less uniform.
• Venation: The parallel venation is reflected in the near-similar sizes of all vascular bundles (except in the main veins).
• Bulliform Cells: In grasses, some adaxial epidermal cells along the veins become modified into large, empty, colorless cells called bulliform cells.
  • When these cells are turgid (full of water), the leaf surface is exposed and flat.
  • During water stress, they become flaccid (lose water), causing the leaves to curl inwards to minimize water loss.