Tissue System

6.3 The Tissue System

We were discussing types of tissues based on the types of cells and their functions. Let us consider how these tissues are distributed in different parts of the plant. Are these tissues organised in specific regions for specific functions? On the basis of their structure and location, there are three types of tissue systems.

• Epidermal tissue system
• Ground or fundamental tissue system
• Vascular or conducting tissue system

6.3.1 Epidermal Tissue System

The epidermal tissue system forms the outermost covering of the whole plant body and comprises epidermal cells, stomata, and the epidermal appendages — the trichomes and hairs. The epidermis is usually single-layered. Epidermal cells are parenchymatous with a small amount of cytoplasm lining the cell wall and a large vacuole. The outside of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water. Cuticle is absent in roots.

Stomata are structures present in the epidermis of leaves. They regulate the process of transpiration and gaseous exchange. Each stoma is composed of two bean-shaped cells known as guard cells. In grasses, the guard cells are dumb-bell shaped. The outer walls of guard cells (away from stomatal pore) are thin and the inner walls (towards the pore) are highly thickened. The guard cells possess chloroplasts and regulate the opening and closing of stomata. Sometimes a few epidermal cells, in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells. The stomatal aperture, guard cells and the surrounding subsidiary cells are together called the stomatal apparatus.

The cells of epidermis bear many hair-like structures called trichomes in the shoot system; root epidermis bears root hairs. The trichomes in the shoot system are usually multicellular. They may be branched or unbranched and soft or stiff. They may even be secretory. The trichomes help in preventing water loss due to transpiration.

6.3.2 Ground Tissue System

All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. Parenchymatous cells are usually present in cortex, pericycle, pith and medullary rays, in primary stems and roots.

6.3.3 Vascular Tissue System

The vascular system consists of complex tissues — phloem and xylem. The xylem and phloem together constitute vascular bundles. In dicot stems, cambium is present between phloem and xylem. 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 monocots, the vascular bundles have no cambium present in them. Hence, since they do not form secondary tissues, they are referred to as closed vascular bundles. When xylem and phloem within a vascular bundle are arranged in an alternate manner along the different radii, the arrangement is called radial as in roots. In conjoint type of vascular bundles, the xylem and phloem are jointly situated along the same radius of vascular bundles. Conjoint vascular bundles usually have the phloem located only on the outer side of xylem.

6.4 Anatomy of Dicotyledonous and Monocotyledonous Plants

6.4.1 Dicotyledonous Root

The internal structure of dicot root (e.g., gram, Arachis) shows: epiblema (epidermis with root hairs) → cortex (multilayered parenchyma) → endodermis (single-layered with Casparian strips) → pericycle (gives rise to lateral roots and lateral cambium) → vascular bundles (radial; usually 2-4 patches; xylem exarch) with conjunctive tissue between them → pith (small or absent).

6.4.2 Monocotyledonous Root

The monocot root (e.g., maize) shows similar layers: epiblema, cortex, endodermis, pericycle. But the vascular bundles are polyarch (more than 6 — often 8-12 xylem groups; many phloem patches alternating). Pith is large and well developed. There is no secondary growth in monocot roots.

6.4.3 Dicotyledonous Stem

The transverse section of a typical young dicot stem shows: epidermis (with cuticle, stomata, multicellular hairs) → cortex (3 zones: hypodermis = collenchyma; general cortex = parenchyma; endodermis = innermost cortex layer rich in starch, also called starch sheath) → pericycle (sclerenchymatous patches above phloem) → vascular bundles (conjoint, collateral, open with cambium; arranged in a ring; xylem endarch) → medullary rays (parenchyma between bundles) → pith (large parenchymatous centre).

6.4.4 Monocotyledonous Stem

The monocot stem (e.g., maize) has: sclerenchymatous hypodermis, large number of scattered vascular bundles embedded in parenchymatous ground tissue. Each bundle is conjoint, collateral, and closed (no cambium). Phloem parenchyma is absent. Water-containing cavities are present within the vascular bundles. The peripheral bundles are smaller and tightly packed; central bundles are larger and widely spaced.

6.4.5 Dorsiventral (Dicot) Leaf

The vertical section of a dorsiventral leaf (e.g., mustard, mango) shows: upper epidermis (with cuticle) → palisade parenchyma (long columnar cells with chloroplasts; main photosynthesising cells) → spongy parenchyma (irregular, loosely arranged cells with air spaces) → lower epidermis (with stomata; cuticle thinner). The mesophyll = palisade + spongy. Vascular bundles run through the leaf in the midrib and veins.

6.4.6 Isobilateral (Monocot) Leaf

The monocot leaf (e.g., maize, grass) is typically isobilateral: stomata are present on both surfaces, mesophyll is NOT differentiated into palisade and spongy — it is uniform. Some grasses have specialized large, thin-walled cells in the upper epidermis called bulliform cells; when full of water they keep the leaf flat; on water loss they shrink, causing the leaf to fold/roll up — minimising further water loss.

Comparison: Dicot vs Monocot Anatomy

Worked Examples