Ground Tissue System: Ground tissue includes all tissues except epidermis and vascular bundles. The ground tissue comprises the bulk of the primary plant body. Parenchyma, collenchyma and sclerenchyma cells are common in the ground tissue.
Parenchyma cells are the most common cell type within ground tissue. They can divide into any plant cell, such as when a cut house plant establishes a new root system. Parenchyma cells contain chloroplasts and palisade mesophyll to facilitate light absorption and photosynthesis within leaves. Within the root system, these cells store sugar or starch. Within seeds, parenchyma cells create the endosperm, the nutritious material surrounding a seed. There are three specialized parenchyma cells within plants:
- Palisade parenchyma cells exist as one or two layers immediately below the leaf’s epidermis and are especially long looking, like columns.
- Spongy mesophyll exists below the palisade parenchyma cells and helps fill space between the dermal and vascular tissue systems.
- Ray parenchyma cells (also known as medullary rays) move nutrients (i.e., sap) across the width of a woody stem.
Diagram of Ground Tissue System
Collenchyma cells have a cell wall that is a bit thicker than that of the parenchyma, but still flexible. They are responsible for structural support throughout the plant’s growing shoot system, yet are only alive once the cells become mature. Botanists find collenchyma cells within the corner of angled stems or bundles of vascular tissue.
Sclerenchyma cells are unable to stretch and, therefore, are hard and tough. These cells die at maturity and are responsible for structural support in a mature stem once a plant (be it woody or herbaceous) has stopped growing. Sclerenchyma cells are the source fiber for rope and linen.
Each of the above cell types has a primary cell wall. The primary cell wall is elastic, allowing the individual cell to grow longer and expand outward. Only sclerenchyma cells, however, have a secondary cell wall. This secondary cell wall typically contains lignin, which is the main substance found in woody vegetation. Lignin makes the cell wall rigid and inflexible, which is necessary for structural support among trees and shrubs.
It is divided into following parts:
(i) Hypodermis: It is situated below the epidermis. It is multilayered and made up of parenchymatous and sclerenchymatous cells.
(ii) General Cortex: This consists of parenchymatous cells with or without chloroplasts.
(iii) Endodermis: Endodermis is single layered made up of parenchymatous cells. The radial and internal walls of endodermal cell are thickened; a band of lignin or suberin knows as casparian strip is sometimes found on the radial and transverse wall of every cell.
(iv) Pericycle: It is single or multilayered and is situated in between endodermis and vascular bundles. It is made up of sclerenchymatous and parechymatous cells.
(v) Pith: The central portion in stems and roots is called pith or medulla. It is made up of parenchymatous cells with intercellular spaces. In dicot stem the pith is large and well developed; in dicot roots the pith is either absent or small; in monocot roots large pith is present; in monocot stem the vascular bundles are scattered and the ground tissue is not marked into different parts.
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By Team Learning Mantras