Tissue system – epidermal tissue system and Functions, Vascular tissue system
Tissue system – epidermal tissue system and Functions, Vascular-tissue-system
The tissue system
A group of tissues performing a similar function irrespective of its position in the plant body is called a tissue system.
In 1875, Sachs recognized three tissue systems in the plants.
They are epidermal tissue system, vascular tissue system and fundamental tissue system.
Epidermal tissue system
Epidermal tissue system is the outermost covering of plants.
It consists of epidermis, stomata and epidermal outgrowths.
Epidermis is generally composed of single layer of parenchymatous cells compactly arranged without intercellular spaces.
But it is interrupted by stomata. In leaves some specialized cells which surround the stomata are called the guard cells.
Chloroplasts are present only in the guard cells of the epidermis. Other epidermal cells usually do not have chloroplasts.
The outer wall of epidermis is usually covered by cuticle.
Stoma is a minute pore surrounded by two guard cells.
The stomata occur mainly in the epidermis of leaves. In some plants such as sugarcane, the guard cells are bounded by some special cells.
They are distinct from other epidermal cells. These cells are called subsidiary or accessory cells.
Trichomes and root hairs are some epidermal outgrowths. The unicellular or multicellular appendages that originate from the epidermal cells are called trichomes.
Trichomes may be branched or unbranched. Rhizodermis has two types of epidermal cells – long cells and short cells.
The short cells are called trichoblasts. Root hairs are produced from these trichoblasts.
Functions of epidermal tissue system
1. This tissue system in the shoot checks excessive loss of water due to the presence of cuticle.
2. Epidermis protects the underlying tissues.
3. Stomata involve in transpiration and gaseous exchange.
4. Trichomes are also helpful in the dispersal of seeds and fruits.
5. Root hairs absorb water and mineral salts from the soil.
Vascular tissue system
The vascular tissue system consists of xylem and phloem. The elements of xylem and phloem are always organized in groups.
They are called vascular bundles. In dicot stem, the vascular bundle consists of cambial tissue in between xylem and phloem.
Such vascular bundle is called open vascular bundle. In monocot stem, cambium is absent in the vascular bundle, hence it is known as closed vascular bundle
In roots, xylem and phloem are arranged in an alternate manner on different radii.
It is called radial arrangement. In stems and leaves, xylem and phloem are arranged at the same radius and form a vascular bundle together.
Such vascular bundle is called conjoint vascular bundle. Depending upon the mutual relationship of xylem and phloem, conjoint vascular bundles are divided into three types.
They are collateral, bicollateral and concentric.
If xylem and phloem in a vascular bundle are arranged along the same radius with phloem towards the outside, such vascular bundle is called collateral vascular bundle.
If phloem occurs on both the outer and inner sides of xylem, the bundle is called bicollateral.
Bicollateral vascular bundles are most typically seen in Cucurbitaceae.
The bundle in which either phloem surrounds the xylem or xylem surrounds the phloem completely is known as concentric vascular bundle.
This is of two types amphicribral and amphivasal. In amphicribral concentric vascular bundles, the phloem completely surrounds the xylem. eg. Polypodium.
In amphivasal concentric vascular bundles, the xylem completely surrounds the phloem. eg. Acorus.
In roots, protoxylem vessels are present towards the periphery and the metaxylem vessels towards the centre. This arrangement of xylem is called exarch.
In stem, protoxylem vessels are towards the centre, while metaxylem towards the periphery. This condition is known as endarch.
Ground or fundamental tissue system
The ground or fundamental tissue system constitutes the main body of the plants.
It includes all the tissues except epidermis and vascular bundles.
In monocot stem, ground tissue system is a continuous mass of parenchymatous tissue in which vascular bundles are found scattered.
Here ground tissue is not differentiated into cortex, endodermis, pericycle and pith.
Generally in dicot stem, ground tissue system is differentiated into three main zones – cortex, pericycle and pith.
The cortex occurs between the epidermis and pericycle. Cortex may be a few to many layers in thickness.
In most cases, cortex is made up of parenchyma tissues. Intercellular spaces may or may not be present.
Cortical cells may contain non-living inclusions like starch grains, oils, tannins and crystal.
In the leaves, the ground tissue consists of chlorenchyma tissues. This region is called mesophyll.
The inner most layer of the cortex is called endodermis. Generally endodermis is made up of barrel shaped parenchyma cells.
These cells are arranged in a single layer without intercellular spaces.
Pericycle occurs between the endodermis and the vascular bundles. It is generally made up of parenchyma cells.
Lateral roots originate from the pericycle. Thus their origin is endogenous.
The central part of the ground tissue is known as pith or medulla. Generally this is made up of thin walled parenchyma cells which may be with or without intercellular spaces.
The cells in the pith generally store starch, fatty substances, tannins, phenols, calcium oxalate crystals, etc.
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