The Permanent Tissue

Plants are composed of three major organs: roots, stems and leaves. All these organs of plants are composed of different types of tissues that work to perform a common function. In this article we will study about different types of plant tissue systems, their place and purpose in a plant. Plant tissue systems are characterised and are classified based on their structure and function. The organs that are formed are organised into patterns within a plant. 

Plant Tissue System

It refers to a collection of cells that performs a specific function. Plant tissues are of many different types and each type performs a certain specialised function. Plant tissue is defined as a fundamental unit that connects all organs of the plant. A tissue refers to an arrangement of similar cells and their extracellular matrix of same origin that carry out certain specific functions. 

Thus an organ is formed by functional grouping of all multicellular tissues together. The study of tissues is known as histology. Xavier Bichat is known as the “father of histology”. In plant anatomy tissues can be broadly classified into three tissue systems: the epidermis, the ground tissues and the vascular tissue let us understand each system in detail.

Epidermal Tissue System

It is the outermost covering of plants. Epidermal Tissue System comprises epidermis, stomata and epidermal outgrowths. Epidermis is usually composed of a single layer of parenchymatous cells that are compactly arranged without any intercellular spaces. In leaves some specialised cells surrounding the stomata are known as the guard cells. Chloroplast is present only in the guard cells of the epidermis. Other epidermal cells generally do not possess chloroplasts. The outer wall of epidermis is mainly covered by cuticles.

Stoma is a minute pore that is surrounded by two guard cells. The stomata occur  usually in the epidermis of leaves. In some plants like sugarcane, the guard cells are bound via some special cells. They are different from other epidermal cells. These cells are known as subsidiary or accessory cells. Trichomes and root hairs represent epidermal outgrowths. 

The unicellular or multicellular appendages that arise from the epidermal cells are known as trichomes. Trichomes appear in branched or unbranched form. Rhizodermis possess two types of epidermal cells like the long cells and short cells. The short cells are known as trichoblasts. Root hairs are formed from these trichoblasts.

Function of Epidermal Tissue System

1. This tissue system in the shoot helps in checking excessive loss of water due to the presence of cuticles.

2. Epidermis helps in protecting the underlying tissues.

3. Stomata is mainly involved in transpiration and gaseous exchange.

4. Trichomes help in the dispersal of seeds and fruits.

5. Root hairs are meant to absorb water and mineral salts from the soil.

Vascular Tissue System

The vascular tissue system comprises xylem and phloem. The elements of xylem and phloem are always present in groups. They are known as vascular bundles. In the dicot stem, the vascular bundle comprises cambial tissue in between xylem and phloem. Such a vascular bundle is known as an open vascular bundle. Whereas, in monocot stem, cambium is absent in the vascular bundle, thus it is referred to as closed vascular bundle.

In roots, xylem and phloem are seen to be arranged in an alternate manner on different radii. It is known as radial arrangement. Inside stems and leaves, xylem and phloem are arranged in the same radius and form a vascular bundle together. This type of vascular bundle is known as conjoint vascular bundle. Based upon the mutual relationship of xylem and phloem, conjoint vascular bundles are classified into three types. They are collateral, bicollateral and concentric.

If xylem and phloem present in a vascular bundle is  arranged along the same radius with phloem towards the outside, this type of vascular bundle is known as collateral vascular bundle. If phloem appears on both the outer and inner sides of xylem, the bundle is known as bicollateral. Bicollateral vascular bundles are mostly seen in Cucurbitaceous. The bundle where the phloem surrounds the xylem or xylem surrounds the phloem completely is called concentric vascular bundle.

This is further divided into two types amphicribral and amphivasal. In amphicribral concentric vascular bundles, the phloem completely covers the xylem. Eg. Polypodium. In amphivasal concentric vascular bundles, the xylem surrounds the phloem completely. 

Eg. Acorus. In roots, protoxylem vessels are present towards the periphery and the metaxylem vessels lie towards the centre. This arrangement of xylem is known as exarch. In the stem, protoxylem vessels lie towards the centre, while metaxylem is present in the periphery. This condition is referred to as endarch.

Ground Tissue System

The ground or fundamental tissue system comprises the main body of the plants. It contains all the tissues except epidermis and vascular bundles. In monocot stem, ground tissue system refers to a continuous mass of parenchymatous tissue in which vascular bundles are found scattered. In this ground tissue is not differentiated into cortex, endodermis, pericycle and pith. Usually in the dicot stem, the ground tissue system can be differentiated into three main zones – cortex, pericycle and pith.

The cortex arises between the epidermis and pericycle. Cortex is a few to many layers in thickness. In many cases, the cortex is composed of parenchyma tissues. Intercellular spaces can be present or absent. Cortical cells may possess non-living inclusions such as starch grains, oils, tannins and crystals.

In the leaves, the ground tissue comprises  chlorenchyma tissues. This region is known as mesophyll. The innermost layer of the cortex is known as endodermis. Usually endodermis is composed of barrel shaped parenchyma cells. These cells can be  arranged in a single layer without intercellular spaces. Pericycle is present between the endodermis and the vascular bundles. It is mainly composed of parenchyma cells. 

Lateral roots arise from the pericycle. Therefore their origin is endogenous. The central portion of the ground tissue is called pith or medulla. Mainly this is composed of thin walled parenchyma cells which may be with or without intercellular spaces. The cells in the pith mainly store starch, fatty substances, tannins, phenols, calcium oxalate crystals, etc.

Conclusion

Permanent tissues are tissues that have reached maturity and no longer have the ability to divide. Tissues that are permanent are made up of cells that do not divide. These tissues’ cells have been changed to execute certain roles. Permanent tissue cells are completely matured, larger in size, and have a specific shape. The metabolic rate of cells in permanent tissues is lower. Meristematic tissue gives rise to permanent tissues.