In this blog, we are going to learn about plant tissue, types of tissues, and their functions. Let us understand about the tissue first! It is a well-known fact that all living organisms are made up of cells. Some are made up of single cells and are called unicellular. In unicellular forms, the same cell acts as a site for diverse life activities such as intake of food (digestion), intake of oxygen (respiration), excretion, reproduction, etc.

While some organisms are multicellular and are composed of millions of different types of cells. In such organisms, the cells cluster together in order to perform specific functions of the body.

For example, In human beings, muscle cells cluster together to perform contraction and relaxation, thus causing movements. Similarly, Nerve cells coordinate to carry messages. Blood cells and plasma flow to transport oxygen, food, hormones and waste materials. Similarly, in plants , cells cluster to transport food and water from one part to the other. Such a cluster (or group) of cells specially positioned and designed to perform a particular function efficiently is called a tissue.

Plant Tissue

Plants and animals have different structures and functions. So, the tissues which they possess are also different. Most of the plants are stationary and remain fixed at one place so they need comparatively less energy. Therefore, most of the tissues of plants are supportive in function and provide mechanical strength. 

These tissues are therefore, thick walled, lignified and dead. Plants grow throughout their life with the help of certain tissues called Meristematic tissues that are present at certain regions in the body. 

Such as, Meristematic tissues at root and shoot apex which divide continuously throughout their life and add new cells to the body. These cells produced by the meristem then differentiate to become permanent tissue which stop dividing and are located at specific regions in plants. 

Types Of Plant Tissue

We already know that tissue is a group of cells which are similar in structure and work together to achieve a particular function in our body. 

Let us now briefly learn about the various types of plant tissues. 

Classification of Plant Tissue

The plant tissues are broadly classified into two types namely:

1. Meristematic tissues
2. Permanent tissues

Meristematic Tissues

What is Meristematic Tissue? Meristematic tissue is a group of living cells located at specific locations which divide continuously to add new cells to the plant body.

Let us now learn a few basic characteristics of Meristematic Tissue.

1. The cells are similar in structure and have thin cellulose cell walls.
2. The shape of cells may be rounded, oval, polygonal or rectangular.
3. The cells are compactly arranged with no intercellular spaces between them.
4. Each cell possesses dense or abundant cytoplasm and a large prominent nucleus.
5. The cells may have a few small or no vacuoles at all.

Let us now learn about the various types of meristematic tissue. We know that the meristematic tissue is restricted to certain specific regions in a plant. Depending on their location in a plant body, meristematic cells are classified as:

Apical, Lateral and Intercalary meristems. 

Let's understand these types one by one.

Based on the region where they are present, meristematic tissues are classified as:

1. Apical Meristem: Located at the growing tips of main and lateral shoots and roots, these cells are responsible for the linear growth of a plant.
2. Lateral: The lateral meristem consists of initials which divide mainly in one plane resulting in an increase in diameter and girth of the stem and root in a plant. 
3. Intercalary Meristematic tissues: This meristem is located between the regions of permanent tissue. Tissue present at the base of leaves or internodes on either side of the node on twigs is intercalary meristem.

Permanent Tissues 

Permanent tissue is a group of cells (living or dead) formed by the meristematic tissue which take up specific roles and lose their ability to divide. This process of obtaining permanent shape, size and function is called differentiation. The permanent tissue is differentiated into different types. When the meristematic cells lose the ability to divide, they form a permanent tissue. Let us now know about the types of Permanent tissues.

The permanent tissues may be of two types- 

1. Simple permanent tissues
2. Complex permanent tissues.

Simple permanent tissues

This type of permanent tissue is called so because it is composed of similar types of cells having a common origin and function. The simple permanent tissues are further classified as the:

1. Parenchyma: 

Parenchyma is a primitive tissue which consists of relatively unspecialized thin-walled cellulosic cells. Let's discuss some more important characteristics of parenchyma. The cells are loosely packed with intercellular spaces between them. The parenchymatous cells are living. They may be isodiametric, spherical or oval in shape. Some parenchyma cells may contain chlorophyll and are called chlorenchyma. The parenchyma cells in aquatic plants possess large air cavities to provide buoyancy for floating. These cells are called the aerenchyma. Parenchyma is widely distributed in various plant organs and occurs mainly in the epidermis, cortex, pith, and the mesophyll of leaves.

Let us now discuss the main functions of parenchyma.

These are:

• To provide mechanical support to the plant,
• To assimilate and store reserve food materials, especially in the parenchyma of stem and root.
• To store waste products such as tannin, gum, crystals, resins, etc.,
• To provide buoyancy to aquatic plants to help them float, and
• To perform photosynthesis if chlorophyll is present.

2. Collenchyma:

Collenchyma is a living tissue which consists of cells which are irregularly thickened at the corners.

 Let us look at some other characteristics of this tissue:

1. The cells of this tissue are generally elongated with oblique end walls.
2. The intercellular spaces are reduced or absent.
3. The collenchyma occurs chiefly in hypodermis of stems and leaves. On the other hand, the monocots and roots are devoid of this tissue. 

Let us now discuss the various functions of collenchyma:

• Collenchyma provides mechanical support, elasticity and tensile strength to the plant body.
• It also helps in manufacturing of sugar and its storage as starch.
• Present in the margins of leaves, it also resists the tearing effect of the wind. 

3. Sclerenchyma, the supportive lignified plant tissue, consists of two types of cells namely: Fibres and Sclereids.

Unlike the parenchyma and the collenchyma, cells of sclerenchyma are dead and consist of thick walls, thickened and hardened due to the deposition of lignin.

Let's have a look at its other characteristics. 

1. Cell walls of sclerenchymatous cells are strong, rigid and impermeable to water. 
2. No intercellular spaces are present.
3. They may be distinguished into long and elongated fibres, or short cells possessing thick lamellate lignified walls called sclereids.
4. Sclerenchyma fibres occur mostly in hypodermis, pericycle, secondary xylem and secondary phloem. 
5. The sclereids are abundant in hard seed coats, and endocarp of almond and coconut.
6. The main function of this tissue is to give mechanical support to the plant parts.

Complex permanent tissue

Unlike the simple tissues, the Complex permanent tissues are made up of more than one type of cells coordinated to perform a common function. 

We also know that the simple permanent tissues are composed of similar types of cells. Do you know what types of cells constitute the complex permanent tissues? 

Let us find out about these complex tissues in this module. 

The complex permanent tissues are composed of groups of more than one type of cells with a common origin and working together as a unit to perform a common function. These tissues have varied functions in plants.

1. The main functions of complex permanent tissues include transportation of water, minerals, nutrients, and food material. 
2. Xylem and phloem are the two important complex tissues in plants. 
3. They are together called the vascular tissues, that is, the conducting tissues.

Let us now discuss these two vascular tissues one by one.

1. Xylem 

In vascular plants, the xylem helps in the conduction of water and mineral salts to different parts of the plant.

Xylem is composed of four kinds of cells namely:

• Tracheids
• Vessels
• Xylem parenchyma
• Xylem fibers

Tracheids are elongated tube-like cells with oblique end walls. They usually appear angular or polygonal and their walls are hard and lignified.

Vessels are placed one upon the other and their end walls are either absent or possess perforations. They form long tubes or channels for the conduction of water and minerals.

Xylem parenchyma is composed of living cells which store food and help in lateral conduction of water.

Xylem fibres provide mechanical strength to the plant body.

Let us now have a look at some important functions of xylem-: 

Xylem is the major conducting tissue in vascular plants which helps in the upward movement of water and mineral salts from root to different aerial parts of the plants. It also provides mechanical strength to the plant body.

2. Phloem 

In vascular plants, the phloem helps in the translocation of organic solutes from the leaf to other parts of the plant. The movement of materials can be in both the directions unlike xylem where only lateral conduction is possible.

The Phloem is composed of four elements- 

1. Sieve tubes
2. Companion cells
3. Phloem parenchyma
4. Phloem fibres

Sieve tubes are long tubular structures composed of elongated sieve tube elements. Its end walls have numerous pores.

Companion cells are living cells always associated with the sieve tubes. Phloem parenchyma is also composed of living cells.

Phloem fibres are dead sclerenchymatous cells.

Functions of phloem-: 

The main function of phloem is translocation of organic solutes from leaves to storage organs and later from storage organs to growing regions.

Read More: What is Cell - Structure, Fuctions And Types