Chapter-6. Tissues

Main-points: 

• In unicellular organisms, a single cell performs all basic functions. For example, in Amoeba, a single cell carries out movement, intake of food and respiratory gases, respiration and excretion.
• In multi-cellular organisms there are millions of cells. Most of these cells are specialised to carry out a few functions. Each specialised function is taken up by a different group of cells.
• Multi-cellular organisms show division of labour.
• Cells specialising in one function are often grouped together in the body.
• A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.

In Human being:

Muscle Cells: contract and relax to cause movement.

Nerve cells: carry messages.

Blood cells: Form blood and blood flows to transport oxygen food hormones and waste material.

In plants:

Vascular tissues conduct food and water from one part of the plant to other parts.

Tissue: A group of cells having similar functions and similar structure is known as tissue.

Plant Tissues:

(i)  Plants are stationary or fixed – they don’t move.

(ii)  Most of the tissues they have are supportive, which provides them with structural strength.

(iii) Most of these tissues are dead, since dead cells can provide mechanical strength as easily as live ones, and need less maintenance.

(iv) The growth in plants is limited to certain regions.

(v) There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Based on the dividing capacity of the tissues, various plant tissues can be classified as growing or meristematic tissue and permanent tissue.

Animal Tissues:

(i) Animals on the other hand move around in search of food, mates and shelter.

(ii) They consume more energy as compared to plants.

(iii) Most of the tissues they contain are living.

(iv) Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non-dividing regions in animals.

Type of Tissue:

(1)  MERISTEMATIC TISSUE:

The growth of plants occurs only in certain specific regions. This is because the dividing tissue, such dividing tissues are located at growing parts of the plants. Such tissues are known as meristematic tissue.

Classification of Meristematic Tissue:

(A) Apical Meristem: Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root.

(B) Lateral Meristem: The girth of the stem or root increases due to lateral meristem (cambium).

(C) Intercalary meristem: Intercalary meristem is the meristem at the base of the leaves or internodes (on either side of the node) on twigs.

Properties Of Meristematic Tissue:

(i)The cells of this tissue are very active,

(ii) They have dense cytoplasm, thin cellulose walls and prominent (Clear) nuclei.

(iii) They have lack in vacuoles.

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(2) PERMANENT TISSUE :

The tissues developed or formed by meristematic tissues take up a specific role and lose the ability to divide further. As a result, they form a permanent tissue.

Cells of meristematic tissue differentiate to form different types of permanent tissue.

Definition: The cells which are differentiated to perform specific function and lose the ability to divide are called permanent tissues.

Differentiation: This process of taking up a permanent shape, size, and a function is called differentiation.

Type of permanent tissue:

(A) Simple permanent tissue:

It compose of single type of cell, which look like each other. Such tissues are called simple permanent tissue. 

E.g: Parenchyma, collenchymas, sclerenchyma etc.

Type of simple permanent tissues:

(1) Parenchyma: A type of permanent tissue, whose few layer of cells form basic packing of tissue. These are called parenchyma.

Features:

(i)  It consists of relatively unspecialised cells with thin cell walls.

(ii) They are live cells.

(iii) They are usually loosely packed.

(iv) therefore large spaces between cells (intercellular spaces) are found in this tissue.

(v) This tissue provides support to plants and also stores food.

(vi) The parenchyma of stems and roots also stores nutrients and water.

(i) Chlorenchyma: In some other parenchyma, it contains chlorophyll and performs photosynthesis, and then it is called chlorenchyma.

(ii) Aerenchyma: In aquatic plants, large air cavities are present in parenchyma to give buoyancy to the plants to help them float. Such a parenchyma type is called aerenchyma.

(2) Collenchyma: This is another simple permanent tissue which gives flexibility in plants. It allows easy bending in various parts of a plant (leaf, stem) without breaking.  Such type of permanent tissue is called collenchymas.

We can find this tissue in leaf stalks below the epidermis.

Features:

(i)  It gives flexibility to plants.

(ii) It allows easy bending in various parts of a plant (leaf, stem) without breaking.

(iii) It also provides mechanical support to plants.

(iv) The cells of this tissue are living, elongated and irregularly thickened at the corners.

(v)  There is very little intercellular space.

(3) sclerenchyma: This also another type of simple permanent tissue, which makes the plants hard and stuff. Such a simple permanent tissue is called sclerenchyma. Example : The husk of a coconut.

This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts.

Features:

(i) The cells of this tissue are dead.

(ii) They are long and narrow as the walls are thickened due to lignin.

(iii) Often these walls are so thick that there is no internal space inside the cell.

(iv) It provides strength to the plant parts.

Lignin: a chemical substance which acts as cement and hardens them.

Differentiation among Parenchyma, collenchymas and Sclerenchyma: