Introduction of Tissues

• A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.
• It is arranged and designed so as to give the highest efficiency of function. Example-Blood, Phloem and muscle of tissues.
• Plants are stationary or fixed- they don’t move. Most of the tissues they have are supportive, which provides them with structural strength.
• Most of these tissues are dead, since dead cells can provide mechanical strength as easily as live ones, and need less maintenance.

There are two types of plant Tissues:-

1)Meristematic Tissues:-Meristematic tissue is the dividing tissue present in the growing regions of the plant, It is similar in structure in structure and have thin cellulose cell walls and it is very compact and do not have intercellular space between them.

FEATURES OF MERISTEMATIC TISSUE:–

• Meristematic tissue acts as a parent tissue from which other tissues develop.
• These tissues take part in growth by formation of new cells.
• With the help of meristems, plants continue to produce new leaves, branches of stem and root, flowers, fruits and root hairs.
• The place of injury in plants is healed up by the formation of new cells by meristems.
• The plant shoots lodged or bent by wind are made to grow upright by activity of intercalary meristem.

There are three types of meristematic tissue-Apical , Lateral and Intercalary

a)Apical meristems:- These are situated at the growing tip of stems and roots, i.e., at shoot apex and root apex and it is also found at apices of the leaves. It results in an increase in the height of the plant.

b)Lateral meristems:- These are found beneath the bark and in vascular bundles of dicot roots and stems. They occur in thin layers. Cambium is the region which is responsible for growth in thickness.

c)Intercalary meristems:- They are located at the base of leaves or internode, e.g., stems of grasses and other monocots. Such tissues also occur below the nodes. It produces an increase in the length leaves and internodes.

2) Permanent Tissue:-Permanent tissues are derived from meristematic tissue once they lose the ability to divide.

This process of taking up a permanent shape, size, and a function is called differentiation. Cells of meristematic tissue differentiate to form different types of permanent tissue. They are classified as simple and complex tissues.

a)Simple Permanent Tissue:-These tissues are composed of cells which are structurally and functionally similar. Thus, these tissues are all made of one type of cells. Different types of permanent tissue are as follows:

1)Parenchyma:- Parenchyma forms the bulk of the plant body. Parenchyma cells are living and posses the power of division. The cells are rounded or isodiametric, that means equally expanded on all sides. The parenchyma cells are oval, round or elongated in shape. The cell wall is thin and encloses a dense cytoplasm which contains a nucleus and surrounds a large central vacuole.

It is widely distributed in plant bodies such as stem, roots, leaves, flowers and fruits. So, it is found in the soft parts of the plant such as cortex of roots, ground tissues in stems and mesophyll of leaves.

Functions of Parenchyma:

• Parenchyma serves as a packing tissue, to fill the spaces between other tissues and maintain the shape and firmness of the plant due to its turgid cells.
• Due to turgidity property, parenchyma acts as a primary support to the stem of herbaceous plants.
• The main function of parenchyma is to store and assimilate food. It serves as a food storage tissue.
• Transport of materials occurs through cells or cell walls of parenchyma cells.
• parenchyma cells are metabolically active; their intercellular air spaces allow gaseous exchange.
• It stores waste products of plants such as tannin, gum, crystals, resins, or inorganic waste, etc.
• If chloroplast is present, the parenchyma tissue is called chlorenchyma and it performs photosynthesis.
• Large air cavities are present in parenchyma to give buoyancy to the plants to help them float. Such a parenchyma type is called aerenchyma.
• The parenchyma of stems and roots also stores nutrients and water.

2)Collenchyma:- It allows easy bending in various parts of a plant (leaf and stem) without breaking. This tissue is find in leaf stalks below the epidermis. The cells of this tissue are living, elongated and irregularly thickened at the corners. There is a very little intercellular space . They often contain a few chloroplasts.

Function of Collenchyma:

• It provides mechanical support and elasticity to plants.
• It allows easy bending in various parts of a plant (leaf and stem) without breaking.
• When cells of collenchyma contain some chloroplasts, they manufacture sugar and starch.

Difference between Parenchyma and Collenchyma.

3)Sclerenchyma: Sclerenchyma cells are dead cells and they are devoid of protoplasm. The cell walls of sclerenchyma are greatly thickened of lignin. Due to excessive thickening of a sclerenchyma cell wall, its cell, its cell cavity or lumen becomes nearly absent. The cells of sclerenchyma are closely packed without intercellular spaces.

This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. The sclerenchyma occurs in abundance either in patches or definite layers.

Function of Sclerenchyma: The sclerenchyma is mainly mechanical and protective function. It gives strength, rigidity, flexibility and elasticity to the plant body and, thus, enables it to withstand various strains.

Epidermis:-

• The outermost layer of cells, called epidermis. The epidermis is usually made of a single layer of cells.
• The epidermis may be thicker since protection against water loss is critical.
• The entire surface of a plant has this outer covering of epidermis. It protects all the parts of the plant . Often secrete a waxy, water- resistant layer on their outer surface.
• It has a protective role to play, cells of epidermal tissue form a continuous layer without intercellular spaces.
• Small pores here and there in the epidermis of the leaf. These pores are called stomata. Stomata are enclosed by two kidney-shaped cells called guard cells. They are necessary for exchanging gases with the atmosphere. Transpiration (loss of water in the form of water vapour) also takes place through stomata.
• Epidermal cells of the roots, whose function is water absorption, commonly bear long hair-like parts that greatly increase the total absorptive surface area.
• Plants like desert plant, epidermis has a thick waxy coating of cutin (chemical substance with waterproof quality)

2)Complex Permanent Tissue:-The complex permanent tissues consist of more than one type of cells having a common origin. All these cells coordinate to perform a common function. Complex tissue transport water, mineral salts (nutrients) and food material to various parts of plant body.

There are two types of complex tissue:-

a) Xylem or wood

b) Phloem or bast

a) Xylem:- Xylem is a vascular and mechanical tissue. It is a conducting tissue. Xylem consists of four different types of cells 1) Tracheids 2) Vessels 3) Xylem parenchyma 4) Xylem fibres. The cells have thick cell walls, and many of them are dead cells. Tracheids and vessels are tabular structures. This allows them to transport water and minerals vertically. The parenchyma stores food and helps in the sideways conduction of water.

b) Phloem:- Phloem is made up of four types of elements : sieve tubes, companion cells, phloem fibres and phloem parenchyma. Sieve tubes are tabular cells with perforated walls. Phloem is unlike xylem in that materials can move in both directions in it. Phloem transports food from leaves to other parts of the plant. Except for phloem fibres, phloem cells are living cells.