Tissues

Tissue

Groups of cells associated together and performing a common function are called tissues.

Plant Tissues

Types

• 1.       Simple tissues
• 2.       Compound Tissues
•          i.            Simple Tissues

When single types of cells perform similar function then they form a simple tissue. These are further divided into two groups.

(i)                  Meristematic Tissues   (ii)  Permanent Tissues

Meristematic Tissues

These are composed of cells, Which have power of division.

Properties

(i)                  They the ability of division.

(ii)                They have dense cytoplasm, large nuclei in the center with small or no vacuoles.

(iii)               They have thin and delicate cell walls, composed of cellulose.

(iv)              They are alike.

(v)                They do not have inter-cellular spaces among them.

Location

On the basis of location, meristematic cells are divided into three groups.

(i)                  Apical Meristems

It means the dividing cells are present at the shoot or root tip (apices).

These are also called growing points. These result in the increase in length of these shoots and roots. Such a growth is called primary growth.

(ii)                Lateral Meristems

It means the dividing cells are present on the lateral sides of shoot and roots. These results in the increase in diameter of these shoot and root.

Such a growth is called secondary growth.

• There are two types of lateral Meristems
• a.       Vascular Cambium: This is present in between the center and phloem. This gives rise to new xylem cells towards xylem and new phloem cells towards on side.
• b.      Cork Cambium: This is present in the outer lateral sides and gives rise to a corky layer.

(iii)               Intercalary Meristems

Patches of this type of Meristems are present among the mature tissues. This helps in the regeneration of removed parts removed by herbivores. Mostly it is present in grasses.

(b) Permanent Tissues

The cells of these tissues lack the ability to divide. They largely originate from primary meristematic tissue.

These are further divided into the following groups

(i)                  Epidermal Tissues (ii) Ground Tissues  (iii) Support Tissues.

• 1.       Epidermal Tissues

These are composed of single layer of cells which are found as the outermost protective covering of leaf, stem and roots.

Properties

(i)                  The cells in epidermal tissue are living having thick walls.

(ii)                They are flattened.

(iii)               They are closely packed with no inter-cellular spaces.

Functions

(i)                  They act as a barrier between the environment and the internal plant tissues. They are also responsible for the absorption of water and minerals primarily in the root region.

(ii)                They secrete cutin (the coating of cutin is called cuticle) on stem and leaves. They cutin prevents overheating of water.

(iii)               Epidermal tissues also have some specialized cells that perform specific functions. For Example:

• (a)    Root Hairs: absorb water and minerals.
• (b)  Leaf hairs: (1-2 cells) reflect light to protect against overheating and excessive water loss. The layer of leaf hair acts to hold in a layer of humidity ‘trapped’. This layer also prevents air moving directly against the stomata which would encourage water loss.
• (c)    Stomata: are made by guard cells and are most abundant on underside of leaves. They regulates diffusion of Co2 into the leaf for photosynthesis as well as regulate loss of water from the leaf.
• (d)   Salt glands: are the waste-bins for the excess salt absorbed from the soil. They form a crust of salt on leaves which reflects light to prevent overheating.
• 2.       Ground Tissues

Ground tissues are simple tissues made up of parenchyma cells.

Parenchyma cells are most abundant cells in plants. Overall they are spherical but flat at point of contact.

Properties of Parenchyma Cells

(i)                  These are most abundant cells in plants.

(ii)                These are large sized living cells.

(iii)               These have thin walls.

(iv)              Some times may develop the ability to divide.

(v)                These have large vacuoles storage of food.

Functions

(i)                  Sometimes perform the function for the storage of food.

(ii)                In leaves, these are sites for photosynthesis and are called mesophyll.

(iii)               In some parts they are the sites of respiration and protein synthesis.

• 3.       Support Tissues

These tissues provide strength and flexibility to the plant. These are of two types.

(i)                  Collenchymas Tissues.

(ii)                Sclerenchyma Tissues.

(i)                  Collenchymas Tissues

(i)                  These are living cells.

(ii)                These have angular thickening of cellulose in the primary cell walls, which become unevenly thicker.

Location

These are found just beneath the epidermis in the cortex of young herbaceous stems and in the midribs of leaves and plants of flowers

Properties

(iii)               These are made of elongated cells.

(iv)              These are flexible.

Function

(v)                These provide support to the young herbaceous parts of plant due to their flexibility.

Sclerenchyma Tissues

Properties

(i)                  These are thick walled cells.

(ii)                Cell walls are heavily impregnated with lignin which provides hardness and strength to the cell and is the main chemical component of wood.

(iii)               These are two types of Sclerenchyma Tissues.

• (a)    Fibrous Cells

These are long and cylindrical.

These are found in xylem and phloem for strength and transport of water.

• (b)   Stone Cells

These are shorter than fibers.

These have uniformly thick cell walls.

These are found in testa (seed coat) and shells of Nuts or endocarp of stone fruits to provide protection.

(ii)                Compound Tissues

These which are compound of different kinds of cells, performing a commom function are called compound or complex tissues.

Xylem Tissues

This vascular tissue provides strength and transports water and dissolved salts from the roots to the stem and leaves in plants.

Types

There are three types of cells in xylem tissues which help each other to perform the function of transport. These are:-

(i)                  Xylem Parenchyma Cells.  (ii)  Vessel Elements

These are thick walled and dead cells. These join end to end to form long pipelines for unidirectional transport of water from roots to leaves.

(i)                  Tracheids

These are also thick walled and dead cells. These are spindle shaped which in addition to being involved in the transport of water, provide strength to root, stem and branches.

Phloem Tissues

These tissues are responsible for the conduction of dissolved organic matter between different parts of the plant body.

Types

These types of cells are found which function together for conduction

These are:

 (i)                 Phloem parenchyma Cells

These store the surplus food.

(ii)                Sieve Tube Cells

These are elongated. Their end walls have small pores so the area at end is called sieve plate. These cells join end to end to form sieve tubes. They posses little protoplasm and lose their nuclei and Ribosomes etc. during development.

(iii)               Companion Cells

In some plants sieve tubes cells are accompanied with nucleated neighboring cells called companion cells. The companion cells contain functional DNA and Ribosomes and they make proteins for the sieve tube cells. Thee regulate or control the movement of food through sieve tubes.