Sunday, July 10, 2011

Class IX, BIOLOGY, "Tissues"

Tissues

A group of cells which perform same function is known as tissue. The tissues are divided into different types on the basis of their form and structure or function.

Plant Tissues

Following are the types of tissues in plants:
1. Simple Tissues
2. Compound Tissues

1. Simple Tissues

Simple tissues consists of only one type of cells. In plants, they are of following types:
i. Meristematic or embryonic tissues
ii. Permanent Tissues

i. Meristematic Tissues
1. Cells of this tissue have ability to divide.
2. Cytoplasm is dense nd nucleus is big in these cells.
3. Vacuoles are smaller if present other wise absent.
4. All cells are identical.
5. There are no intercellular spaces.
6. Their walls are thin and nucleus is present in centre of cell.
7. These tissues found on apex of root or shoot are called apical meristems. The cells of these tissues divide; and redivide to add primary tissue for elongation of setm or root.
This type of growth is called primary growth.
8. Meristematic cells are also found on the lateral sides of roots and stems as lateral(cambium) or intercalary meristem, and these add, secondary tissues. In this way, thickness of stem or root is increased. This type of growth is called secondary growth.

ii. Permanent Tissues
The cells of this tissue lack the ability to divide and they originate from meristems. These are given below:
a. Epidermal Tissues
b. Ground Tissues

(a) Epidermal Tissues
1. They are found as the outermost covering of leaf, stem or root.
2. There are non intercellular spaces.
3. Cells are rectangular in shape.
4. In the epidermal tissues of stem and leaves, there are small openings called stomata for gaseous exchange.

(b) Ground Tissues
1. Most of the portion of body of herbaceous plants consists of ground tissues i.e. parenchyma.
2. They are thin walled.
3. Cells are large in size.
4. Cells sometimes may develop the ability to divide.
5. Their main functions are to prepare and store food and water.

Supporting or Mechanical Tissues
These provide strength flexibility to the plant. They are of following two types:
a. Collenchyma Tissues
b. Sclerenchyma Tissues

(a) Collenchyma Tissues
1. These consist of living cells.
2. Their walls are not uniformly thickened.
3. Usually walls are thickened at angles.
4. These are more flexible or elastic than sclerenchyma.
5. These tissues are found in stem, in midrib of leaves and in cortex of petiole.

(b) Sclerenchyma Tissues
1. These consist of dead cells.
2. Their walls are highly thickened due to deposition of lignin.
3. Lignin provides hardness and strength to the cell.
4. These cells are without protoplasm.
5. Sclerenchyma cells are of two types,

* Stone cells having uniformly thick cell walls; found in testa of seeds.
* Fibrous cells which are elongated cells found in xylem and phloem for strength and transport of water

2. Compound Tissues

These are the tissues which consists of two or more than two types of cells. But all cells perform a common function.
These Tissues are of following types:

Xylem Tissue
1. This vascular tissue transports water in the plants and provides strength to the plant.
2. In this tissue, there are present xylem parenchyma and two types of thick walled dead cells.
Long cells which are called vessel elements or cells. They are joined together to form long pipe-lines. These transport water from roots to leaves.
Spindle shaped cells, which are called tracheicts. These provide strength to root and shoot etc.
3. Xylem conducts water in one direction that is from roots towards the stem and leaves.

Phloem Tissues
1. This vascular tissue transports food in the plants.
2. It helps in two directional conduction of food material i.e. from leaves to roots and vice-versa.
3. This tissue mostly consists of living cells. There are three types of cells
(a) Phloem Parenchyma
(b) Sieve Tube Cells
(c) Companion Cells

(a) Phloem Parenchyma
These cells store surplus water and food. They can start to divide when needed.

(b) Sieve Tube Cells
Their end walls have small pores called sieve plates. These cells join to form long pipelines, which are called sieve tubes. There is no nucleus in these cells. Their main function is to transport food.

(c) Companion Cells
In some plants, each sieve tube cell is accompanied by a companion cell. The companion cell has a nucleus. The corn cell controls the movement of food through sieve tubes.

Animal Tissues

Following are four types of tissues that are found in animals:
1. Epithelial Tissues
2. Connective Tissues
3. Muscle Tissues
4. Nerve Tissues

1. Epithelial Tissues

1. these are found as outer most layers of an organ or as lining of body invaginations.
2. Their cells are long and flat.
3. These may form one or more layers of epithelial tissues of skin which is called squamous epithelial cells.
4. Squamous Epithelium provides protection to skin.
5. Some cells are cubical in shape and known as cuboidal epithelial cells.
6. Cuboidal epithelial cells from the lining of glandular ducts and help in the production of cell secretions.
7. Some cells are small and elongated which are found at certain places in the inner lining of different organs and secret juice. These are called columnar epithelial cells e.g. cells of gastric glands in stomach which secrete the gastric juice.
8. Some columnar cells have cilia at their free surface. These are called ciliated columnar epithelial cells e.g. cells present in trachea. Due to movement of these cilia, mucous and other materials are expelled.

2. Connective Tissues

1. This tissue is made up of semi fluid matrix.
2. These matrixes contain a variety of cells and fibers.
3. These tissues provide support to different body parts and bind them together. These also protect the organs from germs and help in the production of blood cells.
4. These are of two types:
Soft connective tissues e.g. fatty tissues and tendons.
Hard connective tissues e.g. cartilage and bone.
5. Blood is also a special connective tissue with cells suspended in the fluid medium. It transports materials in the body.

2. Muscular Tissues

1. This tissue is made up of special contractile cells or fibers.
2. The cells are elongated and are called muscle fibers.
3. These cells have the ability to contract and relax which results in movements of body and the organs.
4. Following are the three types of muscles in our body.

Skeletal Muscles
These are attached to cartilage and bones. These seem to be striped fibers under the microscope. Therefore these are striped or striated muscles. Their movements are under our control so these are voluntary muscles e.g. muscles of arm and legs which move these parts.

Smooth Muscles
These are found around hollow organs such as blood vessels, gut. These produce slow, sustained contractions but do not fatigue. These re composed of spindle shaped unstriated muscles. These are involuntary and are under the control of the autonomic nervous system.

Cardiac Muscles
These are found in the heart. These are composed of branched fibers and are capable of sustained contraction but do not not fatigue. These are also involuntary in action.

4. Nervous Tissues

1. These are composed of nerve cells which are called neurons.
2. Each neuron consists of a cell body, axon and dendrites.
3. These productive nerve impulse to conduct messages.
4. By this tissue, different body parts have coordination with each other.
5. This tissue also forms brain and spinal cord

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