Group of similar cells performing the same function is called a tissue. If all the functions are performed by one cell then it is called unicellular or acellular eg amoeba, euglena.

In the beginning unicellular organism stuck together to form colonies but they were independent in their own respect. They had a colonial life. Colony of chlamydomonas is called volvox. It is a colonial organism but they were independent. By living in a colony they could protect each other. They live in a jelly-like material.After having a colonial life there was a division of labour like in a honeybee colony. The colony performs the total living function, that developed a formation of single organism into a tissue. When many cells join together to form a single organism it is called multicellular.

Tissues are of two types - plant tissue and animal tissue.

In plants there are two types of tissues - permanent tissue and meristematic tissue.

Meristematic tissues have the capacity to divide. Depending upon the position meristematic tissues are of three types - apical , lateral , intercalary.
Apical meristematic is the tissue present on the tips of root and shoot. By division of the apical meristem the length of the plant increases.

Lateral meristem is present on the lateral side of the plant, all around the plant. By division of the lateral meristem the width of the plant increases.

Intercalary meristem is present on the internodes. The distance between adjacent nodes is called internode. Its function is to increase the length of the internode. This meristem is short lived and it does not last long.
All the permanent tissues are made up of meristematic tissues. There are two kinds of plants dicots and monocots. In dicot the apical and lateral meristem keep on dividing. In monocots only apical is present as it lacks lateral and intercalary . Hence the width of the plant remains the same.
Permanent tissues are of two types - simple tissue and complex tissue. Simple tissues are divided into three types - parenchyma, collenchyma, and sclerencyma.

Parenchyma is made up of only one kind of cells. These cells are thin walled loosely arranged with plenty of intercellular space. Sometimes if chloroplasts are present then they are called chlorenchyma. Its function then is photosynthesis. If they are loosely arranged with plenty of intercellular space then they are called aerenchyma. They are present on the lower side of the leaf. Aerenchyma can also contain chloroplasts. Their main function is the supply of carbon dioxide aerenchyma and chlorenchyma are present to perform additional functions.

In collenchyma the intercellular space is filled with hardening material. Hence collenchyma are more stronger. Hardening material is peptin and pectin. Both are mixed together. The corners being thick it therefore provides mechanical support. It is found in few layers below the epidermis of dicotyledonous stems. It is generally absent in monocot stem root and leaves.
Sclerenchyma cells have thicker walls than collenchyma. Most of the sclerenchyma cells are dead cells. It cannot take any material from outside and cannot give out any material. They cannot grow and only provide mechanical support.
Complex tissues are of two types - xylem and phloem.

Xylem is composed of three types of cells. Xylemparenchyma is formed when xylem and parenchyma combine. Parenchyma is compelled to do the work of xylem.Tracheids are elongated cells and hollow for the transport of water. The adjacent tracheids are joined in a slanting way. These are dead cells.Vessels are elongated cylindrical cells they are hollow and dead cells. Their function is transportation of materials. Xylemparenchyma are present outer to the tracheids and vessels to support them. They are living cells.

Phloem is composed of three types of cells - sieve tubes, companion cells and phloem parenchyma.Sieve tubes are the main types of cells present in phloem through which prepared food is transported. They are thin-walled elongated cells placed one above the other and joining walls are perforated (porous). Sieve tubes have hollow structures for the transportation of material.Companion cells are small cells arranged together with sieve tubes. They contain dense cytoplasm. Cytoplasm is confined to one region. Companion cells help the sieve tubes.
Phloem parenchyma also helps the sieve tubes for the transport of material. These are simple parenchyma cells associated with phloem. Some regions on the sieve tubes are connected to companion cells while some with phloem parenchyma.

ANIMAL TISSUE

Epithelial tissues are divided into four types depending upon the shape size and function. Epithelial means covering body organs as well as internal organs and cavity of organs.

Squamous epithelium cells are of hexagonal disc shaped because the outer region is thin and central region is broad to accommodate a bigger nucleus. It function is to protect the underlying parts of the body from mechanical injury, entry of germs, chemicals and drying. It is located in the lining of cavities of ducts and blood vessels and covering of the skin.

The length of the columnar epithelium cells much more than the width of the cell that is why it is called columnar epithelium. The surface is provided with hair like structures called cilia. It is present in larynx, trachea, salivary gland and liver. It is present in trachea to prevent the passage of duct particles.

Connective tissue connects adjacent tissues or organs. It contains different types of cells. Connective tissue is of four types.
Ligaments and tendons come under areolar connective tissue. Ligament connect bone to bones and tendons connect muscle to bones.

Different types of cells are present in areolar connective tissue like shapeless cells called fibroblasts are present. Fibroblasts are living cells. Its function is to connect different organs. It produces two types of proteins collagen fiber and elastin fiber. Depending upon the amount of collagen and elastin the fibers are divided into two -
More collagen than elastin - white collagen fiber
More elastin than collagen - yellow elastin fiber

Adipose connective tissue is present below the skin so it acts as heat or thermal regulator by storing fat. Fat molecules do not allow body heat to go outside or outside heat to come in. only some heat is released. Sweat formation is a part of heat regulation. Blood releases some extra water as sweat and sweat when evaporates produces a cooling effect. Then the blood vessels also help as they become large to carry more blood. Adipose tissue is an insulator of organs. Fat storage cells are present. It also acts as a shock absorber. It connects skin to internal organs. Here collagen and elastin fibers are absent. In it matrix also fat is deposited. Matrix present inside the cell is called tissue fluid and matrix present outside the cell is called cytoplasm.

Skeletal connective tissue is the framework of the body. Supporting material like cartilage and bones are present. The outer membranous covering of the cell is called perichondrium. Matrix is present inside which several cavities are present called lacunae. They are fluid filled spaces. Cartilage cells present in lacunae are called chrondriocytes. Osteoblasts produces osteocytes. All RBCs ,WBCs, and platelets are formed in the bone marrow.

Fluid connective tissue - blood has two parts plasma and formed elements. The main component of plasma is water and dissolved material, organic or inorganic. Formed elements are of three types RBC, WBC and platelets. RBCs are biconcave because they are enucleated i.e. without nucleus. WBC are having nucleus but they are irregular in shape. The main function of RBC is to carry oxygen to all parts of the body. Function of WBC is to destroy foreign objects. Platelets are fragmented cells i.e. they are cut into smaller pieces. Their shape is irregular but not amoeboid i.e. they cannot change their shape themselves. It does not contain nucleus it is only a small part of the protoplasm. Function of platelets is to help in the clotting of blood.

Lymph is also a part of the blood. It is the clear fluid that has filtered out of the blood. It contains WBC and lacks RBC. Lymph contains nitrogenous waste and no oxygen. In tissue fluid oxygen and RBC are present. Its function is to bring back tissue fluid from the blood vessel otherwise we will suffer from elephantitis.

Muscular tissue - there are three types of muscular tissues striated, unstriated and cardiac. In sraited muscles light and dark bands are present because of this we call it striated as strait means marks. These are voluntary muscles. Muscle cell is covered with a membrane called sarcolemma. The cytoplasm of the muscle cell is called sarcoplasm.
In unstriated muscles the markings are absent. They are also called involuntary muscles, as they are not controlled by will. It is controlled by medulla oblongata (part of brain). It is present in internal organs.
Cardiac muscles show characteristics of both striated and unstriated muscles. Straits are present but they are not present in a proper manner. They are not controlled by the will of the person. It performs non-stop rhythmic contraction. It consists of branches which form a network. A covering is present similar to sarcolemma it is like plasma membrane. Cardiac muscles are uninucleated. Their function is contraction and relaxation of heart. It then contracts the heart chambers. By this the blood is pumped into the body.

Nervous tissue- the schwann nucleus is present for protection. Nervous tissue is formed of neurons. Nephrons is a collection of cells while neuron is one single cell. The region where nucleus and cytoplasm are present is called cyton. From this cyton there are several cytoplasmic projections. These projections are called dendrites. Even dendrites have cytoplasm the biggest dendron is called axon. Dendron is singular while dendrite is plural.
In most cases the axon is covered by myelin sheath. If covered then they are called myelinated neuron while if uncovered then non-myelinated neuron. Myelin sheath increases the speed of conduction of nerve impulse. It will also prevent mixing of impulses because it is a protective covering. Myelin sheath is a part of another cell. It is not continuous and is interrupted in between called Node of Ranvier.

Synapse is the juncture between axon terminal and dendrite of adjacent neurons. Message is transmitted in only one direction not from dendrite to dendrite because the chemical used for transmission of message is present only in axon terminal.

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