Biology     TISSUES An aggregation of similarly specialized cells which together perform specialized functions are called tissues. MERISTEMATIC TISSUES (MERISTEMS)– these are the tissues whose cells divide continuously. Their cells are usually spherical, oval, polygonal or rectangular with a thin cellulose cell wall. The cells are closely arranged so that there are no inter-cellular spaces. Also, their nucle are large and the cytoplasm is in abundance. Vacuoles are either absent or very few in number. They are usually found in the growing tips of roots and shoots and also in the vascular cambium of dicotyledonous. PERMANENT TISSUES- these are meristems which cannot divide further and have attained a definite form and size. They are divided into two two major kinds- simple and complex. SIMPLE PERMANENT TISSUES- these are composed of one type of cells which form a uniform mass. These are further divided into three types of tissues- Parenchyma, Collenchyma and Sclerenchyma. PARENCHYMA- these are isodiametric cells with a nucleus and thin cell walls. Also the cytoplasm within is usually dense with a single large vacuole. They are found in the ground tissues of stems, cortex of roots and mysophyll of leaves. They store and assimilate food and even prepare food in the presence of chlorophyll. They provide mechanical strength by maintaining turgidity and also store waste products like gum and crystals. COLLENCHYMA- these have cells with unevenly thickened cell walls and with cellulose at its corners. They have nucle and the vacuoles are small. The inter-cellular spaces are least and they are found beneath the skin i.e. below the epidermis in the dicot stems. They provide mechanical support to the stem and being extendable, adapt themselves to the rapid elongation of the stem. SCLERENCHYMA- the cell walls are evenly thickened and lignified and usually pointed at both ends. The nucleus is absent and hence the tissues are made up of dead cells. They have simple pits in the walls and the wall between adjacent cells is prominent. It is found abundantly in cells like coconut, jute and hemp. They provide elasticity, flexibility and rigidity to the plant body forming a mechanical support.             COMPLEX PERMANENT TISSUES- these are made of more than one type of cells which combine together to work as a unit. They transport water and food material to various parts of the plant body. They are of two types- xylem and phloem. XYLEM- this complex tissue helps in the transport of water and soluble mineral nutrients from the roots to all the parts of the plant body. It also helps in replacing the water lost during transpiration and photosynthesis. There are different kinds of cells found in the xylem, like the tracheids, trachea, xylem parenchyma and xylem sclerenchyma. PHLOEM- this tissue is found in the vascular plants. Its function is to transport sugars and other food materials from the leaves to the rest of the parts of the plant body. Phloem too is composed of different kinds of cells like the sieve tubes, companion cells, phloem parenchyma and blast fibres. It is found mostly in all the parts of the plant. Xylem and Phloem together form thevascular bundle.     PROTECTIVE TISSUES- these are mostly covers the surface of the leaves and the living cells of roots and stems. These are composed of flattened cells whose top and bottom surfaces are parallel. E.g. upper and lower epidermis of the leaf. VARIOUS KINDS OF PLANT TISSUES:     ANIMAL TISSUES: all animals contain tissues which are divided into four major groups- epithelial,connective, muscular, and nervous. EPITHILIAL TISSUES- these tissues are classified into five major groups namely squamous, cuboidal, columnar, ciliated, and grandular. SQUAMOUS EPITHELIUM- these tissues are found in the outer layer of the skin and linings of cavities and ducts. It consists of compact, plate-like cells with no inter-cellular spaces between them. They protect the underlying parts from injuries and germs. CUBOIDAL EPIYHELIUM- these tissues are found in the kidney tubules and glands. They are composed of cube-like cells. They produce secretions and provide mechanical support to the site of occurrence. COLUMNAR EPITHELIUM- these tissues consist of tall, pillar-likecells found in the lining of the stomach and intestine and are also present in the salivary, sweat and tear glands. They provide mechanical strength and some even absorb digested food materials. CILIATED EPITHELIUM- these tissues are found in the wind pipes of vertebrates and linings of the trachea.these consist of columnar cells and their free surfaces bear the cilia which keep beating rhythmically. The cilia helps in keeping out the unwanted materials. GLANDULAR EPITHELIUM- these are found in certain glands and the lining of the intestines. They secrete mucus and other secretions. CONNECTIVE TISSUES- these tissues bind structures together , store fat, transport substances, provide protection against diseases and help in the repair of tissue damage. They basically form a framework and provide support to the organs and the body as a whole. There are various kinds of connective tissues like the Areolar tissue, Adipose tissue, Fibrous connective tissue, Osseous tissue, Hyaline cartilage and Blood. MUSCULAR TISSUES- these are contractile animal tissues that produce power and locomotion and maintain the movement of body substances. They are made of elongated muscle cells calledmuscle fibres. The contractile protein present in the muscle cells contrct and relax to facilitate movement in the body. There are three types of muscle fibres – Striated, Unstriated and Cardiac. NERVOUS TISSUES- these tissues are made of nerve cells called NEURONS. The brain, spinal cord and nerves are all made up of these tissues. They are specialized cells which have the ability to receive stimulus from outside or within and send impulses to various parts of the body.