Tissue is a group of similar cells that work together with intercellular substances and body fluids to perform a particular function in an organism. Groups of tissues, in turn, form the organs of animals and plants.
Animal tissues
are classified into four main types: (1) epithelial tissue, (2) connective tissue, (3) muscle, and (4) nervous tissue.
Epithelial tissue
consists of tightly packed cells with little intercellular substance. The two main types of epithelial tissues are membranes and glands. Epithelial membranes form the outer layer of the skin. They also line such internal body surfaces as those of the digestive tract and of the blood vessels. Glands produce secretions, such as hormones and perspiration.
Connective tissue
is composed of cells that lie within an abundant intercellular substance. In most connective tissue, the intercellular substance consists of fibers and a clear fluid called ground substance. Some connective tissues surround and connect other tissues and organs. Bone and cartilage are connective tissues that support and protect the body. Blood, which consists of cells suspended in a fluid intercellular substance called plasma, is also classified as a connective tissue.
Muscle
consists of cells that have the ability to contract. Muscle contractions are responsible for the beating of the heart, the movement of substances through the body, and the wide range of body movements.
Nervous tissue
consists of highly specialized cells that conduct nerve impulses throughout the body. Nervous tissue enables the body to coordinate many of its functions and to respond to a wide variety of stimuli.
Plant tissues.
Plants have two major types of tissues, meristematic and permanent. Meristematic tissues consist of immature cells that divide continuously. The growing tips of roots and shoots are composed of meristematic tissue. Permanent tissues consist of mature cells that have special functions. Such tissues include epidermis, parenchyma, xylem, and phloem. Epidermis forms a plant’s protective outer covering. Parenchyma stores food and water in the roots, stems, and leaves of a plant. Xylem conducts water and minerals upward from the roots. Phloem carries sugars and proteins throughout the plant.
