tissue

tissue in biology, aggregation of cells that are similar in form and function and the intercellular substances produced by them. The fundamental tissues in animals are epithelial, nerve, connective, and muscle tissue; blood and lymph are commonly classed separately as vascular tissue. In the higher plants, there are four main types of tissue: (1) meristematic tissue (apical meristem and cambium), composed of cells that grow, divide, and differentiate into all the other cell types; (2) protective tissue (epidermis and cork), composed of thick-walled cells that cover roots, stem, and leaves; (3) fundamental tissues, consisting of cells that make up the bulk of the plant body, including parenchyma (thin-walled cells used for food storage), collenchyma (moderately thick-walled cells used for strength), and sclerenchyma (heavily thick-walled cells used for support in stems and roots); and (4) vascular tissue (xylem and phloem), specialized cells used for conduction. Organs are usually composed of several tissues. In many diseases there are apparent changes in tissue (see pathology ). Histology is the study of the structure of tissues.

tissue A collection of cells of similar structure organized to carry out one or more particular functions. For example, in animals nervous tissue is specialized to perceive and transmit stimuli. An organ, such as a lung or kidney, contains many different types of tissues.

tissue A group of cells of similar type working in a co-ordinated manner towards a common function. In plants, they are normally bound together by their cell walls. Some fluids are also considered to be tissues.