Tissues, aggregates of cells which have certain characters in common. They may be classified in various ways according as attention is directed to their structural, functional, or developmental characters. Thus, in an animal, 'cellular tissues,' in which the unit elements retain their distinctness, may be distinguished from those of muscle and nerve, in which there is usually much modification and integration of the component cells. Or, in plants, 'cellular tissues' may be distinguished from 'vascular tissues,' in which the component cells are fused to form vessels. In these distinctions only structural characters are considered. Again, we may distinguish in an animal nervous tissues, muscular tissues, glandular tissues, skeletal tissues, and so on, the classification here depending on the function. In the same way the tissues of a plant may be distinguished as absorptive, assimilative, conductive, glandular, skeletal, protective, and so on. In short, the physiological conception of a tissue is that of an aggregate of cells in which by division of labour within the body there has come to be a predominance of one function. Or, if we consider the origin of the several tissues from the different layers of the embryo, we distinguish the ectodermic or epiblastic, the mesodermic or mesoblastic, the endodermic or hypoblastic tissues of animals, and similarly in higher plants we distinguish those which arise from the dermatogen, periblem, and plerome of the embryo or growing-point. The commonest classification of animal tissues is perhaps that which distinguishes four sets—(a) epithelial, including covering and lining layers of cells, and their secretory or other modifications; (b) muscular; (c) nervous; (d) connective tissues; but the last includes a great variety—e.g. bone, cartilage, ligaments, and ensheathing membranes. One of the most convenient classifications of vegetable tissue is that which distinguishes (a) cellular tissues—epidermis, parenchyma, prosenchyma; (b) vascular tissues—wood and bast vessels, and laticiferous vessels.
See the articles BARK, BAST, BICHAT, BONE, BRAIN, CELL, CIRCULATION, DIGESTION, EMBRYOLOGY, EPIDERMIS, EPITHELIUM, HORN, KIDNEYS, LEAF, MUSCLE, NERVOUS SYSTEM, PHYSIOLOGY, REPRODUCTION, RESPIRATION, SKIN, VEGETABLE PHYSIOLOGY, WOOD, &c.