The functional classification of tissues is chosen for the purpose of demonstrating the adaptation of cell structure to cell function. If the cells performing a similar function in the different plants were identical in number, distribution, form, color, size, structure, and cell contents, there would not be a science of histology upon which the art of microscopic pharmacognosy is based. It may be said, however, with certainty, that the cells forming certain of the tissues of any given species of plant will differ in a recognizable degree from cells performing a similar function in other species of plants. Often a tissue is present in one plant but absent in another. For example, many aquatic plants are devoid of mechanical fibrous cells. The barks of certain plants have characteristic stone cells, while in many other barks no stone cells occur. Many leaves have characteristic trichomes; others are free from trichomes, etc. Yet all cells performing a given function will structurally resemble each other. In the present work the nucleus and other parts of the living protoplast will not be considered, for the reason that these parts are not in a condition suitable for study, because most drugs come to market in a dried condition, a condition which eliminates the possibility of studying the protoplast. The general structure of the cells forming the different tissues will first be considered, then their variation, as seen in different plants, and finally their functions. CHAPTER II THE EPIDERMIS AND PERIDERM The epidermis and its modifications, the hypodermis and the periderm, form the dermal or protective outer layer or layers of the plant. The epidermis of most leaves, stems of herbs, seeds, fruits, floral organs, and young woody stems consists of a single layer of cells which form an impervious outer covering, with the exception of the stoma. LEAF EPIDERMIS The cells of the =epidermis= vary in size, in thickness of the side and end walls, in form, in arrangement, in character of outgrowths, in the nature of the surface deposits, in the character of wall--whether smooth or rough--and in size. In cross-sections of the leaf the character of both the side and end walls is easily studied. In surface sections--the view most frequently seen in powders--the side walls are more conspicuous than the end wall (Plates 2 and 3). This is so because the light is considerably retarded in passing through the entire length of the side walls, while the light is retarded only slightly in passing through the end wall. The light in this case passes through the width (thickness) of the wall only. The outer walls of epidermal cells are characteristic only when they are striated, rough, pitted, colored, etc. In the majority of leaves the outer wall of the epidermal cells is not diagnostic in powders, or in surface sections.