PLATE 1
Nine figures, showing stages in the cell-division common to the onion root (Allium cepa, L.)
ORIGIN OF MULTICELLULAR PLANTS
All multicellular plants are built up by the repeated cell division of one original cell. If the cells formed are similar in structure and function, they form a tissue. In multicellular plants many different kinds of tissues will be formed as a result of cell division, since there are many different functions to be performed by such an organism. When several of these tissues become associated and their functions are correlated, they form an organ. The association of several organs in one form makes an organism. The oak-tree is an organism. It is made up of organs known as flowers, leaves, stems, roots, etc. Each of these organs is in turn made up of several kinds of tissue. In some cases it is difficult to designate a single function to an aggregation of cells (tissue). In fact, a tissue may perform different functions at different periods of its existence or it may perform two functions at one and the same time; as an example, stone cells, whose primary function is mechanical, in many cases function as storage tissue. The cells forming the tissues of the plant, in fact, show great adaptability in regard to the function which they perform. Nevertheless there is a predominating function which all tissues perform, and the structure of the cells forming such tissues is so uniform that it is possible to classify them.
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.