Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/93

] future observers have been directed iu the proper path of investigation. In the Physiological department of botany the most important researches have been made by French and German botanists. The laboratories in connection with schools in Germany offer facilities for study which do not exist to the same extent in Britain. Physiological researches demand not only a Botanic Garden with its appendages, but apparatus of various kinds, means of pro secuting histological and chemical investigations, physical experiments, and observations by the spectroscope. Our schools require then not only lecture-rooms, but labora tories well fitted up with all needful appliances, and salaried assistants to aid the teachers in their demonstra tions and the pupils in their practical work. The department of Geographical Botany has made rapid advance by means of the various scientific expeditions which have been sent to all quarters of the globe ; and the question of the mode in which the floras of islands and of continents have been formed has given rise to important speculations by such eminent botanical tra vellers as Darwin and Hooker. The latter has published a valuable paper on insular floras. Under this depart ment the connection between climate and vegetation has been carefully studied both by botanists and by meteorolo gists. Among the contributors to this department of botany the following authors may be noticed Humboldt, Schouw, Meyen, Berghaus, Martius, Harvey, Hooker. The subject of Palajontological Botany has been much advanced of late by the researches of botanists and geo logists. The use of the microscope in the examination of tissues has aided much in the determination of fossil plants. The more accurate study of Organography has also been the means of correcting errors in diagnosis. The nature of the climate at different epochs of the earth s history has also been determined from the character of the flora. The works of Brongniart, Goeppert, and Schimper have advanced this department of science. Among others who have contributed valuable papers on the subject may be noticed Heer, who has made observa tions on the Miocene flora, especially in Arctic regions ; Sapor ta, who has examined the Tertiary flora ; Dawson and Lesquereux, who have reported on the Canadian and American fossil plants ; and Williamson, who has made a careful examination of many of the coal fossils, and whose excellent drawings of structure have opened a new light on the character of many of the genera. Delineations of fossils by Witham, Lindley and Hutton, and Carruthers, have tended much to advance our knowledge of the fossil flora of Britain.

Botany may be divided into the following departments : 1. Structural Botany, having reference to the anatomical structure of the various parts of plants, including Vegetable Histology, or the microscopic examination of tissues ; 2. Morphological Botany, the study of the form of plants and their organs (these two departments are often included under the general term of Organography) ; 3. Physiological Botany, by some termed Organology, the study of the life of the entire plant and its organs, or the consideration of the functions of the living plant ; 4. Systematic Botany, the arrangement and classification of plants ; 5. Geographical Botany, the consideration of the mode in which plants are distributed over the different regions of the globe ; 6. Palæontological Botany, the study of the forms and structures of the plants found in a fossil state in the various strata of which the earth is composed.

In the present article we shall confine our attention to the Structure and Morphology of Plants. The limits and classification of the Vegetable Kingdom have been partly considered under (. ). The Classification of Plants will be taken up in extenso under the heading, and the Distribution of Plants in space and time will be treated of in separate articles.

STRUCTURAL ELEMENTS OF PLANTS.

The elementary structure which is the foundation of The cell, all vegetable tissue is the cell. In the young succulent bud of a growing stem each cell consists of an outer firm, elastic membrane of cellulose constituting a cell-ivall ; within this, a gelatinous soft mass of protoplasm, of which there may be a portion distinctly marked off as a nucleus ; and, enclosed by the protoplasm, a cell-cavity containing a more or less watery fluid, the cell-sap (fig. 1). Such may be taken as the structure of a typical vegetable cell, which is thus a closed vesicle or sac with fluid or semi-fluid contents. Of these elements of the cell the protoplasm is that which is e ssential for its growth and development. In it are con tained all the substances requisite for the formation of the cell-wall and the cell-sap; and the nucleus is merely a differentiated portion of it. From it then all the other parts of the cell are formed, and it is essential to the growth of the cell. Hence it has received the appellation of primordial cell; and, indeed, amongst many Algse it exists for some time as a separate cell without any cell- wall or other part. This must be borne in mind when defining the cell as a sac or vesicle. The growth of the cell is usually, at first, uniform throughout, and it has therefore a more or less rounded form ; but, according to the function which it is destined eventually to perform, one or other, or it may be all, of the parts of the cell become modified or specially developed. The cell-wall may be greatly thickened; or it may grow more in one direction than another, so as to be elongated and form protuberances ; or perforations may occur ; or several similar cells arranged in a longitudinal series may, by obliteration of the interposed septa, unite to form a long tube which is then called a vessel. The protoplasm in the process of growth may be completely absorbed ; and when this occurs growth ceases and the cell-walls form merely a framework. It may, how ever, remain a long time, assuming various shapes and often uniting with colouring matters. The cell-sap also may disappear or may remain, containing in solution, or as definite forms in its mass, various assimilative substances, as fat granules, oil globules, starch, mineral crystals, &c. Fig. 1. [ missing text ] In some plants, as amongst Algse (Protococcus), one cell alone performs all the functions necessary for the exist ence of the plant. We have thus in this cell an epitome of vegetable life, and this is the most perfect form of cell. As we pass to the higher forms of plants, where many cells are united, we find a physiological specialization taking place, by which certain cells are set apart for assimilation, some being embryonal, some supporting, and others pro tective, &c. Amongst such plants as Ferns and ordinary Flowering Plants a further differentiation takes place, and some of these cells unite to form true vessels. We thus have a means of arranging all plants in two groups, viz., those whose tissues consist entirely of cells, cellular plants (including Fungi, Algte, Mosses, &c. ) ; and those in which vessels are present, vascular plants (including Ferns, Lycopods, and ordinary Flowering Plants).

1. Cells and Vessels—Cellular and Vascular Tissues

Cells united together constitute cellular tissue (fig. 2). 