XX OUTLINES OP BOTANY. 



185. The number of species included in a genus, or the number of genera in an Order, is very 

 variable. Sometimes two or three or even a single species may be so different from all others 

 as to constitute the entire genus ; in others, several hundred species may resemble each other so 

 much as to be all included in one genus ; and there is the same discrepancy In the number oi 

 genera to a Family. There is, moreover, unfortunately, in a number of lnstances,_ great 

 difference of opinion as to whether certain plants differing from each other in certain particulars 

 are varieties of one species or belong to distinct species ; and again, whether two or raore groups 

 of species should constitute as many sections of one genus, or distinct genera, or tribes of one 

 Order, or even distinct Natural Orders. In the former case, as a species is supposed to have a 

 real existence in nature, the question is susceptible of argument, and sometimes of absolute 

 proof. But the place a group should occupy in the scale of degree is very arbitrary, beingoften 

 a mere question of convenience. The more subdivisions upon correct principles are multiplied, 

 the more they facilitate the study of plants, provided always the main resting-points for constant 

 use, the Order and the Genus, are comprehensive and distinct. But if every group into which a 

 genus can be divided be erected into a distinct genus, with a substantive name to be remembered 

 whenever a species is spoken of, all the advantages derived from the beautiful simplicity of the 

 Liunasan nomenclature are gone. 



Chap. III. Vegetable Anatomy and Physiology. 

 § 1. Structure and Growth of the Elementary Tissues. 



186. If a very thin slice of any part of a plant be placed under a microscope of high magnify- 

 ing power, it will be found to be made up of variously shaped and arranged ultimate parts, 

 forming a sort of honeycombed structure. These ultimate parts are called cells, and form by 

 their combination the elementary tissues of which the entire plant is composed. 



187. A cell in its simplest stateps a closed membranous sac, formed of a substance permeable by 

 fluids, though usually destitute of visible pores. Each cell is a distinct individual, separately 

 formed and separately acting, though cohering with the cells with which it is in contact, and 

 partaking of the common hfe and action of the tissue oE which it forms a part. The membranes 

 separating or enclosing the cells are also called their walls. 



188. Botanists usually distinguish the following tissues : — 



(1) Cellular tissue ot parenchyma, consists usually of thin-walled cells, more or less round in 

 form, or with their length not much exceeding their breadth, and not tapering at the ends. All 

 the soft parts of the leaves, the pith of stems, the pulp of fruits, and all young growing parts, 

 are formed ■ of it. It is the first tissue produced, and continues to be formed while growth 

 ntinues. and when it ceases to be active the plant dies. 



(2) Woody tissue ot prosenchyma, differs ia having its cells considerably longer than broad, 

 usually tapering at each end into points and overlapping each other. The cells are commonly 

 thick walled ; the tissue is firm, tenacious, and elastic, and constitutes the principal part of 

 wood, ot the inner bark, and of the nerves and veins of leaves, forming, in short, the framework 

 of the plant. 



(3) Vascular tissue, or the vessels or ducts of plants, so called from the mistaken notion that 

 their functions are analogous to those of the vessels (veins and arteries) of animals. A vessel 

 in plants consists of a vertical row of cells, which have their transverse partition-walls obliterated 

 so as to form a continuous tube. All phffinogamous plants, as well as terns and a Jew other 

 cryptogamous plants, have vessels, and are therefore called vascular plants ; so the inajority of 

 cryptogams having only cellular tissue are termed cellular plants. Vessels have their sides very 

 variously marked ; some called spiral vessels, have a spiral fibre coiled up their inside, which 

 unrolls when the vessel is broken ; others are marked with longitudinal slits, cross bars, minute 

 dots or pits, or with traverse rings. The size of vessels is also very variable in different plants ; 

 in some they are of considerable size and visible to the naked eye in cross sections of the stem, 

 in others they are almost absent or can only be traced under a strong magnifier. 



189. Various modifications of the above tissues are distinguished by vegetable anatomists 

 under names which need not be enumerated here as not being in general practical use. Air- 

 vessels, cysts, turpentine-vessels, oil-reservoirs, etc., are either cavities left between the cells or 

 large cells filled with peculiar secretions. ' 



190. When tissues are once formed, they increase, not by the general enlargement ot the 

 whole of the cells already formed, but by cell-division, that is, by the division of young and 

 vitally active cells, and the enlargement of their portions. In the formation of the embryo the 

 first cell of the new plant is formed, not by division, but around a segregate portion of the 

 contents of a previously existing cell, the embryo-sac. This is termed /ree cefZ-fomation in 

 contradistinction to cell-division. ' 



191. A young and vitally active cell consists of the outer wall, formed of a more or less 

 transparent substance called cellulose, permeable by fluids, and ot ternary chemical composition 

 (carbon, hydrogen, and oxygen) ; and of the cell-contents usually viscid or mucilaginous, consist- 

 ing ot;)rofopZasm, a substance of quaternary chemical composition (carbon, hydrogen,' oxygen 



