XXvi OUTLINES OF 



Chap. III. "Vegetable Anatomy and Physiology. 

 § 1. Structure and Growth of the E/emsniary Tissues. 



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

 magnifying 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 state is 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 life and action of the tissue 

 of 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, or parenchyma, consists usually of thin-walled cells, more or less 

 round in form, or with their length not much exceeding their breadth, and not taper- 

 ing 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 continues, and when it ceases to be active the 

 plant dies. 



(2) Woody tissue, or prosenchyma, differs in 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, of 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 dtuts of plants, so called from the mistaken no- 

 tion 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 trans- 

 verse partition-walls obliterated, so as to form a continuous tube. All phcenogamous 

 plants, as well as ferns and a few other cryptogamous plants, have vessels, and are 

 therefore called vascular plants; so the majority 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 transverse 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 ana- 

 tomists under names which need not be enumerated here as not being in general prac- 

 tical 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 of 

 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" 

 segregate portion of the contents of a previously existing cell, the embryo-sac. This 

 is termed free cell-formation, in contradistinction to cell-division. 



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

 leas transparent substance called cellulose, permeable by fluids, and of ternary chemicw 

 composition (carbon, hydrogen, and oxygen) ; and of the cell-contents, usually viscid 

 or mucilaginous, consisting of protoplasm, a substance of quaternary chemical compo- 

 sition (carbon, hydrogen, oxygen, and nitrogen), which fills an important part in cell- 

 division and growth. Within the cell (either in the centre or excentrical) is usually* 

 minute, soft, subgelatinous body called the nucleus, whose functions appear to be in* 1 



