xlii OUTLINES OP BOTANY. 



The definitions above given of Varieties, Races, Species, Sections, etc., 

 must be taken in a general sense, as the distinctions between them are not 

 always so absolute as they were once thought to be. 



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 mi- 

 croscope 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 combi- 

 nation 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 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 continues, and when it ceases to be active the plant dies. 



(2) Woody tissue, or prosenchyma, differs in having its cells consider- 

 ably longer than broad, usually tapering at each end into points and over- 

 lapping 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 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 phsenogamous 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 thei* 

 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 anatomists under names which need not be enumerated here as 

 cot being in general practica.1 use. Air-vesvcls, cysts, turpentine-vessels, 



