xlii OUTLINES OF BOTANY. 
The definitions above given of Varieties, Races, Species, Sections, ete., 
must be taken in a general sense, as the distinctions between them are not 
always so absolute as they were once thought to be. i 
Cuap. IIT. Vucrraste ANATOMY AND PuystoLoGy. 
§ 1. Structure and Growth of the Hlementary Tissues. 
186. If a very thin slice of any part of a plant be placed under a mi- 
eroscope 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 tisswes 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 fiuids, 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, oi 
the inner bark, and of the nerves and veins of leaves, forming, in short, the 
framework of the plant. 
(8) 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 phzenogamous plants, as well as ferns and 
a few other cryptogamous plants, have vessels, and are therefore called 
vascular plants ; so the majority of eryptogams 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 anatomists under names which need not be enumerated here as 
not being in general practical use. Air-vessels, cysts, turpentine-vessels, 
