INTRODUCTION TO BOTANY. XXill 
or the student who uses a Flora for the purpose of ascertaining the names 
of plants, has not necessarily much concern. In this outline, therefore, we 
shall treat of these cognate sciences very briefly. 
§ 1. The Elementary Organs. 
151. If a very thin slice of a plant (say, of a succulent leaf or fruit) be 
magnified, it will be found to be made up of variously-shaped and arranged 
ultimate parts or elementary organs, forming a sort of honeycombed 
structure. The ultimate parts are called cedds. 
152. A ced, in its simplest state, is a closed membranous sac, formed of 
a substance permeable by fluids, though usually destitute of visible pores. 
When cells are combined, the mass is called a ¢isswe ; but each cell isa 
distinct individual, separately formed and separately acting, though co- 
hering 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 mem- 
branes separating the cells are called their walls. 
153. Botanists usually distinguish the following tissues :— 
(a) cellular tissue, called also pulp and parenchyma, consists of 
roundish, oblong, cylindrical, hexagonal, or stellate thin-walled 
cells, and is found in every plant. All the soft part of leaves, 
the pith of stems, the pulp of fruits, and all young growing 
parts are formed of cellular tissue; and very many crypto- 
gamic plants possess no other tissue. In it also are centred 
the most active functions of the living vegetable. It is the 
first tissue formed, and continues to be formed while growth 
continues, and when it ceases to be active, the plant dies. 
(0) woody tissue, or pleurenchyma, consists of long, slender cells 
tapering to each end, of a thicker, stronger, and much tougher 
substance than cellular tissue, but otherwise similarly or- 
ganized. It constitutes the principal part of the wood, of the 
fibrous inner bark, and of the nerves and veins of leaves, sepals, 
and petals. It isnot found in the lower Orders of Cryptogams. 
(c) vascular tissue, or trachenchyma, consists of very thin-walled, 
elongated tubes, variously marked. This tissue is of two prin- 
cipal kinds, the spira/ vessel and the duct. Spiral vessels, often 
called ¢rachee, contain highly elastic spiral fibre, usually capable 
of being unrolled; they meet or overlap at the ends, and where 
two such vessels adhere, the intervening membrane is absorbed, 
and they communicate freely. They are found round the pith 
in stems, and in all parts that emanate from it, especially along 
the nerves and veins of leaves; very rarely they occur in the 
wood or bark. Ducts are tubes usually of much greater diameter 
and length than the spiral vessel, containing a spiral fibre in- 
capable of being unrolled, and often broken into imperfectly 
spiral bars, or rings, or dots, or disposed like the rungs of a 
ladder. They occur chiefly in the wood; are abundant in the 
wood of Ferns; but absent from the wood of Conifers; their 
functions are not clearly ascertained. 
(d) laticiferous tissue, or cinenchyma, consists of uninterrupted, ana- 
stomosing, thick-walled tubes, which contain a peculiar fluid 
called datex, usually turbid ; often coloured red, white, or yellow, 
but often colourless. The use of this tissue is unknown. 
154. Various modifications of cellular, woody, and vascular tissues are 
distinguished by vegetable anatomists, but need not be here enumerated. 
Other false elementary organs, such as air-vessels, eysts, turpentine-vessels, 
oil-reservoirs, ete., are all either intercellular cavities, or large cells filled 
with peculiar secretions. 
