INTRODUCTION TO BOTANY. XXlll 



or the student who uses a Flora for the purpose of ascertaining the names 

 of plants, has not neceasarily much concern. In this outline, therefore, we 

 shall treat of these cognate sciences very briefly. 



§ 1. The FJcmoitary Organs. 



151. If a very thin slice of a plant (say, of a succulent leaf or fruit) be 

 magnified, it will be fomid to be made up of variously -shaped and arranged 

 ultimate parts or ekmoitary organs, forming a sort of honeycombed 

 structm-e. The ultimate parts are called cells. 



152. A cell^ 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 tissiie ; but each cell is a 

 distinct individual, separately formed and separately acting, though co- 

 hering with the cells with wliich it is in contact, and partaking of the 

 common life and action of the tissue of wdiich it forms a part. The mem- 

 branes separating the cells are called their walls. 



153. Botanists usually distinguish the following tissues : — 



(«) ctllular tisme, called also piilj) and parenchyma., consists of 

 rovmdish, 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 fi-uits, and all young growing 

 parts are fonned of cellular tissue ; and very many cr\T)to- 

 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. 

 {b) woody tissue, or plcurenchynia, 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 is not 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 spiral vessel and the duct. Spiral vessels, often 

 called trachecc, 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 spii^al vessel, containing a spii'al fibre in- 

 capable of being unrolled, and often broken into imperfectly 

 spu-al bars, or rings, or dots, or disposed like the ruags 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. 

 {cl) laticiferous tissue, or cinenehyma, consists of vminterrupted, ana- 

 stomosing, thick- walled tubes, which contain a peculiar fluid 

 called latex, usually tm'bid ; often coloured red, white, or yellow, 

 but often colourless. The use of this tissue is unknown. 

 154. Vaiious modifications of cellular, woody, and vascular tissues are 



distinguished by vegetable anatomists, but need not be here enmnerated. 



Other false elementary organs, such as air-vessels, cysts, ttirpentine-vessels, 



oil-reservoirs, etc., are all either intercellular cavities, or large cells filled 



with peculiar secretions. 



