OUTLINES OF BOTANY. XXXIX 



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 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 ducts of plants, so called from the mis- 

 taken 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 their inside, which unrolls when the vessel is broken ; others are 

 marked with longitudinal slits, cross bars, minute dots or pits, or with trans- 

 verse 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, 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 en- 

 largement 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 i3 

 formed, not by division, but around a 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 less transparent substance called cellulose, permeable by fluids, and of 

 ternary chemical composition (carbon, hydrogen, and oxygen) ; and of the cell- 

 contents, usually viscid or mucilaginous, consisting of protoplasm, a substance 

 of quaternary chemical composition (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 a minute, soft, subgelatinous 

 body called the nucleus, whose functions appear to be intimately connected with 

 the first formation of the new cell. As this cell increases in size, and its walls 

 in thickness, the protoplasm and watery cell-sap become absorbed or dried up, 

 the firm cellulose wall alone remaining as a permanent fabric, either empty or 

 filled with various organized substances produced or secreted within it. 



192. The principal organized contents of cells are 



sap, the first product of the digestion of the food of plants ; it contains the 

 elements of vegetable growth in a dissolved condition. 



sugar, of which there are two kinds, called cane-sugar and grape-sugar. 

 It usually exists dissolved in the sap. It is found abundantly in growing 

 parts, in fruits, and in germinating seeds. 



