XXViii INTBODUCTION. 



oontinues to be formed while growth continuea, and when it ceases to be actire the 



(2) Wbod^ tissue, or prosench/ma, 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 mistaken no- 

 tion 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 trans- 

 verse partition-walls obliterated, so as to form a continuous tube. All phsenogamous 

 plants, as well as ferns and a few other eryptogamous plants, have vessels, and are 

 theretore caHeA. vascular plants ; so the majority of cryptogams having only cellular 

 tissue are termed cellular plamis. 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 ana- 

 tomists under names which need not be enumerated here as not being in general prac- 

 tical use. Air-vessels, cysts, tv/rperdirte-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 enlargement 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 is formed, not by division, but around a 

 segregate portion of the contents of a previously existing cell, the embryo-sac. This is 

 termed ^ee 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 fiuids, 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 compo- 

 sition (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 mucleus, whose functions appear to be inti- 

 mately 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 ele- 

 ments of vegetable growth in a dissolved condition. 



sugar, of which there are two kinds, called cane-sugar and gra/pe-swgoA: It usually 

 exists dissolved in the sap. It is found abundantly in growing parts, in fruits, and in 

 germinating seeds. 



dextrine, or vegetable mucilage, a gummy substance, between mucilage and starch. 



stoA-dh ovfecula, one of the most universal and conspicuous of cell-contents, and 

 often so abundant m farinaceous roots and seeds as to fill the cell-cavity. It consists 

 of minute grains called starch-grarmles, which vary in size and are marked with more 

 or less conspicuous concentric Unes of growth. The chemical constitution of starch 

 IS the same as that of cellulose ; it is unaffected by cold water, but forms a ielly with 

 boihng water, and turns blue when tested by iodine. When fally dissolved it is no 

 longer starch, but dextrine. 



cUoropTvgll, ve^ minute granules, containing nitrogen, and coloured green under 

 the action of sunlight. These granules are most abundant in the layers of cells imme- 

 diately below the surface or epidermis of leaves and young bark. The green colouring 

 matter is soluble in alcohol, and may thus be removed from the granules. 



