

XXvi INTRODUCTION, 



the principal part of \voocl, of tlie inner bart, and of the nerves and veins of leaves, 



forming, in sliort, tlie 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 tliose of the vessels (veins and arteries) of 

 animal::. 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 phscuogamous 

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

 therefore called cascidar plants ; 80 the majority of cryptogams having only cellular 

 tissue are termed celhtlar plants. Vessefs 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 m 

 cross sections of the stem, in others they arc almost absent or can oidy 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, turpentine-vessels, oil-reservoirs, etc., are either cavities 

 left between the cells, or large cells filled with peculiar secretions. 



190. "When tissues arc 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, tlie 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//*6<? 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 siibstance called cellulose, permeable by fluids, and of ternary chemical 

 composition (carbon, hydrogen, and oxygen) ; and of the cell-contents, usually viscid 

 or mucilaginous, consisting oi protoplasm, a substance of qiiaternary chemical compo- 

 sition (carbon, hydrogen, oxygen, and nitrogen), which fills an important pai't in cell- 

 division and growth, Within the cell (either in the centre or excentrical) is usually a 

 minute, soft, subgelatinous body called tlie nucleus, 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 princii)al organized contents of cells are 



sap, the first product of the digestion of the food of plants j it contains the ele- 

 ments 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 m growing parts, in finiits, and in 

 germinating seeds. 



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



starcli orfecula, one of the most universal and conspicuous of cell-contents, and 

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

 of minute grains called sf arch-granules, which vary in size and are marked with more 

 or less conspicuous concetitric Hnes of growth. The chemical constitution of starch 

 is the same as that of cclhdose ; it is unaffected by cold water, but forms a jcUr with 

 boiling water, and turns blue when tested by iodine. When fully dissolved it is no 

 longer starch, but dextrine. 



chlorophyll, very minute granules, containing nitrogen, and coloured green unrler 

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

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

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



chromule, a name given to a similar colouring matter when not green. 



xvax, oils, camphor, and resinous matter, are common in cells or in cavities in the 

 tissues between the cells, also various mineral substances, either in an amorphous state 

 or as microscopic crystals, when they are called Eaphides, 



