XXVI OUTLINES OF 



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lular 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, crosa 

 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 

 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 cavi- 

 ties left between the cells, or larije cells filled with peculiar secretions. 



lyO, When tissues are once formed, they increase, not by the general enlarge- 

 ment 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 free cell -formation, in contradistinction to cell- 

 division. :; 



191. A young and vitally active cell consists of the outer ivall, 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 im- 

 portant part m cell-division and growth. Within the cell (either in the centre or ex- 

 centrical) is usually a minute, soft, subgelatinous body called the nucleus, whose func- 

 tions 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 cellidose wall alone remaining as a per- 

 manent 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 j^roduct of the digestion of the food of plants: it contains the 

 elements of vegetable growth in a dissolved condition. - 



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

 usually exists dissolved in the sap. It is found abundantly in erowini? parts, in 

 fruits, and in germinating seeds. 



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



starch or fecida, one of the most universal and conspicuous of cell-contents, 

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

 consists of minute grains call starch -granules, which vary in size and are marked 

 with more or less conspicuous concentric lines. The chemical constitution of starch 

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

 with boiling water, and turns blue when tested by iodine. 



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

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

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

 colouring matter is solu]>le in alcohol, and may thus be removed from the granides. 



chromnle, a name given to a similar colouring-matter when green. 



wax, 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 amor- 

 phous state or as microscopic crystals, when thev are called JRavhides. 



§ 2. Arrangement of the Elementary Tissues. 

 Leaves 



t 



during the first year of their existence consist anatomically of '- -, . ? 



^^^ ¥, a cellular system-, or continuous mass of cellular tissue, which is developed^ 



both vertically as the stem or other part 



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