OUTLINES OF BOTANY. xliii 



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 enlarge- 

 ment 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 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 inti- 

 mately connected with the first formation of the new ^eli. As this cell in- 

 creases 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 organised sub- 

 stances produced or secreted within it. 



192. The principal organised 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. 



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

 and starch. 



starch or fecula, 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 called starch-granules, which vary 

 in size and are marked with more or less conspicuous concentric lines of 

 growth. The chemical constitution of starch is the same as that of cellu- 

 lose ; it is unaffected by cold water, but forms a jelly 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 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 soluble in alcohol, and may thus 

 be removed from the granules. 



chromule, a name given to a similar colouring matter when not 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 amorphous state or as microscopic crystals, when they are called 



§ 2. Arrangement of the Elementary Tissues, or Structure of the Orgam 

 of Plants. 



193. Leaves, young stems, and branches, and most parts of phaeno- 



