71 



Other substances are tannins, fats, ethereal oils, resin, 

 caoutchouc, and mineral crystals the latter usually consisting 

 of Calcium Oxalate 



65. So far as is known at pre- ceil 

 sent, every cell originates from a pre-existing cell and this 

 is usually brought about by normal cell-division. The whole 

 cell about to divide increases in size, the filaments contained 

 in the nucleus contract and become converted into a few 

 thick threads of equal length. The nuclear membrane en- 

 closing the nucleus then disappears, while each of the threads 

 divides into two longitudinal halves, the latter separating and 

 passing to opposite ends of the cell. The halves collected at 

 each end again coil up and each mass forms a new nucleus 

 provided with a membrane, while between them the forma- 

 tion of a cell-wall completes the division into 2 cells. In this 

 way an accurate division of the nuclear substance contained 

 in the cell is insured. 



66. The cell-wall is at first very Thickening of 



thin, but when the cell has attained its full size the wall is *J j^."^* 11 

 more or less thickened, the thickening substance being applied tion of pits, 

 in layers to the original membrane. Such thickened walls 

 are therefore, as a rule, distinctly stratified. Parts of the 

 original cell- wall, however, usually remain unthickenecl. Some- 

 times very small areas are thus left and narrow channels 

 through the thickening layers consequently arise, forming the 

 so-called pits. The pit in the wall of one cell is usually 

 continued through the thickening layers of the wall of the 

 adjacent cell, so that the cells are connected as it were by 

 a narrow channel which is, however, blocked in the middle 

 by the membranous primary wall. A bordered-pit is a modi- 

 fication of the simple pit, the channel of which is wide in 

 the centre and narrow at the ends, the centre of the original 

 w^all which blocks the middle of the channel being thickened 

 and forming the so-called torus, exactly facing the narrow en- 

 trances to the channel. In this case the thickening layers are 

 built out into a dome, instead of being closely adpressed to 

 the primary wall as is the case with a simple pit. The 

 entrance to the bordered-pit is at the apex of the dome. 



In other cases when the greater part of the original wall 

 remains unthickened, the thickening portions take the form 

 of narrow bands which sometimes anastomose and form a net- 

 work. The cell- wall consists chiefly of cellulose, a carbohydrate 

 which after treatment with sulphuric acid turns an iodine 

 solution blue, but as it thickens it is usually more or less 



