ELEMENTARY STRUCTURE. 11 



by the successive deposit of new matter upon their inner surface. 

 This new matter is generally deposited in layers proceeding 

 from without inwards, which thus gradually diminish the 

 ca\'ity of the cell, and even in many cases cause it to be com- 

 pletely or nearly filled up (^zgr. 18). This increase in thick- 

 ness may be especially observed in the cells of the wood 

 and inner bark ; also in the hard cells of the stone of the 

 peach, cherry, and other similar fruits, as well as in the 

 shells of many other fruits. This thickening however of 

 the young cell-membrane, by successive layers of deposit 

 in its interior, is by no means confined to the cells of the 

 wood, or the other cases above mentioned, but it may be ob- 

 served more or less in all cells where active chemical changes 

 are going on. Thus it may be especially seen in those of the 

 pith of Hoya carnosa (Jig. 18). It is jPj„ j^g^ 



these deposits which give hardness ^^^ ^ ^* 



and firmness to the wood of plants ^~~ 



and to the stones of fruits, and hence 

 the name of Sclerogen (from a Greek 

 word signifying hardness) has been 



given to them. The term Lignine is V^ i^ 



also frequently appUed to them from \^' ^' 



their common occuri'ence in wood. 

 Whether Sclerogen or Lignine be es- 

 sentially diflferent in its chemical com- ^iS-}S- Transverse section of a 

 ..•', ni 1 ^i_ ^n thick-walled cell of the pith 



position to cellulose has not been tuUy of Boy a camosa. FromMohl. 

 ascertained; but, according to Mulder, 



there are three or more definite compounds forming the incrust- 

 ing layers of the cells of the wood and other parts, all of which 

 are richer than cellulose in carbon and hydrogen. Further 

 experiments are wanted before we can arrive at any definite 

 conclusions on the above points. 



These thickening layers, which are commonly called second- 

 ary layers or deposits, never consist however of pure Lignine 

 or Sclerogen, but they contain, combined with it, various mine- 

 ral substances, which have been absorbed dissolved in water by 

 the roots of the plant, and deposited in those layers in the pas- 

 sage of the fluid through the cells ; also various organic pro- 

 ducts, as colouring matters, &c., which have been formed in the 

 cells themselves, or have been conveyed to them from other 

 parts. 



Porosis, Pitted^ or Dotted Cells. — Li almost all cases where 

 the cell-membrane has thus become thickened by incrusting 

 matters, it presents (instead of the smooth and uniform appear- 

 ance, as is the case, as we have seen, when it is in a young 

 condition) a greater or less number of dots, pits, or slits of 

 various kinds (Jigs. 19 and 20, e), which were formerly con- 

 sidered as actual openings in the waUs of the cells, and hence 



