CELLULAR TISSUE. 23 



tion in its cells ; and every degree of its deposition may be 

 seen from a slight increase of thickness of the cell-wall to 

 the entire filling up of the cavity of the cell. The difference 

 between sapwood and heartwood is caused, in fact, by a dif- 

 ference in the amount of the incrusting matter in the cells, 

 and by an increase of which the former is converted into the 

 latter. The indurated tissue which forms the cells of the 

 stones of the peach, cherry, and other fruits is produced in 

 this way. 



18. Sometimes nearly the whole of the interior surface of 

 the cells is thus thickened, the stratification of the matter 

 being distinctly visible on the cross-section, as is well seen in 

 the woody cells of the liber of the birch (Fig. 3), the cells 



Fig. 8. 



being nearly filled with a deposit of solid matter in concentric 

 layers j occasionally, however, certain parts of the primitive 

 layer of protoplasm only have the power of attracting from the 

 fluid contents of the cells the incrusting matter, and of becom- 

 ing thickened. This produces those dots and lines which are 

 seen to mark the cell-walls of many species of plants, and 

 which have been taken for pores or slits in the cell-wall ; but 

 the primitive membrane of the cell-wall is never perforated 



