THICKENING OF THE WALLS OF CELLS. 



35 



lining. Every degree of this secondary deposition occurs, from a 

 slight increase in the thickness of the membrane to the filhng 

 up of the greater part of the cavity of the cell. Any hard wood 

 furnishes illustrations of this. Indeed, the difference between sap- 

 wood and heart-wood in trees is principally owing to the increase 

 of this deposit, which converts the former into the latter ; as may 

 be seen by comparing, under the microscope, the tissue of the older 

 with that of the newest rings of wood, 

 taken from the same tree. Figures 

 196-199 show this in a piece of oak 

 wood. Fig. 29 represents a highly 

 magnified cross-section of some wood- 

 cells from the bark of a Birch, with 

 their calibre almost obUterated in this way. It is by the same 

 process that the stone of the peach, cherry, &c. acquires its extreme 

 hardness. Similar indurated cells of tlie same kind are met with 



even in the pulp of some 

 fruits, as in the gritty grains, 

 which every one has noticed 

 in the flesh of certain pears, 

 especially of the poorer sorts. 

 A section of a few cells of the 

 kind is represented in Fig. 

 27, with their cavity much 

 reduced and rendered very 

 irregular by this internal in- 

 crustation. Similar cells may 

 be found in some parts of the tissue even of such juicy fruits as the 

 cranberry and the blueberry (Fig. 28). 



42. The tliickening matter, when pure, is of the same nature as 

 the original membrane of the cell, that is, it consists of cellulose 

 (27). But with this are nungled some mineral matters, — small 

 quantities of which must needs be dissolved in the water which 

 the plant imbibes by its roots, and be deposited in the cells of the 



TIG. 27 Magnified section of the gritty cells of ttie pear ; the cavity almost filled with an 

 internal deposit. 28. Similar cells found in the pulp of the hlueberry {Vaccinium corym- 

 bosum). 



riG. 29. Highly magnified cross-section of a bit of the old liber of the hark of the Birch ; 

 • the tubes nearly filled with a deposit of solid matter in concentric layers. (From Link ) 



FIG, 30. Highly magnified wood-cells (seen in transverse and longitudinal section), from 

 the root of the Date Palm ; showing the thickening deposit in layers, and some connecting 

 canals or pits. {From Jussieu, after Mirbel.) 



