368 



STRUCTURE OF PHANEROGAMIC PLANTS. 



commonly associated with it, is found to be allied in chemical 

 composition to cellulose) is formed in successive layers, one 

 within another (Fig. 154, a), which present themselves as con- 

 centric rings when the cells containing them are cut through ; 

 and these layers are sometimes so thick and numerous, as almost 

 to obliterate the original cavity of the cell. By a continuance 

 of the same arrangement as that which shows itself in the single 

 layer of the dotted cell, — each deposit being deficient at certain 

 points, and these points corresponding with each other in the 

 successive layers, — a series of passages is left, by which the 

 cavitj' of the cell is extended at some points to its membranous 

 wall ; and it commonly happens that the points at which the de- 

 posit is wanting on the walls of two contiguous cells, are coinci- 

 dent, so that the membranous partition is the only obstacle to 

 the communication between their cavities (Figs. 154-156). It 

 is of such tissue that the " stones" of fruit, the gritty substance 



Fig. 156. 



Fio. 155. 



Section of Clterry Stone, cuuiiij;: Ihe 

 cells transversely. 



Seclion of C'cquiUa Nut, in the 

 direclioii of the long diameters 

 of the cells. 



which surrounds the seeds and forms little hard points in the 

 fleshy substance of the pear, the shell of the cocoa-nut, and the 

 albumen of the seed of Phytelephas (known as "vegetable 

 ivory"), are made up ; and Ave see the use of this very curious 

 arrangement, in permitting the cells, even after they have at- 

 tained a considerable degree of consolidation, still to remain per- 

 meable to the fluid required for the nutrition of the parts which 

 such tissue encloses and protects. 



228. The deposit sometimes assumes, however, the form of 

 definite fibres, which lie coiled up in the interior of cells, so as 

 to form a single, double, or even a triple or quadruple spire 



