94 



TISSUES. 



[CHAP. 



Pith of a young branch of any tree, especially Elder. 

 The cells are often closely packed, and consequently 

 polygonal. 



Lax cellular tissue from stem of Common Rush 

 (Jitncus). Hence the " intercellular cavities " occupy 

 as much space as the cells, which become stellate 

 drawn out at their points of contact like so many star- 

 fish. 



FIG. 67. Cellular tissue from the 

 pith of the Elder, magnified. 



FIG. 68. Cellular tissue, with the 

 walls of the cells much thickened, 

 as in the stone of stone-fruits; 

 magnified. 



Piece of the stone of a stone-fruit, or shell of a nut, as 

 Walnut, ground 'excessively thin, by rubbing it with the 

 finger upon a hone. The cells have very thick sides, so 

 thick that sometimes the cavity is almost obliterated. 

 The "pits" are lengthened out into long "canals/* 

 which radiate from the cavity of each cell ; and the 

 thick walls, seen in section, present more or less dis- 

 tinctly, under a high magnifying power, the appearance 

 of concentric stratification, as though the thickening 

 deposit had been thrown down upon the inside of the 

 cell-wall in successive layers. This thickening of the 

 walls of cells takes place by intussusception, additional 

 molecules being intercalated between those already built 

 in, and the appearance of concentric zoning is due simply 

 to the alternation of belts richer and poorer in water, and 

 so varying in their refractive power, when viewed by 

 transmitted light, as transparent objects. 



Potato. Apple. Thin-walled cells. 



Young shoots of Brake Fern (boiled to a pulp). Large 

 vessels, marked with cross bars. 



