102 



ELEMENTS OF STRUCTURAL BOTANY. 



longer soft must, then, have untleigonc a change of 

 some kind. Let us try to understand the nature of 

 this change. It has been stated that the walls of new 

 cells are extremely thin ; as they become older, however, 

 they, as a rule, increase in thickness, owing to dejiosits 

 of cellulose upon their inner surface. It sometimes 

 happens, indeed, that the deposits are so copious as to 

 almost completely fill up the cavity of the cell. The 

 idea will naturally suggest itself, that this thickening of 

 the walls must impede the passage of the sap, but it is 

 found that the thickening is not uniform, that there 

 are, in fact, regular intervals which remain thin, and 

 that the thin spot in one cell is directly opposite a cor- 

 responding thin spot in the wall of its neighbour. 

 Eventually, however, these altered cells cease to convey 

 sap. 



167. The hard parts of plants, then, differ from the 

 Foft parts in the different coiisistencij of their cell-iialh. 

 But they differ also in the form, of the celh 

 ihemselccs. In those parts where toughness 

 and strength will be required, as, for ex- 

 ample, in the inner bark, in the stem, and 

 in the frame-work of the leaves, the cells 

 become elongated and their extremities as- 

 sume a tapering form, so that they overlap 

 each other, instead of standing end to end as 

 in ordinary cellular tissue (Fig. 1G5). To 

 this drawing-out process, combined with the 

 hardening of the walls, is due the firmness 

 of wood generally, and the tissue formed by these modi- 

 fied cells is known as woody tissue. On account of 

 the gi-eat relative length of the cells found in the inner 



