TILE CELL. 



75 



pit). These pits or channels may be simple or branched 

 (Fig. 180), and in cross-section appear oval, round, etc. ; 

 or they may be elongated fissures. The woody tissue of 

 the Pine family is characterized by bordered pits (Figs. 

 174-178), where each pit is surrounded, when viewed from 

 without, with a ring (Figs. 173, bor, and 174) ; this result is 

 brought about thus: a cir- 



cular portion of the wall 

 remains thin (Fig. 175), and 

 around this a wall arises and 

 grows inwards (Fig. 176), 



^ — - 



Q\ 



oi: 



174 



17 



175 176 178 



Fip. 173. Diagrammatic section through ducts and cells, showing various modes 

 of thickening : P, pilh ; jry, wood ; C, cambium ; Pk, bast ; Cx, cortex ; par, ^^t- 

 enchyma ; /?-f7j, prosenchyma; sie, sieve tubes ; ^;V, pitted; bar, bordered pits: 

 sea, scalariform : ret, reticulated; an, annular; s^, spiral. Figs. 174-178, Bor- 

 dered pits, and their development. 



