THE CELL. 



75 



Ph. 



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), 



f — ■ 



{2\ 



174 



177 



175 170 



Fi^. 173. Diagrammatic section through diicts and cells, showing various modes 

 of thickening ; P, pith ; jry, wood ; C, cambium ; Pk, bast ; Or, cortex ; par, par- 

 enchyma ; pros, prosenchyma ; sie, sieve tubes ; pit, pitted ; bor, bordered pits ; 

 scd, scalariform ; ret, reticulated; an, annular; sp, spiral. Figs. 174-178. Bor- 

 dered pits, and their development. 



