BORDERED PITS 



277 



But in some cells, particularly the vessels and tracheids 

 of the water-conducting system (see below), the pit is 

 narrower where it opens into the general cell cavity 

 than at the bottom on the pit membrane. The walls 

 thus overhang the pit, which is then said to be bordered 



Fig. 44. — ^Mechanical tissue elements of the Seed Plant. A, coUen- 

 chymatous cells in longitudinal section, showing great thickening 

 of parts of the walls forming longitudinal " pillars " of wall 

 substance. B, the same in cross-section ; w, thickened walls. 

 C, thick walled " stdhe cell " with branching pits. D, part 

 of three fibres in longitudinal section. E, fibres in cross-section. 



(Fig. 45, I). Some bordered pits are not included in 

 the general thickness of the cell wall, but are built out, 

 so to speak, into the cell cavity in the form of a flattish 

 dome, with the pit opening at its summit. The two 



