THE VASCULAR TISSUES. 



65 



lumen of the pit, is surrounded by a raised thickened bOPdeP (see Figs. 



47 and 48). 



ii. In tangential sections of the stem of Pinus bordered-pits may 



be studied in section. It is found that the raised borders coincide in 



position on corresponding parts of the cell-wall in two adjacent traoheides 



(see Fig. 48), and that these thicKned 

 parts enclose between them a space, 

 the pit - chamber, into which the 

 lumina of the two halves lead. Stretch- 

 ing across this chamber is the middle 

 lamella, in the centre of which occurs 

 a thickening, fusiform in section, and 

 known as the torus. By this arrange- 

 ment the lumina of the "pit" may 



Fig. 44. — a, Portion of a Rbtioulo- 

 PITTED Vessel (Bog-oak). 



6, Portion or a Vessel with 

 Small Bordered Pits, different 

 in structure to those of Pinus 

 (Bog-oak). 



c. Portion of a Scalarifokm 

 Vessel (Pteris). 



Fig. 45. — A Pitted Vessel (re- 

 cently-formed).— e. End-walls 

 which become absorbed ; p, 

 pits in surface view ; p', 

 in section. 



pits 



