THICKENINGS OF THE WALL. 



gated fissures, as in Fig. 14, or of any form intermediate 

 between these. Pits with elongated fissures may be twisted, 

 giving them, when seen in front view, the appearance of two 

 fissures crossing one another (Fig. 14^4, E). 



30. In the thickening of the cells of the wood of the 

 Ooniferae bordered pits are formed (Fig. 15). Here large 

 round areas of the wall remain thin, and the thickening 

 mass arches over them on all 

 sides in such a way as to form 

 low domes (Fig. 16, F) ; at the 

 top of each dome a small round 

 opening is left, and this permits 

 free communication between the 

 cavity of the cell and the pits 

 formed by the dome. This pro- 

 cess takes place in exactly^ the 

 same way upon both sides of the 

 common Avail of contiguous cells 

 (Fig. 16,,t,t, and (7). When 

 the partition separating opposite 

 pits breaks away, as it generally 

 does quite soon, the resulting cav- 

 ity is doubly convex in shape 

 (Fig. 16, E). When a pit of 

 this structure is seen in front 

 view, it has the appearance of two 

 concentric circles (Fig. 15, t", 

 and Fig. 16, D) ; the outer one 



' ' ' 



being formed by the bottom Of <', <", V". bordered pits, increasing in 



.. -i ,1 i ,i a?e ;*/, large pits where c Us of the 



the pit, and the inner by the medullary rays lie next to the wood- 



opening at its top. 



The bordered pita of pines, firs, and other Ooniferse may be readily 

 examined by making a longitudinal radial section. They are not found 

 in abundance on the tangential surfaces of the cells. 



The reel structure of the bordered pits of the Conifers; was not under- 

 stood until quite recently.* Von Mohl, apparently not noticing the 



* Schacht, in 1859 (BotaniscJie Zeitung, pp. 238, 239), and in a memoir 

 in 1860 (" De Maculis in Plantarum Vasis Cellulisque Lignosis"), gave 

 the first correct explanation of the structure of bordered pits. 



Fi? 15. Pinus sylvestris ; longi- 

 tudinal radial eection through the 

 wood of a rapidly growing branch ; 

 c, ft, e.wnbial wood-cells (trachetdes) ; 

 a toe, older wood-cell* (trache?des); 



