ELEMENTARY STRUCTURE. 



13 



Cells with Bordered Pores. — In the wood-cells of some trees we 

 find their walls present, in addition to the ordinary pits, large cir- 

 cular dots or discs which encircle them, so that each pit looks as 

 if it had a ring surrounding it (Jig. 21) ; hence such cells have 

 heen termed cells with bordered pores or pits, or disc-bearing 

 wood-cells, or punctated wood-cells. Such an appearance is pro- 

 duced by the walls of the cells having a number of circular 

 depressions on their outside, each of which is shaped hke a 

 watch-glass (fig. 22). When two cells lie side by side, the 



Fig. 21. 



Fig. 22. 



Fig. 23. 



Fig. 21. Disc-bearing wood-cells of the pine, with a single row of discs on 



each cell Fig. 22. Diagram showing the watch-glass depression on the 



outside of a wood-cell of the pine Fig. 23. Diagram showing disc- 

 bearing wood-cells in combination. 



depressions on the one accurately correspond to those upon the 

 other (fig. 23), by which a number of lenticular cavities are 

 formed between them, so that when viewed by transmitted light 

 they appear like discs. The central pit is formed in the same 

 manner, and owes its appearance to the same cause which leads 

 to the ordinary pit of cells. The existence of these lenticular 

 cavities between the sides of certain cells was first accurately 

 proved by the late Edwin Quekett, who succeeded in separating 

 from the discs of a piece of fossil wood, double convex masses 

 of solid matter. 



Cells presenting such an appearance appear to be of universal 

 occurrence in the wood of the Coniferae or cone-bearing plants, 

 where they are also most distinctly observed. It was formerly 

 supposed that they were confined to such plants, but it has now 

 been proved that similar discs also exist in the wood-cells of 

 some other trees. Thus in the Winter's Bark Tree ( Wintera 

 aromatica ) and in the Star Anise (lllicium floridanurn) such 

 discs may be observed (fig. 24), but the central pit is absent in 

 these cases. 



