22 



ORGANOGRAPHY. 



perforations are well seen in the Sphagnum, where they are suffi- 

 ciently large as to allow of the passage through them of animal- 

 cules and minute granular matters. 



Cells with Bordered Pits. — In the 'cell-walls of the wood-cells 

 of certain trees we find, in addition to the ordinary pits, large 

 circular discs which encircle them, so that each pit looks as if it 

 had a ring surrounding it {fig. 38); hence such cells have been 

 termed cells with bordered pits, or disc-hearing wood-cells, or 

 functated wood-cells. Such an appearance is produced by the 

 walls of the cells having a number of circular depressions on 

 their outside, each of which is shaped like a watch-glass {fig. 39). 



Fig. 38 



Fig. 39. 



Fig. 40. 



Fig. 38. Disc-bearinff wood-cells of the Pine, with a single row of discs on 



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



the outside of a wood-cell of the Pine. Fig. 40. Diagram showing 



disc-bearing wood-cells in combination. 



When two cells lie side by side, the depressions on the one accu- 

 rately correspond to those upon the other {fig. 40), 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 appear- 

 ance to the same cause which leads to the ordinary pit of cells. 



Cells presenting such an appearance appear to be of universal 

 occurrence in the wood of the Coniferse, and in other Gymno- 

 spermous plants (see 0\'ule), where they are also most distinctly 

 observed. It was formerly supposed that disc-bearing wood- 

 cells 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 {Wi^itera aromatica) and 

 in the Star Anise {Illicium anisatum) such discs may be observed 

 {fig, 41), but the central pit is in these cases absent. 



