PITTED CELLS.—DISC-BEARING WOOD-CELLS. 43 
their young state) by the originally thin membrane of which it 
is at such points composed (figs. 79 and 81, a, a), and thus 
give to the parts of the cell-wall in which they are found, when 
viewed by transmitted light under the microscope, a more 
transparent appearance than that possessed by the thickened 
membrane surrounding them. Such cells are, therefore, im- 
properly called porous, and hence are now correctly termed 
pitted or dotted cells. These canals thus terminating in the 
wall of one cell correspond exactly with the ends of those of 
the adjoining cells ; and thus the sap can readily pass through 
the intervening cell-wall notwithstanding the general thicken- 
ing which the walls have otherwise undergone (jig. 81). It 
frequently happens that two or more canals unite together at 
varying distances from the wall of the cell, and thus form a 
eommon opening into its cavity (jig. 79). 
Although, as thus shown, the dotted appearance is not caused 
by holes or perforations in the original walls of the cells, yet as 
the latter advance in age, and lose their active vitality, they 
Fre. 81. 
Fic. 80. 
Fig. 80, Pitted cells—— Fig. 81. Thick-walled cells from the fruit of a Palm, 
a,a. Cell-walls. 6b, b. Concentric layers of thickening. c. Canals extend- 
’ ing from the central cavity to the inside of the wall of the cell. d. Cavity 
of the cell, e, e. External dotted appearance, From Unger. 
frequently become perforated, in consequence of their thin pri- 
mary wall becoming more or less absorbed, or breaking away. 
Such perforations are well seen in the Sphagnum, where they 
are sufficiently large to allow of the passage through them of 
minute granular matters. 
Cells with Bordered Pits or Disc-bearing Wood-cells.—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. 82) ; 
hence such cells have been termed cells with bordered pits or 
disc-bearing wood-cells. This appearance is produced by circular 
patches of the cell-wall remaining thin after the general thicken- 
ing has commenced and the rim growing obliquely inwards, 
leaving only a narrow orifice in the centre; or, in other words, 
the opening of the canal into the interior is narrow, while the 
