44 CELLS WITH BORDERED PITS. 
outer opening by the cell-wall is broad (fig. 83, a, b,c). As 
these thickenings occur always in twos, that is, one on each side 
of the cell-wall, they appear as two watch-glasses would do if 
placed rim to rim, and separated by a thin sheet of paper. To 
carry out the comparison, however, completely, the watch-glasses 
must be supposed to be perforated in their centres (jig. 83, b). 
The central lighter spot when examined by transmitted light is 
caused by the light having to pass only through the thin un- 
thickened cell-wall or membrane (jig. 83, c, w), while the darker 
colour of the border is caused by the light having to pass through 
the thicker substance of the rim. It frequently happens that 
this intervening membrane (fig. 83, b and d) becomes absorbed, 
and then direct communication takes place between the adjoiming 
cells. 
These bordered pits or discs occur either in single rows (fig. 
Fic. 82. Fic. 83. 
OO 
JOO © 
5 
fig. 82. Bordered pits of the wood-cells of the Pine, with a single row of 
discs on each cell.——Fig. 83. Bordered pits of the wood cells of the Pine 
(diagram). a. Young stage with unthickened cell-wall or membrane, 
6. Older stage where the intervening membrane has been absorbed. ec, 
Semi-profile view, showing position of membrane, w. d. The same where 
the membrane has been absorbed. After Sachs. 
82), or in double (figs. 84 and 85), or triple rows (fig. 86). 
In those cases where there is more than one row of bordered 
pits, those in each row may be either on the same level, as is 
more commonly the case (fig. 84), or at different levels, and 
hence alternate to each other, as in the Araucarias and allied 
trees (figs. 85 and 86). 
Cells presenting such a characteristic appearance are of 
universal occurrence in the wood of the Conifer and Taxacez, 
where they are also most distinctly observed. But somewhat 
similar bordered pits of smaller size may also be found in many 
of the Phanerogamia. 
Fibrous Cells.—It frequently happens that the thickening of 
the cell-wall (instead of taking place so as to give the appear- 
