THE CELL-WALL 45 
again would be a vestibule, and it would not be until he had emerged from this 
through the aperture in the second moulding that he would reach the interior 
of the adjoining cell. Seen from in front, the outline of one of these windows, 
or rather the outline of the common floor of the vestibules, appears as a circle, 
whilst the aperture or opening in the moulding—which is exactly in the centre 
of this circle—is seen as a bright dot or pit encompassed by the circle which 
defines the limits of the vestibule. Hence these curiously protected window 
structures are named bordered pits. They are shown in fig. 10! and 10°, and 
are to be seen in great perfection in the wood-cells of pines and firs. 
Whenever bordered pits are formed, the thickening of the cell-membrane is 
comparatively slight; the frame of the window in the cell-wall is never more than 
Fig 10.—Connecting Passages between adjacent Cell-cavities. 
1, Bordered pits. 2, Section of a bordered pit. 3, Mode of connection of adjacent cells in the bundle-sheath of Scolopendrium. 
4, Sieve-tubes. 5, Group of cells from seed of Nux-vomica, the protoplasts of adjoining cell-cavities connected by fine 
protoplasmic filaments. 
five times as thick as the window-pane itself. In other cases, however, the cell-wall 
becomes twenty or thirty times as thick as it was at first, and the interior of the 
cell is thereby seriously diminished in size. But even if, little by little, the cell-wall 
augments in thickness a hundredfold, any spot where thickening has not taken place 
from the first, and where, accordingly, a little depression occurs, is not subsequently 
covered with cellulose, but is carefully kept open by the protoplast as it builds. 
A greatly thickened wall of this kind resembles a fortification provided here 
and there with deep, narrow loopholes. Where two cells thus provided adjoin one 
another, the windows in the one occur, normally, exactly opposite those of its 
neighbour, and the result is the formation of canals, very long relatively, which 
penetrate through the two adjacent cell-walls and connect the neighbouring cell- 
cavities together (fig. 10°). A canal of this kind is still closed, it is true, in the 
middle by the original cell-membrane as though by a lock-gate; but this slight 
obstruction may be removed later by solution, and the contiguous cells have then 
perfectly open connection through the canal. 
Very frequently provision is made in the very first rudiments of a cell-mem- 
