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. ^, Mode of connection of adjacent cells in the bundle-sheath of Scolopendrium. 

 \ Sieve-tubes. ^, 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 subsequentlj' 

 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- 



