THE CELL-WALL 



4» 



afjfiin would be ;i vcstiliule, and it would not be until he had emerged from this 

 tlirough 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 mouMing — 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. Thej^ are shown in tig. 10' and 10", and 

 ai-e to be seen in great perfection in wood-cells. 



Whenever bordered pits are formed, the thickening of the cell-membrane is com- 

 paratively slight; the frame of the window in the cell-wall cannot be more than 



Fiy 10. —Connecting Passages between adjacent Cell-cavities. 



'. Biinlered pits. '', .Section of a bordered pit. ', Mode of connection of adjacent cells in tlie bundle-sheath of Scolopendmmx. 

 <, .sieve-tubes, s, Group of cells from seed of Xiix-mmica, 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 whei-e, accordingly, a little depression occurs, is not subsequently 

 filled up with cellulose, but is carefullj' kept open by the protoplast as it builds. 

 A greatly thickened wall of this kind resemliles a fortification provided here 

 and there with deep, narrow loopholes. If 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 

 midiUe 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. i^ 



Very fi-equently provision is made in the veiy first rudiments of a cell-mem- 



