THE VEGETABLE CELL 



291 



interior of the cell is narrow, and the pit itself is funnel-shaped 

 (fig. G39, a, b, c). As these thickenings occur always in pairs, 

 that is, one on each side of the cell-wall, they appear a,s 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 supj)osed to be perforated 

 in their centres {fig. 639, b). The central spot, which appears 

 lighter than the rest of the pit, when examined by transmitted 

 light is caused by the light having to pass only through the thin 

 unthickened cell-wall or membrane (fig. 639, c, w), while the 

 darker colour of the border is caused by the light having to pass 

 through the thicker substance of the rim. The membrane has a 

 separate central thickening called the torus, which thus lies 

 inside the pit, and is about as large as the central opening. 



Fig. 638. 



Fm. 639. 



Fig. 638. Bordered pits of the wooil-cells of the Pine, with a single row of 



discs on each cell. Fig. 639. Bordered pits of the wood-cells of the Pine 



(diagram), a. Yomig stage with unthickened cell-wall or membrane. 

 h. Older stage showing the membrane with its torus, c. Semi- profile view, 

 showing position of membrane, ic. After Sachs. 



which it can therefore block up when great tension exists on 

 either side of the membrane, forcing it outwards. In very old 

 wood this intervening membrane often becomes absorbed, 

 and then direct communication between the adjoining cells is 

 possible. 



These bordered pits occur either in single (fi^. 638J or in 

 double or triple rows. In those cases where there is more than 

 one row of them, those in each row may be either on the same 

 level, as is usual, or at different levels, and hence alternate to 

 each other, as in the Araucarias and allied trees. 



Cells presenting such a characteristic appearance are of 

 universal occurrence in the wood of the Coniferse and Taxacea;, 

 where they are also most distinctly observed. But somewhat 

 similar bordered pits of smaller size may be found also in many 

 other Phanerogamia. 



