Fig. 7.2-4. Longitudinal section of fern stem (Pteridium). This micrograph shows excellent scalariform pitting. Each long, horizontal white area is the pit aperture (actually, we are looking through the two apertures of each pit-pair) and the red is the Safranin-stained secondary wall. Note that almost all of the primary wall is covered with secondary wall in a cell like this – there is so much secondary wall that the cell is strong enough to provide considerable support to the stem. The two lines pointed out by the arrows indicate the corners of the cell that are not penetrated by the scalariform pits. Because these corners are not pitted, they are especially strong and prevent the cell from being stretched by the surrounding tissues.