Fig. 7.2-7. Longitudinal section of wood of American hornbeam (Carpinus caroliniana), a dicot. The two arrows point out circular bordered pits in the side wall of a vessel element. There is no torus or margo; instead, the white dot in the center of each pit is where we are looking through the pit aperture. The gray region around each aperture is the border – we are looking through the area where the secondary wall is lifted up away from the primary wall. It looks gray because of the amount of light coming through that region – it is actually stained red. The dark red lines that outline the shape of each circular bordered pit is the region where the secondary walls are firmly attached to the primary walls. Although these are properly called “circular bordered pits,” they are actually hexagonal in shape; however, there is no such term as “hexagonal bordered pits.” For every pit we see in this vessel element, there is a corresponding pit in the vessel element behind it. Rather than just pits, we are seeing the front half of pit-pairs.
Notice just how porous these walls are – there are many, many sites at which water can pass from one tracheary element to another. But keep in mind that these are not complete holes – there is a pit membrane (the two primary walls and middle lamella) separating each pit of the pit pair. That pit membrane reduces the likelihood of any air bubble passing from one cell to another.