Fig. 7-2-6. Longitudinal section of pine wood (Pinus). All cells shown here are tracheids, and all the circular structures are circular bordered pits. The outer white area is known as the margo, the inner dark area is the torus. The margo is an area where the pit membrane is very thin, partially digested away in some species such that water passes relatively easily through it. The torus, in contrast, is a thickened area that can seal off the pit if one of the tracheids becomes filled with air. Compare this with Fig. 7.13 on page 117 in Plant Anatomy (Mauseth).