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).