Thallus: Foliose, composed of flattened overlapping branches which are only loosely attached to the bark; the individual branches are 1-4 mm. wide and relatively thick (0.5-1.5 mm.) but hollow; upper surface smooth, dull pale gray to greenish- gray when dry, similarly colored when wet; lower surface black or very dark brown, often wrinkled; lacking rhizines.

Apothecia: Rare.

Soredia: Common, formed in ring-shaped soralia at. the ends of branches (Figs. A & B) 

Isidia: Lacking.

Comments: Hypogymnia tubulosa and Bypogymnia physodes are differentiated from the other pale gray foliose lichens on our list by their thicker hollow branches with black lower surface.  The branches lack rhizines and thus are easily lifted from the bark.  In H. tubulosa the soredia are formed in ring-shaped soralia at the ends of branches;  whereas in H. physodes the soralia are lip-shaped or crescent-shaped or on the undersides of the tips of branches.