Thallus:  Minutely foliose, composed of tiny, closely packed, flattened, irregularly-lobed branches.  (Use a hand lens) The individual branches are 0.1-0.5 mm wide; mustard-yellow when dry, greenish-yellow when wet; similarly colored on both upper and lower sides; turn purple or purple-brown when wet with dilute (ca. 10% w/v) potassium hydroxide (lye).

Apothecia: (Fig. A & B) Disk- shaped or saucer-shaped;  up to 3 mm (3/32 in) across; bright orange with a yellow margin when fresh, becoming orange-brown when dry; present on almost all older thalli.

Soredia: Lacking.

Isidia:  Lacking.

Comments:  The bright yellow to orange color distinguishes this species from most other lichens growing on oak or ash twigs in our area.  Lepraria candelaris is similarly colored, but it is crustose and lacks apothecia.  Xanthoria polycarpa will tolerate more pollution than most species.  Thus, it is often found in suburban or lightly industrialized areas.  In areas with cleaner air it may be crowded out by less tolerant species.