Collema lichens are characterized by their medium to large size, with a diameter of . They have a
leaf-like, membranous structure that does not swell noticeably when wet. The upper surface of the
thallus is dark olive-green to brown-black and is not hairy. The are 2 to 15 mm broad, rounded, and usually flat or partially raised, with smooth or surfaces adorned with elongated ridges or folds. Both upper and lower are absent in
Collema, and the photobiont
Nostoc forms chains of cells throughout the thallus without creating a separate .
Isidia can be present or absent, while
soredia are not found in this genus. The reproductive structures, known as
apothecia, have a pale brown to red-brown that can be flat or convex. The is whole and sometimes displays isidia. The consists of cells that can be either uniformly sized or elongated. The is indistinct and can be colourless or pigmented, while the
hymenium is colorless and turns blue when treated with
iodine. A more or less colourless is also present. The comprises paraphyses that separate in a solution of
potassium hydroxide and may be unbranched or branched, often connecting near their tips, which can be club-shaped or round and exhibit a yellowish to reddish-brown hue. The
asci are club-shaped (clavate) with a strongly thickened apex, and both the and the downwardly projecting annulus and apical cap react blue to iodine. Each ascus produces eight spores that are narrowly
ellipsoidal to spindle-shaped or nearly cylindrical with transverse
septa. , or , are immersed within the thallus and can be located on the margins or lamina with a pale
ostiole. The are rod-shaped with a slightly enlarged apex. No
lichen products have been detected in
Collema using
thin-layer chromatography. ==Species==