Microbryum W. P. Schimper, 1860.

Traditional classifications of the family Pottiaceae placed in the genus Pottia nearly all the very small and largely ephemeral plants with broad leaves and a single abaxial stereid band in the costa. Zander (1993) presents a strong case for a reduction of the size of the genus with many of the previously included members now viewed as members of Tortula. That realignment means that the type of Pottia is now in Tortula, and so, a new name is required.

Key to Microbryum

Mosses included in this section are very small bulbiform acrocarps growing in winter and early spring on seasonally moist and sunny soil. They have erect and exserted sporophytes with elliptic to short cylindric capsules, and the leaves are papillose.

Species included in this key are all in Pottiaceae:
Microbryum davallianum (J. E. Smith) Zander
Microbryum starckeanum (Hedwig) Zander
Microbryum vlassovii (Lazarenko) Zander, recognized from CA since 2004

The species of Microbryum are among the smallest of the mosses in the Pottiaceae, and they are unlikely to be collected except when sporophytes are present. The peristome of the various species of Microbryum is absent or very much reduced usually to truncate stubs. The spores of Microbryum are typically more than 20 µm in diameter, and they are usually strongly papillose to warty-papillose. Even when sterile, Microbryum can be recognized by the very bulbiform plants with each leaf ending in a short and colored mucro. Such plants should be tested with a drop of 10% KOH for the very red coloration that is immediately visible upon application. Under the compound microscope the poor demarcation of the alar region of Microbryum sets it apart from most of Tortula.

A. Costa and adjacent lamina with distal portion covered on adaxial side with a cushion of bottle-shaped and papillose cells; capsule immersed to slightly emergent, cleistocarpous on a short seta .....Microbryum vlassovii
A. Costa without such an adaxial cushion; capsule exserted .....B

B. Leaves mostly more than 2:1, with a reddish apiculus; spores densely spinulose .....Microbryum davallianum
B. Leaves mostly less than 2:1, with a short yellowish apiculus; spores finely papillose .....Microbryum starckeanum