Bryum is a large and
extremely variable genus of acrocarpous mosses generally found on soil or
humus. The plants often have a rosette growth form with the upper leaves larger
and better developed than the lower leaves. The more robust species of Bryum,
which often have a strongly rosulate habit, have recently been segregated into
the genus Rosulabryum (Spence 1996). Typically the leaves are bordered
by linear cells, have large rhomboidal to oblong‑rhomboidal upper cells,
and percurrent to excurrent costae. Its capsules are usually pendent, more or
less pyriform, and have well‑developed, often perfect, diplolepidous
peristomes. Bryum is considered taxonomically difficult due to its
usually small size and generally non‑descript, often technical
characters. Adding to the difficulty, leaves taken from different parts of
single plants may have dissimilar morphologies and character states. Within Bryum,
groups of seemingly related forms can sometimes be recognized, but " ....
a clear division into subgenera or sections is not indicated". (Andrews
1940). Characters that are important in the taxonomy of Bryum include:
1. leaf cell areolation (upper leaf cell length vs width; thickness of cell
walls; basal leaf cell size and shape), 2. costal length; 3. leaf margin stance
and border development, 4. plant sexuality, 5. leaf decurrency development, 6.
capsule shape; 7. endostome development.