Macromitrium is a sizable
genus, about 350–400 species, of medium- to large-sized pleurocarpous mosses.
The last treatment of the genus that included Central America was Grout (1946),
which attributed 24 species to the region. The genus is often found on upper
tree branches in the forest canopy, but it also commonly occurs on tree trunks
and can be found on rocks and soil in open, drier habitats. Most Macromitrium
species have elimbate leaves with short upper leaf cells and elongate to
linear, tuberculate basal leaf cells, mitrate calyptrae and short, truncate
exostome teeth that are fused for most of their lengths. However, the genus is
morphologically complex, and there are Macromitrium species with various
combinations of limbate leaves, long upper leaf cells, short basal leaf cells,
non-tuberculate basal cells, cucullate calyptrae, and long, narrowly
triangular, non-fused exostome teeth. As a result the genus is difficult to
characterize or cleanly separate from a number of segregate genera (e.g., Groutiella,
Macrocoma, Cardotiella) recognized for groups of species with short basal
leaf cells in combination with some other distinctive feature. Groutiella
has a basal leaf limbidium and so appears well differentiated from Macromitrium,
but there are many limbate Macromitrium species. Macrocoma
differs in having slender plants with imbricate leaves. Cardotiella has
strongly tuberculate cells on well developed leaf decurrencies, however, these
tuberculate cells seem to be only a remarkable amplification of the basal
marginal teeth that characterizes an entire subgroup in Macromitrium.
Vitt (1994) recognized three
distinctive groups for the ten Macromitrium species in Mexico. The 31
Central American Macromitrium species are more diversified and variable
than those of Mexico. As would be expected from such a species-rich genus, Macromitrium
has many distinctive characters. However, it is difficult to subdivide the
genus because its character states occur in reticulating combinations making it
impossible to find any significant correlation of characters. The 31 Macromitrium
species in Central America are arranged below into 7 informal groups bringing
together species that share at least one of seven distinctive features. The
groups are intended to help in the handling of this large genus and may not
represent monophyletic units.
Group 1. Macromitrium leprieurii and M dubium.
Group 2. Macromitrium
flavopilosum, M standleyi, M. subcirrosum, M.
trichophyllum, and M. ulophyllum.
Group 3. Macromitrium
carionis, M. contextum, M. fragilicuspis, M. guatemalense,
M. oblongum, and M. podocarpi.
Group 4. Macromitrium
microstomum and M. richardii.
Group 5. Macromitrium
aureum, M. cirrosum, M. frustratum, M. greenmanii, M.
longifolium, M. parvirete, M. picobonitum, and M. sharpii.
Group 6. Macromitrium
crosbyorum, M. echinatum, M. fulgescens, M. fuscoaureum,
M. mcphersonii, and M. scoparium.
Group 7. Macromitrium
punctatum and M. sejunctum.
Table 1. Distinguishing features of
the Macromitrium subgroups. Character states: plant size, s
small, m medium, l large; leaf border, + present, ‑ absent; upper
leaf cells, lin linear, e elongate, iso isodiametric; basal leaf cells, l
linear, s short rectangular; swollen teeth on basal margin, + present, ‑
absent; calyptra, m mitrate, c cucullate; capsules, p puckered, x
smooth or furrowed.
upper leaf cells
basal leaf cells