45. WEISSIODICRANUM Plate
63.
Weissiodicranum Reese in
Reese & Buck, Bryologist 94: 308, 1991. Type: Weissiodicranum insularum
Reese in Reese & Buck.
Plants forming low
cushions or turfs, green above, tan below. Stems branching occasionally,
ca.0.5(1.0) cm in length, transverse section rounded-pentagonal, central
strand distinct and rather large, sclerodermis absent, hyalodermis absent; axillary
hairs 410 cells in length, hyaline or weakly thick-walled basally;
rhizoids sparse. Leaves incurved, tubulose, contorted when dry,
spreading when moist, longly ligulate to lanceolate, ca. 2.5 mm in length,
upper lamina broadly channeled across leaf, margins sharply incurved to
involute, entire, apex narrowly acute; base scarcely differentiated or
rectangular; costa shortly and sharply mucronate, superficial cells quadrate,
elongate and smooth dorsally, with 4 rows of cells across costa ventrally at
midleaf, costal transverse section nearly round to elliptical or ovate,
2 stereid bands present, differentiated epidermis present both ventrally and
dorsally, guide cells (2)4 in 1 layer, hydroid strand absent; upper
laminal cells subquadrate, ca. 810 ΅m in width, 1:1, walls evenly
thickened, superficially strongly bulging ventrally, nearly flat dorsally;
low molariform papillae present on upper part of costa ventrally; basal
cells differentiated across leaf, rectangular, 1013 ΅m in width, 24:1, walls
thin to evenly thickened, several alar cells at insertion inflated in 13
rows, extending from margin about 2/3 way to costa, decurrent as a pad
on stem. Probably dioicous, only archegonia known. Perichaetia terminal,
inner leaves little different. Perigonia and sporophytes unknown. Laminal
color reaction to KOH yellow.
A monotypic genus
found on soil at low to medium elevations in the West Indies (Puerto Rico and
Jamaica) and the Galapagos Islands of Ecuador.
As Reese and Buck (1991) indicated, this
genus is distinctive in its inflated alar cells (Pl. 63, f. 8), similar to
those of the genus Dicranum Hedw. (Dicranaceae), differing as well from
a very similar species, Weissia breutelii, by the absence of a dorsal
costal epidermis and by the bulging dorsal surface of the costa (Pl. 63, f. 9).
The alar cells are enlarged and banana-shaped, grading below into a unique pad
of inflated cells that are 12 cells in thickness on the stem. This is
apparently an extreme development of a tendency already present in Weissia
(a synapomorphy not used in the Data Set), in which leaf insertions consist of
one or two rows of swollen cells that occasionally partially detach from the stem
laterally when the leaf is removed for examination and giving the appearance of
differentiated alar cells. Examined species of Weissia with small groups
of distinctly differentiated but inconspicuous alar cells include: W.
abbreviata, W. balsansae, W. crispum, W. rutilans, and
W. veviridis; there are presumably others. It is almost certain, because
of the complex and characteristic costal anatomy, and because somewhat
differentiated alar cells are found in many species of Weissia, that Weissiodicranum
is derived from ancestors of pottiaceous stock, not of Dicranaceae.
Number of accepted species: 1.
Species examined: Weissiodicranum
insulanum (BUF).
