re-evaluation of tortella - Missouri Botanical Garden

eside the costa and broadly incurved above. Tortella
rigens has bistratose cells juxtacostally in the median
portion of the leaf and therefore the laminae spread out
in a broadly concave leaf cross section, the distal cells
14 µm or more in width.
Although Tortella rigens was not recorded as
part of the Canadian moss flora by Ireland et al. (1987),
it does occur in the most recent checklist of the mosses
of North America north of Mexico (Anderson et al.
1990). The species has previously been reported from
the United States for Colorado by Weber (1973), and
several specimens have been collected by Hermann and
determined as that species. Since all of these collections
occurred in the Rocky Mountains far from alvar
substrates, it was felt necessary to examine European
material to clarify the concept of the species.
Specimens from Scandinavia of Tortella
rigens, including authentic material identified by
Albertson, were loaned to the author from the Museum
of Natural History, Stockholm (S). The specimens were
compared with the species description as given by
Nyholm (1989).
R. Zander (pers. comm.) pointed out the
elongate, smooth cells on the apical margins of the
leaves of some plants of Tortella rigens in Swedish
material—but these occurred only on specimens with
long-lanceolate leaves that showed a narrowed region in
the apex of the leaf. Many reduced stems with
correspondingly reduced leaves occur in these Öland
and Gotland collections. Larger stems and leaves start to
display an abrupt apical limb, which may be interpreted
as an apical propagulum since these are deciduous. It is
only these apical elements that are fragile. The apical
leaf regions also have elongate smooth clear cells on the
margins and, in fact, together with the stiffly erect
leaves and tomentose stems, strongly resemble T.
fragilis. (Note that these marginal cells are frequently
eroded and not evident, but many samples exist where
they are as distinctive as in T. fragilis.) These
collections include populations with short stems and no
apical differentiation, grading to long stems that are
hardly distinguished from the latter species. Smaller
leaves without apical modifications do not appear to be
fragile at the apex.
Tortella inclinata has frequently been
associated with T. fragilis through "intergrading forms"
(Limpricht 1890; Dixon 1924). Tortella rigens was
probably one of those forms before being removed from
the concept of T. inclinata. Persson (1947) indicated
that Tortella rigens may have a "hybrid origin in the
Postglacial" since (in Europe) it is morphologically
intermediate between T. fragilis and T. inclinata. The
deciduous apices and marginal cells noted above seem
to substantiate this suggestion.
In the few American specimens available for
study, the apex may become narrowed, but so far no
bordering tendency has been observed in distal portions
of the leaf.
Tortella rigens appears to be much more
polymorphic than one might expect from the key and
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description of it given by Nyholm (1989). The
species can resemble T. inclinata s.l. in having short
plants with only cucullate leaves. It can resemble T.
tortuosa in having lanceolate leaves that are undulate
and tipped with a short mucro. These can have
anomalous leaf cross sections that are not bistratose
juxtacostally but have the large leaf cells. Taller plants
can resemble T. fragilis by reason of the fragile leaf tips
of specimens with elongated leaf apices and overall by a
more rigid leaf-stance in dry material. The stablest
characters are those that show the affinity with leaves of
T. inclinata s.l.: particularly the relatively naked adaxial
surface of the costa, the inrolled to tubulose to cucullate
leaf margins as well as the impression that the proximal
cell region occupies less of the overall area of the leaf.
Definitive characteristics are the larger size of the leaves
(than T. inclinata), the bistratose area beside the costa in
leaf cross-sections resulting in a broadly tubulose
outline, and the alvar substrate. The plants are generally
taller, and of a darker green (less yellow) color.
Specimens noted to date from North America
more resemble the Tortella tortuosa facies in larger size
and leaf characteristics, especially in undulate leaves,
which are not fragile. The leaves are all tubulose,
however, with occasional cucullate or subcucullate leaf
apices, with distinctive cross sections alread noted
(bistratose juxtacostally with large lumens beside the
costa becoming sharply reduced in size toward the
margins). Specimens in herbaria identified from the
American and Canadian west, possibly because of an
expectation that the species would resemble T. fragilis
based on European material and literature, were actually
depauperate examples of that latter species (discussed
above).
Tortella inclinata is a rather small plant,
occasionally attaining 15 mm in height. Plants that
resemble that species, but which are much larger
overall, such as to 25 mm, and which display variable
leaf apices, rather than the uniformly stubby, nearly
fistulose leaves of T. inclinata, together with their
association with a limestone pavement substrate, rather
than gravel, sand and silt, are most likely to be T.
rigens.
The observations listed in the above
description of the species parallel in many respects the
Latin description of the type by Albertson (1946), which
is roughly translated as follows:
"Tufts robust more or less rigid, 1.5–3, more
rarely to 5 cm tall composed of stems not or little
branched with red-brown radicles loosely scattered.
Moistened leaves erect-patent, dry ones strongly
contorted or crisped; in young plants short, lanceolatetriangular
prominently canaliculate, mitriform in the
apex, mature leaves 4–5 mm long (at the most to 6 mm,
in T. inclinata normally 2–3 mm, at most 4 mm long), at
least in the proximal part canaliculate oblonglanceolate,
more or less elongate in the distal part, yet
not subulate, very often broken, suddenly contracted.
The nerve strong, in quite mature plants usually redbrown,
in all parts distinctly separated from the cells of