Flora of North America species comparison

Uniquely among species of the genus in the flora area, except for var. arctica of Tortella tortuosa, in T. humilis the proximal laminal cells of the leaf are typically gradually differentiated from the distal cells with a zone of cells intermediate in color, cell size, cell wall thickness and papillosity, the angle of hyaline marginal cell differentiation typically shallow across the leaf base, then extending up the margins in a U shape. This character is typical of the genus Trichostomum, with which Tortella is closely allied. Small (depauperate) plants of T. flavovirens may share this character. Typical basal cell differentiation in Tortella is abrupt, the angle of differentiated cells steep, the proximal echlorophyllose and distal chlorophyllose cell boundary sharp.

In cross section, the costa of Tortella humilis diminishes in size toward the leaf tip: sectioning distally along the leaf, the epidermal layer first disappears, leaving up to two stereid cells in width exposed. In some instances at the extreme apex, even the stereid layer disappears leaving the guide cells exposed. The well-developed peristome—elongate and conspicuously twisted—will distinguish Tortella humilis from specimens of T. flavovirens and Trichostomum species, whose peristomes are either not or little twisted, or are rudimentary. Any specimen without fruit is unlikely to be Tortella humilis. H. A. Crum and L. E. Anderson (1981) well described and illustrated the rare species Trichostomum spirale from the Great Lakes region (Ontario, Wisconsin, Minnesota) that, like Tortella humilis, is a nearly stemless plant with gradually differentiated proximal cells and a similar leaf shape. It also has stalked perigonial buds and is autoicous. It may be distinguished by the peristome teeth said to be short, erect and smooth or marked with spiral lines rather than spiculose papillae, the latter characteristic of peristomes of the genus Tortella.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Tortella alpicola is associated with a western corridor of both montane and valley habitats, including the western Great Plains, extending in an arc from Arizona to Ellesmere Island. It is the smallest of the North American Tortellae. The plants are noted by their small size, absence of tomentum on typical (sterile) stems, broken tips on most of the leaves, vividly shining white leaf bases on the stem in contrast with the deep or bright green of the limb, and large leaf cells (to 14 µm wide). Small fertile plants of T. tortuosa and T. inclinata have a similar aspect; these also have stiff, awned perichaetial leaves distinct from the cauline leaves. The large leaf cells and presence of a distinct stem central strand in T. alpicola will differentiate ambiguous specimens from similarly small stems of T. tortuosa and T. fragilis. The costal anatomy of leaves of T. alpicola shows two sets of characteristics, one for sterile plants and one for fertile (perichaetiate). In sterile plants, the costal layers (epidermis, stereids and guide cells) are weakly differentiated and in many cases, not differentiated at all; the costa is undifferentiated in the proximal part of the leaf where the laminae are fully extended and 1-stratose. The laminae are never fully 2-stratose but occasionally exhibit 2-stratose patches. In sterile plants, the epidermal and guide cell layers may be chlorophyllose and there are papillae on the abaxial surface of the costa. In T. fragilis, the costal cells become undifferentiated in the nonlaminate subula while the lamina just proximal to the propagulum is 2-stratose.

In sterile plants the abaxial stereid band disappears along with the other layers. In perichaetiate plants the abaxial and adaxial stereid bands are definite, with thick-walled cells. The adaxial stereid layer, however, appears to quickly disappear in the distal region of the leaf, rather than the epidermal layer, the reverse of every Tortella species in North America (excepting many specimens of T. tortuosa var. fragilifolia). The two other Tortella species in North America with stem central strands, T. humilis and T. flavovirens, have two well-differentiated, strong stereid bands abaxial and adaxial to the guide cell layer. The propaguloid modifications of the leaves are different in kind between T. fragilis and T. alpicola. The propagules of the former species fall as a single unit from leaves disposed all along the stem, whereas those of the latter fall in numerous, fragile, barrel-shaped caducous units of about equal length from the leaf apices.

(Discussion copyrighted by Flora of North America; reprinted with permission.)