4c. Tortella tortuosa var. fragilifolia (Juratzska) Limpricht, Laubm. Deutschl. 1: 605. 1888.
Barbula tortuosa var. fragilifolia Juratzska, Laubm.-Fl. Oesterr.-Ung., 123. 1882
Plants green or yellow-green, appearing brown proximally, loosely foliose, some leaf bases exposed, only slightly comose at stem apex. Stems conspicuously tomentose. Leaves in loose, simple spirals or once circinate when dry, appearing firm or rigid, often fragile and erose, leaf tips often absent; inconspicuously undulate, especially when dry, fragile or not, erect- to erect-spreading when wet; undulate or plane; proximal laminal cells thin-walled and hyaline, sharply differentiated in shape and size from the papillose distal cells, which are papillose in the area of contact; leaves keeled in section, costa in apical region exposed adaxially by up to two stereid cells in width or completely covered with an adaxial epidermis of quadrate papillose cells; leaf cross section with 2-stratose areas beside the costa, the lamina irregularly 2-stratose in patches, lamina tattered, costa occasionally appearing undifferentiated in apical region of the leaf, adaxial stereid layer occasionally disappearing toward the apex, epidermal layer may be continuous throughout the leaf length.
Soapstone quarry, limestone pavement; moderate elevations; low to high elevations; Alta., B.C., Que.; Alaska, Calif., Mich., N.Y., Vt., Wash.; Europe.
Variety fragilifolia is characterized by extremely fragile leaf apices that may be all absent, leaf cross-section irregularities (2-stratose areas juxtacostally), relatively broad leaves that are rather shortly mucronate with quadrate cells on the adaxial surface of the costa extending to the leaf apex. The adaxial stereid band disappears distally, but the adaxial epidermal layer remains intact, an unusual characteristic for species of the genus. Although the stem in section appears rather larger than is typical for the var. tortuosa, a distinctive central strand is rarely present. The stems usually have a dense rufous tomentum. The leaves differ by being little or not undulate or crisped when dry, but may be undulate when wet; they are usually only once circinate and without the delicate appearance of the typical variety. The leaves, within the range of variation of the species, tend to be somewhat shorter and broadly lanceolate in the proximal two-thirds (not setaceous), and the laminae can seem to deteriorate and appear fragile; yet they may also be extremely long and narrow (setaceous), especially in western North American populations. The variety is easily confused with Tortella fragilis, having a setaceous, deciduous (but nonpropaguloid) apex. While some leaves have the typical long, glossy mucro of var. tortuosa, in others the mucro is just a small tip surmounting what appears to be a prolongation of the leaf apex into a canaliculate extension, this frequently broken. Infrequently some apices have a solid and circular form which, like a finger, together with a vague border of cells free of papillae, show a striking affinity with T. fragilis. That the abaxial surface of the costa in the apical portion of the leaf is often roughened with low papillae is also indicative of the propaguloid modifications of T. fragilis, as is the tendency toward undifferentiated cells in the costa in the apical region. The laminal cells in the distal half of the leaf can become very small and obscure: 5-7 µm on average with transversely flattened marginal cells. The laminal cells may also be very large, reaching 17 µm. The leaf cross section also frequently resembles a smaller celled version of Tortella rigens: small marginal cells grading into larger ones beside the costa, there being a bi-to multi-stratose area juxtacostally. This variety has been confused with the excluded European species Tortella nitida; see discussion thereunder for differences.