Stems

The stem of the Pottiaceae when highly developed (e.g. Timmiella spp.) has four morphologically distinctive layers, the central hydroid strand (hereafter usually termed simply the central strand) of thin-walled, often partially collapsed cells; the central cylinder of parenchyma; the cortex, often differentiated as a sclerodermis of stereid or substereid cells; and the hyalodermis of enlarged epidermal cells, usually thin-walled and often collapsed in mature parts of the stem (e.g. Pl. 1, f. 7). There is much variation in the presence or absence of these features, and in the degree of expression, but these are often taxonomically important and examining stem sections must become standard practice.

Central strand. Hilpert (1933) found that the central strand was sometimes absent in species in which it was otherwise usually present, Trichostomum tenuirostre being a case in point. Saito (1975a) indicated that Didymodon asperifolius always lacked the central strand, but Zander (1978e) questioned this. In Hymenostylium recurvirostrum, one variety occasionally has a central strand, but it is otherwise absent (Pl. 32, f. 1) in the typical variety. In spite of some variation, however, presence or absence of the central strand is generally a good character at the generic level in the Pottiaceae, being, for instance, always absent in Leptodontium (Pl. 36, f. 2–3; 37, f. 10) and Scopelophila (Pl. 47, f. 2). Occasionally, the stem is hollow (Pl. 3, f. 2), in which case a central strand was probably present (section near the stem apex to check this). “Satellite” strands visible in sections in the cortex of the stem occur in Aloinella and Calymperastrum, and are connected with the leaf strand, but not with the central strand of the stem.

Central cylinder. The central cylinder of cells is usually of medium-sized, thin-walled parenchymatous cells, but may occasionally be anatomically distinctive. Its cells may be uncommonly large, as in species of Barbula (Pl. 43, f. 2), or thick-walled, as in many species of Trichostomum (Pl. 11, f. 7), Tortella (Pl. 18, f. 16) and related taxa.

Cortex. The cortex of the stem of Pottiaceae usually consists of cells with smaller lumens than those of the central cylinder. It is sometimes differentiated as sclerodermis, which is well developed in many genera of Pottiaceae and consists of small, thick-walled, longitudinally fusiform cells, the “stereids.” In most cases, the presence of a sclerodermis is easily determined. In Barbula, especially, the sclerodermis is usually strongly differentiated from the enlarged parenchyma of the central cylinder, but occasionally, the outer cortex consists of “substereid” (Pl. 31, f. 2; 39, f. 3) cells difficult to interpret.

Hyalodermis. This consists of usually one layer of enlarged, thin-walled cells on the surface of the stem, commonly in combination with a sclerodermis or at least an outer cortex of small-lumened cells. Like the central strand, this character is variable in expression in some taxa, both as to presence and degree of differentiation, yet it is often of value taxonomically. In Leptodontium (Pl. 36–37), the hyalodermis is characteristic of certain sections of the genus, but not of others (Zander 1972). A collapse of the thin outer walls of the hyalodermis in mature stem parts of many but not all taxa may give a “fluted” appearance to the stem section.

Axillary hairs. The filaments (Pl. 44, f. 21; 59, f. 15–16) borne in the axils of the leaves of Pottiaceae are generally from 2–15 cells in length, consisting of hyaline, uniseriate, cylindrical cells or rarely these bulging and thus producing a moniliform filament (cf. Molendoa and Gymnostomiella). The basal 1–2 cells are either similar to the rest or are more thick-walled and brownish or yellowish. The terminal cell is apparently never terminally pored as in the axillary mucilage hairs of species of certain other families (e.g. Brachymitrion jamesonii Tayl. of the Splachnaceae), but occasionally may be similarly swollen (e.g. Globulinella globifera). Ligrone (1986) has described the ultrastructure and cytochemistry of the mucilage-secreting axillary hairs of Timmiella.

Leaves

The cauline leaves of the Pottiaceae are typically lanceolate to spathulate. Although the basal cells are differentiated in most genera, leaves with more or less sheathing bases are less common, and are characteristic of the Leptodontium (Pl. 36–37) group, Pleurochaete (Pl. 16), and certain other genera—these taxa usually of relatively large physical size. Such leaves may be squarrose-recurved, reflexed at the top of the basal sheath (Pl. 3–4) or rarely wasp-waisted as in Timmiella (Pl. 1). Leaf shape may be variable in a characteristic fashion such that, within the same species, there appears to be a standard differential development of character states. In species with much variation in leaf shape, leaf width commonly varies far less than leaf length, perhaps because stem width also varies proportionately less than leaf length. Thus, in some one species or genus, shorter leaves are ovate, and are characteristically stiff and erect in stance when dry, and longer leaves are generally scarcely wider to sometimes twice as wide, but may be three to four times as long as are the short leaves, and then mostly lanceolate in shape, more easily twisting or curling when dry simply because they have a high length to width ratio. So what may be perceived as a number of characters (including leaf length and stance), correlated with plant size may all be based on a single, simple developmental feature found in many species with variable leaf shapes. Leaves that twist, flex or curl or become tubulose when dry may in that way lessen heat stress. In vascular plants, research on Spartina pectinata Link (Gramineae) by S. Heckathorn and E. DeLucia (Anon. 1990b) demonstrated that leaf rolling reduced surface exposure to sunlight, lowering leaf temperature by five degrees Cent.; J. Lebkuecher and W. Eickmeier (Anon. 1990b) found that leaf curling in Selaginella lepidophylla (Hook. & Grev.) Spring. (Selaginellaceae) reduced high radiation damage that occurred during desiccation. See also discussion of surface wax by Proctor (1979b).

There are at least six distinct trends in the family towards elaboration of specialized photosynthetic leaf tissue:

1. Corrugated or rasp-like leaves, such as are developed in Anoectangium aestivum variants (see Zander 1976).

2. Costal lamellae on the ventral laminal surface, as in Pterygoneurum (Pl. 72, f. 9) or species of Acaulon (Pl. 102, f. 12, 24–25), or the dorsal surface as reported in species of Hymenostyliella.

3. Ventral costal filaments, as in Aloina (Pl. 75, f. 8, 14), Aloinella, Crossidium and Pseudocrossidium.

4. Bulging ventral costal surface, as in the ventral surface of certain species of Tortula sect. Tortula, especially T. atrovirens (Pl. 85, f. 8) and T. revolvens (Pl. 89, f. 4), with the epidermal cells generally higher than wide in transverse section.

5. Protected tissue inside leaf margin revolutions consisting of swollen, thin-walled and coarsely hollow-papillose tissue in species of the genus Pseudocrossidium (Pl. 27, f. 13, 17), and in Hilpertia (Pl. 112, f. 4–5) forming a densely chlorophyllose cylinder along each lateral margin.

6. Ventrally bulging and dorsally nearly flat upper laminal cells that may enhance photosynthesis in periods of dim light and dew in arid habitats by focusing light on the chloroplasts. Papillae may obscure the asymmetric bulge of such cells, and sections are often necessary to demonstrate these. Leaves with this distinctive areolation are found, for example, in genera of the Hyophila complex (Hyophileae sensu this treatment; Pl. 54, f. 7, 16) and in Gertrudiella and Timmiella. Upper laminal cells that are strongly bulging on both sides, as in some species of Syntrichia (Pl. 105, f. 7), may be an equivalent adaptation to dim light. Delgadillo (1984) listed several taxa characteristic of the Yucatan Peninsula of Mexico with a similar phenotype including ventrally bulging upper laminal cells.

Costa

Wyatt (1985) has reviewed the complex and various terminology that has been applied to moss costal anatomy.

Genera that consistently lack a ventral epidermal layer are Aschisma (Pl. 81, f. 7–8, 17), Gymnostomiella, Leptodontiella, Leptodontium (Pl. 36, f. 8), Reimersia, Streptotrichum, Trachyodontium, and Triquetrella. This is also characteristic of Hymenostylium (Pl. 31, f. 7, 16) except in one variety of H. recurvirostrum (Pl. 32, f. 6).

The costal hydroid strand (the Begleiter cells of many earlier authors, and hereafter referred to in context simply as the hydroid strand) when present is found between the guide cells and the dorsal stereid band, or rarely also between the guide cells and the ventral stereid band (e.g. Calymperastrum, Pl. 9, f. 9; Barbula riograndensis, Pl. 45, f. 12). The hydroid strand is usually single, but in some taxa (e.g. Pseudocrossidium, Pl. 27, f. 8, 13, Timmiella, Erythrophyllopsis), there may be two or more distinct strands all dorsal to the guide cells. In very young leaves, the several individual, thin-walled cells of the strand may be seen in section (Pl. 9, f. 9; 69, f. 10–11), but in mature leaves, the strand is usually represented by an angular, three-armed or sometimes star-shaped (with concave sides) opening between the central guide cells and the stereid band (Pl. 3, f. 5; 23, f. 9). Taxa that consistently lack a stem central strand apparently never have a leaf hydroid strand. In taxa with both stem and leaf strands, there is no evidence of a hydroid “leaf trace” connecting the leaf and the stem strands. In some taxa with a stem central strand, the leaf strand may be seen (in stem sections) penetrating the outer cortex for a short distance (Pl. 77, f. 2).

The location of the hydroid strand in the center of the single stereid band of Hennediella stanfordensis (Pl. 98, f. 11), Phascopsis rubicunda (Pl. 100, f. 8–10), Acaulon robustum and other taxa, and the central location of the single stereid band in the costa of Scopelophila ligulata (Pl. 47, f. 6) and Streptopogon spp., may both be explained as the guide cells being absent in essentially double stereid-banded species, but correlation of other characters indicates that this is probably not the case, except possibly the latter in Scopelophila. Also, developmental series studied by Saito (1975a, p. 388) indicate that the ventral stereid band of Barbuleae is elaborated from the same tissue that forms the parenchymatous layer between the guide cells and ventral epidermis in Pottieae. Additionally, certain taxa with variable development of the ventral stereid band (e.g. species of Streptocalypta, Pseudocrossidium and Tortula) show what is apparently a thickening of the walls of pre-existing cells ventral to the guide cells, giving the appearance of what seems to be a second stereid band.

The shape of the dorsal stereid band is considered important here in taxonomically distinguishing between advanced genera of the Pottieae. It must be noted, generally, that the dorsal stereid band in species with much reduced leaves and apparently narrowed costa will be reduced from a crescent-shaped state to a rounded state (e.g. Gyroweisia). Taxa with rounded dorsal stereid bands in the upper part of the leaf may show crescent-shaped bands in the lower part of the leaf (e.g. Leptobarbula, Pl. 78, f. 9–10, and species of Tortula). This character is, as are many in the Pottiaceae, of greatest value with plants of comparatively large stature and, in any case, sections should be taken at midleaf, most definitely above a sheathing leaf base.