Upper Laminal Cells

Cell wall thickness may have a significant effect on survival in some species. Gerson (1987) found that thickened cell walls in mosses prevent feeding by mites with short mouth parts.

For consistency, measurements of laminal cells in this paper are of the width (perpendicular to the length of the leaf) with indication of length to width (in that order) ratio following. Norris and Koponen (1989) indicated that laminal cells arranged in rows was characteristic of the Pottiaceae, and could be used to distinguish such genera as Streptopogon from, for instance, those of the Splachnaceae.

Trigones. Cell wall thickenings where three walls are contiguous (the so-called “corners” of cells, Pl. 52, f. 4) are characteristic of species of several taxa, including Calyptopogon, Hymenostylium, Hymenostyliella, Leptodontium, and Reimersia, correlated with hygric habitat. Leptodontium, Hymenostylium and Reimersia are apparently closely related (see phylogenetic analysis).

Papillae and mamillae. The laminal cells of most taxa may be described as papillose, but this is an overly general term for a rather complex superficial ornamentation. Ignoring papillae sensu stricto for the moment, examination of transverse sections of laminae demonstrates that species generally have characteristically superficially bulging or flat or slightly convex superficial cell walls. This character has been little used taxonomically (the exceptions are treatments of Tortula by Kramer 1980 and Mishler 1986a) firstly because it is masked by papillae when they are present and, secondly, because secondary thickening of the cell walls can modify the degree of convexity. Some taxa (e.g. species of Tortula) that are quite mamillose, with deep grooves around each laminal cell such that the cells meet only in a narrow band around their periphery, are commonly not described as being intensely bulging because of obscuring high papillae.

Taxa with ventrally mamillose and dorsally flat or weakly convex laminal cells are distinctive in appearance (Pl. 1, f. 2; 52, f. 6; 55, f. 11; 60, f. 5; 64, f. 8–9; 65, f. 8), and the lens-like (commonly non-papillose) nature of the ventral bulge may play some part in focusing light during photosynthesis. Recently, R. Donahue (Anon. 1990a) used fiber-optic light detectors, SEM techniques, and a gas-exchange system to trace light paths within leaves of Thermopsis montana Nutt. (Leguminosae). Shade plants had cells with significantly more convex surfaces than sun plants, capturing diffuse light at greater angles and focusing it on a particularly sensitive photosynthetic region of the cell. In the same report (Anon. 1990a), G. Martin used finely detailed molds of leaf surfaces to make replicas in agarose gel. He found that shade leaves can intensify captured light up to 26 times better than sun leaves, while in intense light potentially damaging radiation may be focused on a protective, absorbing layer. As for papillae, Simon (1987) has pointed out that the papillae of Tortula ruralis increase its leaf surface area by a factor of 30 to 40. Proctor (1979a) discussed papillae as efficient capillary systems in the Pottiaceae.

How these observations might apply to the majority of species of Pottiaceae, which are well known to survive in harsh environments, would be worthwhile to investigate. The character of ventrally convex and dorsally flattened upper laminal cells is often recognized as contributing to distinctiveness at the generic level (e.g. Gertrudiella, Luisierella, and other genera and species). Weissia condensa, however, exhibits variation in degree of ventral mamillosity, this additionally obscured by its papillae. Weissia breutelii, W. veviridis and other species, likewise, have ventrally mamillose leaf cells, yet are not otherwise distinguished as a group from other members of the genus. Hyophila is a genus with ventrally mamillose and dorsally flat cells characteristic of some species, but not others. There is a strong possibility that eperistomate taxa of Trichostomum are masquerading as Hyophila species, and, especially because of the large size of the genus Trichostomum and its evident heterogeneity, Hyophila will probably be significantly reduced in size by some future monographer.

The papillae are often taxonomically important features at the infrageneric level, but, like cell wall convexity, their appearance may be modified by secondary thickening of the cell walls. Hollow papillae, e.g. those characteristic of species of Leptodontium sect. Verecunda, may be “filled in” with secondary wall thickening in some specimens. Likewise, papillae that are characteristically bifid may, in some thick-walled specimens of the same species, appear to be fused into massive “multiplex” papillae (Pl. 38, f. 13).

Hagen (1929, p. 14) described three distinctive types of laminal papillae in the Pottiaceae: (1) large, rounded or platelike, scattered over the lumens, (2) cylindrical and centered over the lumens or otherwise occurring in groups, and (3) ring or horseshoe-shaped papillae. In the literature, many species are commonly described as having “cee-shaped” papillae. In most cases, these are actually hollow, simple papillae (like blisters). Because they are arranged on a convex cell wall surface, they are viewed from above at an angle, and are seen in optical section because of the shallow depth of field at high magnification. The optical section of the walls of these blisters takes the form of the letter “c” or “o”. Andrews (1945) correctly called “ring-shaped or horseshoe-shaped” papillae an “optical illusion,” urging more careful focusing on the part of the microscopist.

Simple papillae in the Pottiaceae exclusive of Leptodontioideae occur usually only singly or in mixed ones and twos over each upper cell lumen (Pl. 10, f. 8); plants morphologically similar to Pottiaceae and with many simple papillae over each lumen may be looked for in the Orthotrichaceae or Rhachitheciaceae. Bifid papillae (Pl. 22, f. 8; 107, f. 17) are common in the Pottiaceae, occurring usually two or more over each upper laminal cell lumen. There may be a high saddle of tissue between the two salients, and in this case the pairs form bifid papillae that are genuinely cee-shaped in both morphology and gross appearance, i.e. the cee shape is not an optical section of a thin hemisphere. Some species, e.g. Leptodontium flexifolium, may have optically cee-shaped papillae in thin-walled plants and bifid papillae in thick-walled plants, so there may be a developmental relationship between the two types. Scanning electron micrographs of the papillae of the Pottiaceae have been published by Mishler (1985b, 1986a, 1987a), Proctor (1979a), Robinson (1974) and Zander (1972), among others.

Laminal thickness. The laminae of some taxa are typically multistratose (Pl. 1, f.2; 3, f. 5), yet this character is variable at the specific level in others. Some genera may have some species with bistratose laminae (Pl. 108, f. 15) and some species with unistratose laminae, but most genera have unistratose laminae. Considerable variation is evident in the genus Didymodon (Zander 1982a), especially the species related to D. rigidulus and D. vinealis. Sayre (1952) briefly described similar variation in Grimmia (Grimmiaceae). When laminal cells are bistratose in patches medially, each of the cells is usually only half as thick through (measured from ventral surface of the leaf to the dorsal) as the cells of the unistratose portion, giving the appearance of a single laminal cell with an interior periclinal wall (Pl. 4, f. 8; 13, f. 15). When leaf margins are bistratose, however, the cells are generally each as thick through as the medial cells, inflating the border whether the marginal cell walls are thickened (Pl. 17, f. 8; 33, f. 8) or similar to the medial cells in thickness (Pl. 8, f. 8; 9, f. 9; 23, f. 9).

Basal Laminal Cells

The basal laminal cells are sometimes called hyalocysts (cf. Edwards 1980a) when much enlarged and lacking chlorophyll. They are usually also epapillose or weakly papillose and sharply differentiated from the smaller, green, papillose upper cells. Hyalocysts are characteristic of Calymperaceae, but are also found occasionally in the Pottiaceae in various species of such genera as Leptodontium, Bryoerythrophyllum and Tortula. In most species of Pottiaceae, however, the basal laminal cells more or less gradually intergrade in size and degree of papillosity with the upper cells. The basal cells may be wide or little wider than the upper cells, and are generally smooth and elongated. Well-differentiated basal cells are commonly bordered along the leaf margins by narrow, rectangular cells that may serve to inhibit breakage of the leaf, the basal cells apparently being weak structurally. Occasionally a distinct area of unusually thickened rectangular cells is discernible in the leaf just distal to the area of hyaline basal cells, as in species of Trichostomum subg. Oxystegus.

Edwards (1980a) noted that resorption pores are found in the hyalocysts of all three major traditional groups of the Pottiales, in the leucobryaceous leaf, and in Sphagnum. He reviewed past references to such pores by other authors, and was surprised that they have not been mentioned more commonly. He found, in Calymperaceae of west tropical Africa, variation in number per cell, position and shape, such characters being of taxonomic value at the family and infrageneric levels. Zander and Cleef (1982) described such pores in Kingiobryum Robins., now recognized in the Dicranaceae but which may belong to the Pottiaceae.