5. CHIONOLOMA Plate 6.

Chionoloma Dix., J. Bot. 60: 102, 1922. Type: Chionoloma induratum Dix.

Plants growing in a dense turf, yellowish green above, light reddish brown below. Stems branching occasionally, ca. 4.0 cm in length, transverse section rounded-pentagonal, central strand distinct, often strong, sclerodermis not differentiated from central cylinder, which is of incrassate, large-lumened cells, hyalodermis present, usually collapsed in mature parts of stem; axillary hairs of several hyaline cells, basal 1–2 also hyaline, but with somewhat thicker walls; radiculose and also closely invested with a verrucose or papillose red tomentum. Leaves incurved, curled and often tubulose when dry, spreading when moist, long-linear-lanceolate, ca. 6–7(–8) mm in length, upper lamina broadly channeled to shallow-grooved along costa near apex, margins plane to weakly incurved, often sharply incurved near apex, distantly weakly denticulate along border, bordered in lower 1/2–3/4 of leaf by 2–7 rows of unistratose, rectangular, epapillose, thick-walled, porose cells; apex very narrowly acute; base long-elliptical, weakly sheathing; costa excurrent as a cylindrical smooth mucro, superficial cells quadrate and papillose ventrally, elongate dorsally, 10–12 rows of cells across costa ventrally at midleaf, costal transverse section elliptical to rounded-triangular, stereid bands two, the ventral dorsal band stronger than the reniform, epidermis present ventrally, entirely absent dorsally, guide cells 8–10 in 1 layer, hydroid strand absent; upper laminal cells small, rounded-quadrate, often short-rectangular, 6–9 µm in width, 1–2:1, walls thick, obscured by papillae, medially free walls bulging ventrally and weakly convex dorsally, bulging marginally on both sides of lamina; papillae massive, multiplex-capituliform, crowded, with many small salients; basal cells differentiated (but not sharply so) across leaf, not or weakly grading into marginal border, rectangular, mostly 10–13 µm in width, 3–5:1, thin-walled and hyaline near insertion grading to incrassate and porose in upper part of leaf base. Perichaetia, perigonia and sporophyte unknown. Laminal KOH color reaction deep yellow or yellowish orange above, reddish brown below.

Found on limestone in Burma, Thailand, Malaysia and Borneo.

This genus has much the appearance of Tortella, but differs in having four areas of very distinct laminal areolation: quadrate upper laminal cells; a marginal border of long-rectangular, porose, incrassate cells (Pl. 6, f. 5); short-rectangular, incrassate and porose medial upper leaf base cells (similar to those characteristic of Trichostomum subg. Oxystegus), and hyaline, thin-walled rectangular cells of the lower leaf base (Pl. 6, f. 6). The border of Chionoloma is similar to that of Pleurochaete in that it does not clearly meld with the basal cells to reach the costa below, and the upper medial laminal cells are also often bulging strongly ventrally but weakly so dorsally; but Pleurochaete differs from Chionoloma by the costal section reniform, the ventral stereid band smaller than the dorsal, the leaf margin plane or broadly channeled near the apex, and the marginal leaf border of relatively thin-walled cells. Pleurochaete is unusual in being pleurocarpous, and the sexual position in Chionoloma is unknown. The incurved upper margins (Pl. 6, f. 14), the rather thick papillae (best seen in section), and the leaf border of thick-walled cells of Chionoloma are similar to those of Hypodontium.

Only the quadrate upper leaf cells, the upper ventral costal cells, and cells of the uppermost portion of the leaf base are papillose. The stem section (Pl. 6, f. 2) is distinctive and similar to that of many Trichostomum species, with all cells incrassate except those of the central strand and the hyalodermis. The leaf section shows an unusually large number of ventral epidermal cells (in costal sections) and of guide cells (but both are each in one layer), and the section is otherwise similar to that of Pseudosymblepharis in the ventral stereid band being stronger than the dorsal and the costal section often rounded-triangular. The red tomentum (Pl. 6, f. 3) is also a distinctive feature, reminiscent of that of some species of Leptodontium, a genus which, however, differs in the lack of a central strand and of a differentiated ventral costal epidermis. The extreme upper laminal margins are sometimes sharply and narrowly incurved like those of Weissia, but that genus does not have the marginal border or characteristic costal section; W. jamaicensis is similar in size, the distinct rectangular leaf base, and its ventral stereid band is commonly larger than the dorsal, but the costal section is round and the upper laminal cells are mainly isodiametric, with walls not so highly thickened. The three species of Chionoloma are not especially different from one another. The genus was synonymized with Pseudosymblepharis by Eddy (1990, cf. Menzel 1992) as merely “exceptionally robust plants in which the pellucid leaf border ascends high into the leaf limb, but the latter character is extremely variable, even within a single species.” The denticulate border of Chionoloma will immediately distinguish this genus from Pseudosymblepharis.

Additional literature: Dixon (1922a).

Number of accepted species: 3.

Species examined: C. induratum (BM), C. latifolium (BM), C. longifolium (BM).