toothed plagiomnium moss

Toothed plagiomnium moss, also called baby tooth moss and in the UK woodsy thyme-moss, is a common, tuft-forming (acrocarpous) moss. It occurs in Europe, Asia, and North America. It occurs throughout the United States and southern Canada, but it is uncommon from the West Coast through the Great Plains and in the south. In Minnesota, it is common in the forested eastern regions, uncommon in the western prairie region except along wooded river corridors.

Toothed plagiomnium moss is found in deciduous and mixed forests, forested peatlands, shrubby areas, savannas, fens, cattail marshes, and river shores. It grows on the ground on soil and on woody debris, and occasionally on tree bases, tree stumps, and soil-covered rocks. It can form a dense mat or an open, often widely spreading tuft.

The stems are green or yellowish green when young, and they usually turn brownish as they age. The main, dominant, stem is a vegetative (sterile), creeping, stolon-like, “plagiotropic shoot”. It is unbranched, it lies flat on the ground (prostrate) and it can be up to 2¾″ (7 cm) long. The leaves are arranged on two sides of the stem, not whorled, and they are oriented in a flat plane (complanate). The leaves are smaller at the tip of the stem. There are sometimes numerous, brown, root-like filaments (rhizoids) on the bottom of the stem. The fertile stems are erect, unbranched, and ⅜″ to 1⅜″ (10 to 35 mm) tall, and the leaves are crowded at the top of the stem. The leaves appear to be connected to the stem at the same level, but they are not.

The leaves are ⅛″ to 3⁄16″ (3 to 5 mm) long, 1⁄32″ to 1⁄16″ (1 to 2 mm) wide, and usually egg shaped or more or less diamond shaped, occasionally elliptic. They are dark green or yellowish green, somewhat translucent, and spreading when wet, shriveled and contorted when dry. Dry leaves will return to their previous state when moisture is provided. The base of the leaf blade runs far down the stem. There is one midrib (costa), and it is prominent. The tip of the blade is gradually tapered to a long, sharp, flexible point (cusp). The costa extends into the cusp. The cusp is sometimes toothed. Under magnification, the leaf cells can be seen to be short and elongated. They are sometimes arranged in longitudinal rows, rarely in diagonal rows. The cells in the 2 to 4 marginal rows are smaller, linear, and lighter in color. The leaf margin is toothed only on the upper half, the lower half is smooth. The teeth are sharp and prominent.

Male and female parts are produced together (synoicous) on each fertile stem. A single spore-bearing reproductive structure (sporophyte) rises from near the tip of each fertile stem. The sporophyte is a small capsule at the end of a long stalk (seta). The seta is ¾″ to 13⁄16″ (2 to 3 cm) long, slender, smooth, and yellow, brown, or sometimes reddish. The capsule is egg shaped or cylinder shaped, drooping (pendant), and 1⁄16″to ⅛″ (2 to 3.5 mm) long. It is green at first, soon becoming yellow or yellowish brown.

Acrocarp

⅜″ to 1⅜″ (10 to 35 mm)

Deciduous and mixed forests, forested peatlands, shrubby areas, savannas, fens, cattail marshes, and river shores

The fruit ripens in late spring.

Sources

3, 4, 10, 24, 29, 30.

Janssens, J.A. 2014. Noteworthy Mosses & Liverworts of Minnesota, Part II: Species Fact Sheets. Minnesota Department of Natural Resources 2014, 208 pp.

Native

Common and widespread

This species was formerly placed in the genus Mnium. It was long known that the genus needed revision. In a generic revision of the family Mniaceae (Koponen, 1968), the author proposed dividing the family into ten genera and the genus Mnium into five genera. Two of those five genera are uncommon, three are widespread and common. Of those three, Plagiomnium leaf margins have single teeth, Mnium leaf margins are double toothed, and Rhizomnium leaf margins have no teeth.

Plagiomnium cuspidatum (toothed or “baby-tooth” plagiomnium moss and woodsy thyme-moss)
Rostyslav Yurechko

Plagiomnium cuspidatum, a "baby tooth moss" on a cliff side. Giant city state park.
Mike's thoughts on plants.

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Nancy Falkum
5/18/2023

Location: Whitewater WMA, Main Branch Unit