Common and easy to recognize, tooth mosses in genus Plagiomnium have several common names: tooth mosses, thyme mosses, calcareous mosses, and more. The large, transparent, bright green or yellow-green leaves have a beautiful glassy appearance. Often, branches of these mosses intrude into the mats of other mosses.
In most species, the leaves are large (for a moss), oval or oblong, and transparent. Note the distinct midrib and the fine teeth along the margins of the blade. There are two types of stems: vegetative (sterile; not fertile) and reproductive (fertile). The vegetative branches lie flat or arch downward. The leaves (unusually, for a moss) are alternate and two-ranked (positioned on two opposing sides of the stem; not whorled). When these mosses dry, the leaves shrivel and darken considerably, giving the vegetative branches a chainlike appearance.
This is a genus of acrocarpous (cushion-forming, nonbranching) mosses. The sporophytes arise only from the stem tips. The stems that produce male gametes are held erect and have larger leaves that, at the tip of the stem, form a rosette that looks like a tiny green flower with a dark center.
Sporophyte: The stalks can be yellow, yellow-green, brown, or reddish. The capsule is cylindrical to ovate, and it may nod (bend down) on its stalk or be held horizontally. The pointy, side-split tip (calyptra) falls off and you can see tiny teeth guarding the opening where the spores come out.
Learn more about Missouri’s mosses on their group page.
Species and similar species: Missouri has 10 species in the mnium family. Four have remained in genus Mnium, while five others are segregated into genus Plagiomnium. One other species in the family is Rhizomnium punctatum.
- Species Plagiomnium cuspidatum is one of the easiest to recognize mosses in Missouri. It’s also very common. Common names include woodsy moss, baby tooth moss, woodsy thyme moss, and toothed plagiomnium. A key ID character are the finely toothed margins in only the upper half or two-thirds of the blade. Part of the leaf blade runs down along the stem where it attaches to the stem. The chubby, keg-shaped capsules arise on stalks in early spring. The capsules are nodding (bend downward); when young, they are light green; as they mature (in May), they dry and become tan with a rust-colored stalk.
- Other fairly common Missouri species in the family are Mnium marginatum, M. stellare, Plagiomnium ciliare, P. ellipticum, P. rostratum, and Rhizomnium punctatum.
- Mnium hornum, M. thomsonii, and Plagiomnium medium are rare or very localized in distribution and therefore are listed as species of conservation concern in Missouri.
The two-ranked leaf arrangement of mosses in this family is unusual but not unique. Plume mosses (Fissidens spp.), for instance, are similar, but they have an unusual leaf anatomy, and the basal parts of the leaves clasp the stem.
To distinguish leafy liverworts from mosses that have two-ranked leaves, remember that moss leaves have midribs and can have teeth.
Stem length: to about 3 inches. Height: fertile stems to about ¾ inch.
Statewide. Different species may have different ranges.
Habitat and Conservation
Our most common tooth moss species occur in a variety of habitats, including damp rocks and soil and rotting wood.
Baby tooth moss (Plagiomnium cuspidatum), in particular, is widespread in America and even globally. It is frequently found — a stem here and there — growing among other mosses. It’s always a pleasure to come across this appealing moss. The only thing that seems to restrict its habitat is lack of moisture.
Life Cycle
As with other mosses, members of this genus have alternating generations. The plant we usually think of as the moss is the gametophyte generation (it’s a gamete-, or sex-cell-bearing plant). In separate organs, it produces sperm and eggs. When water, even a thin layer of rainwater, is present, the sperm can swim to the eggs and fertilization occurs. The resulting offspring plant, called a sporophyte (spore-bearing plant), grows out of the female organs of the gametophyte.
In this genus, the sporophyte takes the form of a stalked capsule growing out of the tip of the gametophyte stem. The stem forms a rosette of leaves at the tip where the sporophyte will arise. The sporophyte obtains nutrients from the gametophyte. When mature, the capsule opens to release spores, which can grow into new gametophyte plants.
Mosses also commonly reproduce asexually when a portion of the plant breaks away, gets moved elsewhere, and continues growing as a separate plant.
Human Connections
These pretty little mosses look a lot like tiny ferns, and it would be easy to guess that they're some kind of vascular plant instead of a moss. What a wonderfully diverse world we live in!
Learning about mosses, liverworts, and lichens is fun. At least they don’t run or fly away when you try to get a good look at them! Today, with lots of information on the Internet and with cameras that take good closeup pictures, it’s easy to dive into this fascinating realm of nature.
Ecosystem Connections
Many kinds of birds, as well as other animals, use mosses in nest building, since they can provide cushioning, insulation, camouflage, or all three:
- Birds that have been documented using tooth mosses in nests include the American robin, wood thrush, Carolina wren, eastern phoebe, barn swallow, and prothonotary warbler.
- Four-toed salamanders have been documented nesting in areas with tooth mosses.
- Even bumblebees use mosses in their nests, and tooth mosses are doubtless among the mosses they collect and incorporate into their waxy nurseries.
Mosses, liverworts, hornworts, and lichens seem rather similar, but these organisms are in very different groups. Mosses, liverworts, and hornworts are small, low plants usually found in damp habitats. Unlike more familiar plants, they lack veinlike structures and do not produce flowers or seeds — instead, they produce spores. Meanwhile, lichens are not plants at all: they are a collection of different fungi that have photosynthetic algae living within their tissues.