Mosses
Phylum Bryophyta
Mosses are the most familiar group of bryophytes, and the most successful spore-bearing plants currently on Earth. They are non-vascular and produce spores like other bryophytes, but are unique in exhibiting spirally-arranged leaves. These plants are usually small, but some mosses (e.g. Dawsonia) can get quite tall: 60 in. In these latter cases, mosses have evolved special water-conducting cells, called hydroids, which function like xylem, but lack thickened and lignified walls.
Ecology and form
Mosses are small plants in stature, due to the lack of vascular tissues
Many mosses are epiphytes (living on trunks) or epiphylls (living on leaves), but some live on soil, rotting wood, rocks, or in stream beds.
Sphagnum moss forms thick mats in peat bogs, which serves as the substrate for many plants that exhibit special adaptations to this acidic environment
Gametophyte (=gamete-forming) phase
Vegetative features
The gametophyte is green, leafy, multicellular, and mostly macroscopic
The gametophyte stage starts when spores germinate
These spores produce algae-like, filamentous strands called protonema
These protonema strands will eventually create buds that grow into a leafy moss
Stems
Moss stems can be upright or creeping, and these forms can help identify mosses
Upright mosses are called acrocarpous
Creeping or prostrate mosses are called pleurocarpous
Even though mosses are non-vascular, they can have water-conducting cells, called hydroids, which are found in both gametophyte and sporophyte
These hydroid cells are not true xylem, because they lack lignin
Leaves
Mosses have spirally-arranged leaves
These leaves are arranged 360 degrees around the stem (compare to leafy liverworts which have their leaves in two rows)
Moss leaves are sometimes called "phyllids", to differentiate them from leaves in vascular plants
"Roots"
Mosses lack true roots, but possess root-like rhizoids
These rhizoids anchor the plant, but do not contribute much to water absorption
Moss rhizoids are multicellular and brown (compare to the unicellular and clear rhizoids of liverworts)
Above: protonema from moss
Reproductive features
Sexual reproduction
Sperm- and egg-forming structures (=gametangia) grow in special clusters at tips of plant, sometimes called splash cups
Sperm (=male gamete), produced in antheridium, swim through water to the female (see image below)
An egg (=female gamete) is produced in the archegonium on a female gametophyte (see image below)
Sperm swim to and into the archegonium and fertilizes the egg, although there is some evidence that tiny arthropods may assist in fertilization
The result is an embryo, which is a young sporophyte, starts to grow off the tip of this female gametophyte
Above: "Splash cups" of a moss, that contain antheridia or archegonia
Above: Archegonia with eggs on a female moss
Above: Sperm inside an antheridium
Spore-producing phase (=sporophyte)
The sporophyte is multicellular and macroscopic (few centimeters long) growing off the tip of the female gametophyte, but not leafy
The sporophyte is composed of a single unbranched leafless stalk with a spore case (sporangium) at the tip
The stalk is called a seta, and elevates the spore capsule to allow for spore dispersal into wind currents
The seta or stalk can be photosynthetic in mosses, but the sporophyte is mostly dependent on the female gametophyte for nutrition
Unlike vascular plants, bryophyte sporophytes do not branch. In other words, they are always a single pole-like stalk with a single spore case at the tip
Spore case (=sporangium)
The spore case or capsule is a complex structure with 3 sections
Theca: the central container that holds the spores
This is the area where meiosis would occur to create haploid spores
There is a central column of sterile tissue, called a columella, found in the theca
A columella is also found in the hornworts, as well as the extinct horneophytes † and Aglaophyton †
Operculum: this is the lid that pops off to allow for the dispersal of the spores
The spores are released with help of peristome teeth, which fling out spores
Apophysis: this is the "neck" of the capsule connecting the capsule to the seta
Stomates are found in apophysis of the capsule.
Above: Mosses with sporophytes
Above: peristome teeth on a moss sporophyte capsule
Diversity
The most species-rich group of bryophytes (~12,000 spp.)
1 order, 1 family, 1 genus (Takakia), 2 species (T. ceratophylla; T. lepidozioides)
Native to western North America and central and eastern Asia
Takakia has one of the lowest known chromosome count (n=4) per cell
Peat Mosses: mosses that store water and secrete acid in bogs
1 extant order, 3 families (Ambuchananiaceae, Flatbergiaceae, Sphagnaceae), 4 genera (Ambuchanania, Eosphagnum, Flatbergium, and Sphagnum), 200 species
Granite Mosses: grow on rocky habitats ranging from tropical to arctic climates, on which they form tufted colonies, typically with reddish to blackish shoots
1 extant order, 1 family, 2 genera (Acroschisma, Andreaea), 100 species
Andreaeobryopsida
Endemic to Alaska and western Canada
1 order, 1 family, 1 species (Andreaeobryum macrosporum)
Oedipodiopsida
Distributed in cooler climates of Eurasia, as well as from Alaska, Washington state, British Columbia, Yukon, Greenland, Newfoundland, Tierra del Fuego and the Falkland Islands
1 order, 1 family, 1 species (Oedipodium griffithianum)
Nematodontous or Hair-cap Mosses
1 order, 1 family, 20 genera (Alophosia, Atrichopsis, Atrichum, Bartramiopsis, Dawsonia, Dendroligotrichum, Hebantia, Itatiella, Lyellia, Meiotrichum, Notoligotrichum, Oligotrichum, Plagioracelopus, Pogonatum, Polytrichadelphus, Polytrichastrum, Polytrichum, Pseudatrichum, Psilopilum, Steereobryon)
Tetraphidopsida
Commonly found in northern latitudes
1 order, 1 family, 2 genera (Tetraphis and Tetrodontium), 4 species (Tetraphis - T. geniculata and T. pellucida; Tetrodontium - T. brownianum and T. repandum)
True or Arthrodontous or Jointed-toothed Mosses
24 orders, 103 families, 11,500 species
Additional Resources
The oldest and fastest evolving moss in the world might not survive climate change (Phys.org 9Aug2023;
Adaptive evolution of the enigmatic Takakia now facing climate change in Tibet (Hu et al. 2023)
An epic global study of moss reveals it is far more vital to Earth’s ecosystems than we knew (The Conversation 1May2023)
Tiny arthropods may help mosses reproduce (Discover 2013)
400 year old bryophytes resprout from under glacier (PNAS 2013)
Desert mosses' water collection features (Utah State University 2016)
The world's tallest moss, Dawsonia (In Defense of Plants, 2016)
50,000 year old Sphagnum in Hawai'i (In Defense of Plants 2018)
Tiny World of Buxbaumia mosses (In Defense of Plants 2019)
The Many Meanings of Moss (The Guardian 3Nov2022)