Mosses & Liverworts

General characteristics

• gametophyte dominant life cycle
• non-vascular
  • some have water conducting hydroid cells in center of stem
    and food-conducting leptoid cells surrounding hydroids
  • some [Sphagnopsida] have closely appressed descending branches
    that function by wick action as water-conducting structures
  • Sphagnopsida can lift water approximately 18 cm above water surface (LaFrance, 1970. unpubl.)
• male gametes flagellated; swim through dew to female gametangia
• female gametes remain in gametangium, even after fertilization
• sporophytes grow attached to gametangium
• germinating spores produce protonema
  then typical gametophytes
• diploid zygotes grown on petri dishes can become gametophytes rather than sporophytes
  produce diploid gametes, and quadraploid zygotes
  up to 32n gametophytes.
• haploid spores grown in 3-d matrix sometimes resemble sporophytes
  with abortive spore formation

Classification

• Hepaticophyta (liverworts)
  • thalloid liverworts
  • leafy liverworts
  • protonema filamentous if present
  • gametophyte exhibits prostrate growth
  • thalloid gametophytes, a flattened leaf-like thallus
    dichotemous branching
    1-celled rhizoids on lower surface
    asexual reproduction by spore-like gemmae
  • leafy gametophytes, a stem-like axis with 2 rows of leaf-like thalli
    unequal branching
    few 1-celled rhizoids on axis
    asexual reproduction by terminal decay
• Anthocerophyta (hornworts)
  • gametophyte, a filmy thallus less than 2 cm diameter
  • sporophyte, an erect, thin (less than 0.5 cm), greenish, curved rod up to 5 cm tall
    central columella (believed to be conducting, hydroid?)
• Bryophyta (true mosses)
  • gametophyte with branched stem [branches are leaf-like]
  • leaves are almost microscopic, 1-cell thick, triangular shaped
  • male and female gametangia on separate branches or separate plants
  • male gametes [2- or 4-flagellae] swim through dew
    to female eggs in gametangia, typically apical
  • Sporophyte usually colorless, golf tee shaped structure attached to gametophyte
  • top (lid) of sporophyte opens explosively, releasing spores
  • protonema typically resembles branched filamentous algae
  • asexual reproduction by terminal decay produces clumps of clones of the parent
  • small fragments of gametophytes may be also serve for asexual reproduction
    by forming protonema, then new gametophytes.
  • orders of Bryophyta:
    • Bryales (true moss)
    • Sphagnales (peat moss)

fossil history

The non-woody Bryophyta fossilize poorly, so the fossil record is scant.

• oldest known thallose liverwort = Pallavicinites devonicus, Metzgeriales
  Devonian, biostratigraphic date: 350 Myr BP; USA
• oldest known true moss = Muscites plumatus, Bryales
  Carboniferous, 350-270 Myr BP; England
• oldest known [putative] peat moss = [species not stated], Protosphagnales
  Permian, 270-220 Myr BP; [location not stated]
• data from J.-P. Frahm 1994. Moose-lebende Fossilien,
  Biologie in unseren Zeit vol. 24, pp. 120-124.
  English translation in Botanical Electronic News vol. 168 (July 5, 1997)
  [www.ou.edu/cas/botany-micro/ben/]

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