The Bryophytes

There are about 20,000 species of Bryophytes, the plants which lack vascular tissue. They are found throughout the world. Although more prevalent in moist and shady areas, Bryophytes can be found in alpine regions, where they are subjected to freezing, and some in deserts, where they are desiccated most of the time. Many Bryophytes are highly specialized, and found in restricted habitats. One of the most abundant, Sphagnum, is found in bogs world wide, and forms peat, a source of fuel when compacted, and a soil texturizer when less "decomposed". Associations of Bryophyes with mychorrizae are common.
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There are three Divisions of Bryophytes:

  • Bryophyta (Mosses)
  • Hepaticophyta (Liverworts)
  • Anthocerotophyta (Hornworts)
    Distinguished from each other by:
    Major Characteristics of Bryophytes
     Lack Vascular Tissue --> Impact on Structure
     Bryophytes lack vascular tissues so Bryophytes must absorb all water and nutrients at the surface and pass them from cell to cell. A few have modified water transport cells, call hydroids , and a few have solute conducting cells, called leptoids , but this is rare. Depending on cell to cell diffusion for transport places size restriction on Bryophytes.
    No cuticle on above ground parts (generally) so most are "soft" and pliable.
    Sperm are motile
    Vegetative reproduction is common in Bryophytes:
    Gametophyte generation is the predominant and assimilative stage.

    General Life History (Some variations in the three Divisions)

     
    Sperm (n)	+	Egg (n)
|
Zygote (2n)
Growth by mitosis produces:
|
Embryo (2n)
|
Sporophyte (2n)
|
Sporangium (2n) 
Meiosis in Sporangium produces:
|
Spore (n)
Growth by mitosis produces:
|
Gametophyte (n)
|			|
Antheridium (n)		Archegonium (n)
|					|
Sperm (n)			Egg (n)
    Notes
    Division - Bryophyta (Mosses)

    Characteristics of the True Mosses
     Mosses have radial symmetry (usually). The gametophyte has a stem like axis with spirally arranged "leaves", which are properly called phyllodes . Mosses attach to their substrate with multicellular rhizoids .
    They may be erect or prostrate (axis along the ground)
    Moss "leaves" have a costa (midrib)
    Mosses are found in a range of habitats, although moist and shady habitats are more common. Some mosses are found on rocks and in arid locations. As mentioned, mosses form huge masses in bogs throughout the world. Mosses are often epiphytes.
    Most water absorption is from the surface cells of the moss plant. Like most Bryophytes, mosses can desiccate, or dehydrate for long periods without permanent damage.

    Sexual Reproduction
     Water is required for transfer of the motile sperm to egg. Most antheridia are in terminal disk-shaped clusters to facilitate water capture for sperm transfer.

    The vase-shaped archegonia produce one egg. After fertilization, the sporophyte grows out of the archegonium, and nutrients for the developing sporophyte are provided by the gametophyte.

    The sporophyte consists of a foot , anchored in the archegonium, a seta, or stalk, which elevates the sporangium, or capsule . Typically, a portion of the gametophyte, called the calyptra , protects and covers the developing capsule.

    Meiosis in the capsule produces haploid spores. When spores are mature, the lid of the capsule, called the operculum, opens, and a row or rows of hygroscopic teeth, the operculum, respond to changes of humidity to open and release spores.

    Each spore germinates and divides to form a filamentous protonema , which develops into the gametophyte.

    Life History 
    Spore
   |
Protonema
   |	As protonema grows: 
   |		Lower portion forms rhizoids
   |		Upper portion forms axis and "leaves"
Leafy Gametophyte
   |
   Gametangia
  |           |	May be 	Lateral or Terminal
  |           |	May be 	Homothallic 
  |           |		(both sexes on one plant) or
  |           |		Heterothallic (separate plants)
  |           |
Antheridium  Archegonium (Both have sterile hairs, paraphyses associated)
(Club-shaped)	(Elongate vase-shaped)
    |            |
Motile Sperm    Egg (One egg)

            Zygote
              |
    Embryonic Sporophyte
              |   Sporophyte consists of:
              |       Foot
              |       Elongate Seta (Stalk)
              |       Capsule*
              |
      Spore parent cells  *Capsule which consists of:
              |               Central Column
           Meiosis            Operculum with Calyptra
              |               Peristome (Teeth) which are
           Spores               hygroscopic for dispersal
    Vegetative Reproduction in Bryophytes
    Fragmentation Gemmae (rare)

    Human role of mosses

  • Decorative stone walls, rock gardens
  • Pioneer ecological species
  • Bog ecology Division - Hepaticophyta (The Liverworts)
     Two groups:
    Characteristics
     Thallose Liverworts
    A common genus is Marchantia , one which has lobes with a central notch and lengthwise surface groove. The meristem is located at the notch, and branching occurs forming two new lobes.

     Life History
     All liverworts exhibit the basic alternation of generations. Meiosis produces half male and half female spores, so that a gametophyte is either male or female. In Marchantia , for example, the archegonia and the antheridia are elevated above their respective thallus structures on stalks called archegoniophores and antheridiophores. Several archegonia and antheridia are found in each stalk. Other thallose liverworts will have the antheridia and archegonia embedded in the thallus tissue, rather than on stalks.

    The sporophyte is a simple club-shaped sporangium which grows from a basal foot embedded in the archegonium. At maturity, the sporangium splits open, and the release of spores is assisted by special hygroscopic hairs, called elaters.

    Vegetative Reproduction

    Leafy Liverworts
    Plant body along an axis (looks like leaves and stem) with the "leaves" overlapping in pairs along the axis
    Prevalent as epiphytes in warm moist tropics. They are not uncommon in the Pacific Northwest. We just mistake them for mosses. They are not as hardy as mosses, and can best be found in the late spring, when the sporophytes mature.

    Life History
     The sporophyte capsule, found on the surface of the gametophyte is "ephemeral". The seta (sporophyte stalk) elongates rapidly and then the spores are dispersed

    Division - Anthocerotophyta (Hornworts)

    Hornworts have rounded, small, thallose-like gametophytes which are found in moist shaded soils. The gametophytes may be unisexual or bisexual, depending on the species.

    The sporophyte is "horn-shaped", and grows from a basal sheath beneath the surface of the gametophyte thallus. The sporophyte continues to grow from a basal meristem, producing spores clustered around a central stalk. The sporophyte tip splits releasing spores. Spores continue to mature for some time, and the Sporophyte continues to split.

    Hornworts are less common than liverworts or mosses, with only about 100 species identified. Some are found in Washington.

    Vegetative reproduction occurs in the hornworts by fragmentation

    Hornworts have just one chloroplast per cell, which is unique among the true plants. One chloroplast per cell is common in many algae.

    This page most recently modified on 9/6/04.