Bryophytes - Amphibians of the Plant Kingdom

Bryophytes are called the 'amphibians of the plant kingdom' because, although they live on land, they need water for sexual reproduction. They grow in moist, shaded places.

  • The plant body is thallus-like, attached to the substratum by rhizoids; it has no true roots, stem or leaves (only root/stem/leaf-like structures).
  • The dominant, photosynthetic phase is the gametophyte (haploid).
  • Sex organs are multicellular: the male antheridium produces biflagellate antherozoids; the female archegonium is flask-shaped and produces a single egg. Antherozoids swim in water to the archegonium (water needed for fertilisation).
  • The zygote forms a sporophyte (foot, seta, capsule) that is attached to and dependent on the gametophyte. The capsule makes spores by meiosis; a spore germinates into a protonema, then the leafy gametophyte.

Liverworts (e.g., Marchantia) are thalloid and reproduce asexually by fragmentation and by gemmae - green multicellular buds in gemma cups. Mosses (e.g., Funaria, Sphagnum, Polytrichum) have a gametophyte with two stages - the protonema and the leafy stage.

Economic importance: Sphagnum provides peat (fuel) and is used as packing material (it holds water); mosses with lichens are pioneer colonisers of bare rock and help reduce soil erosion.

Pteridophytes - The First Vascular Plants

Pteridophytes are the first terrestrial plants to possess vascular tissue (xylem and phloem). They are found in cool, damp, shady places.

  • The main plant body is the sporophyte with true roots, stem and leaves. Leaves may be small (microphylls, as in Selaginella) or large (macrophylls, as in ferns).
  • Sporophylls bear sporangia, which produce spores by meiosis. A spore develops into a small, free-living, photosynthetic gametophyte called a prothallus - it needs cool, damp, shaded conditions, which restricts the distribution of pteridophytes.
  • The prothallus bears antheridia and archegonia; water is needed for fertilisation. The zygote forms the new sporophyte.

Homosporous plants make one kind of spore; heterosporous plants (e.g., Selaginella and Salvinia) make two kinds - microspores and megaspores. The megaspore is retained on the parent and its female gametophyte develops there - an event regarded as a precursor to the seed habit.

Classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), Pteropsida (Dryopteris, Pteris, Adiantum).