Bryophyta

Bryophyta -- from the Greek bryon (moss) and phyton (plant) -- are simple land plants of moist, shaded habitats. The group is large, comprising roughly a thousand genera and tens of thousands of species.

Why they are called 'amphibians of the plant kingdom'. Although bryophytes grow on land -- often forming soft green carpets on damp soil, tree bark or rock -- they remain tied to water for one crucial step of their life cycle: fertilisation. Their motile male gametes must swim through a film of external water to reach the egg, so bryophytes can only complete reproduction where at least occasional moisture is available. This dependence on both land and water, much as amphibian animals depend on both habitats, is exactly why bryophytes are described as the 'amphibians of the plant kingdom'.

Ecological role. Even on otherwise bare ground, bryophytes are important pioneer organisms. They are often among the first plants to colonise fresh rock or soil, helping to break it down and prepare it for the plants that follow in ecological succession.

The plant body. The bryophyte body is more organised than that of algae, though still simple compared with vascular plants. It may be flat and prostrate (lying close to the ground) or erect (standing upright), and it lacks true roots, stems and leaves -- what it has instead are root-like, stem-like or leaf-like structures that merely resemble these organs. Anchorage and absorption are carried out by rhizoids, which are unicellular in liverworts and multicellular in mosses. The visible plant body is haploid and is called the gametophyte, since it is the phase that bears the sex organs and produces gametes.

Sex organs and fertilisation. The sex organs of bryophytes are multicellular, unlike the comparatively simple reproductive structures of algae. The male organ, the antheridium, produces motile male gametes called antherozoids, each bearing two flagella. The female organ, the archegonium, is flask-shaped and encloses a single egg. Released antherozoids swim through a film of surface water to reach the archegonium, where fertilisation produces a diploid zygote.

From zygote to sporophyte. The zygote does not undergo meiosis immediately. Instead it develops into a multicellular sporophyte, which stays attached to, and draws nourishment from, the parent gametophyte rather than living independently. Certain cells of the sporophyte eventually undergo meiosis to produce haploid spores, which germinate to regenerate the gametophyte, completing the alternation of generations.

Liverworts (Hepaticeae). Regarded as the most primitive bryophytes, liverworts have a gametophyte that is a flat, green, dorsiventral (distinct upper and lower surface) thallus, lying prostrate and anchored by unicellular rhizoids. Example: Riccia, Marchantia. A related group, the hornworts (Anthocerotae), also has a flattened thallus but bears an upright, horn-shaped sporophyte -- for example, Anthoceros. …