Q.Describe the structure and function of rhizoids in bryophytes, and state how they differ between liverworts and mosses.

Bryophyta ('moss-plants') are simple, mostly terrestrial land plants of moist, shaded places that still depend on a film of external water to complete fertilisation -- hence their description as the 'amphibians of the plant kingdom'. The visible plant body is the haploid gametophyte, lacking true roots, stems and leaves, and anchored instead by rhizoids (unicellular in liverworts, multicellular in mosses).

The male sex organ, the antheridium, produces motile, biflagellate antherozoids; the female sex organ, the flask-shaped archegonium, encloses a single egg. Fertilisation requires the antherozoid to swim through surface water to reach the egg. The resulting zygote develops into a sporophyte that stays permanently attached to, and nutritionally dependent on, the gametophyte -- so unlike pteridophytes and gymnosperms, the gametophyte, not the sporophyte, is the dominant generation.

Bryophyta divides into liverworts (Hepaticeae) -- primitive members with a flat, dorsiventral thallus (e.g. Riccia, Marchantia) -- and mosses (Musci) -- more advanced members whose gametophyte passes through a filamentous protonema stage before developing into an erect leafy stage (e.g. Funaria, Sphagnum). Economically, mosses (especially Sphagnum) provide peat fuel, water-retentive packing material, and help colonise bare rock and bind soil.