Chapter 22.  Plantae: Bryophytes
Note -- We will emphasize the Mosses

• Bryophytes - liverworts, hornworts and mosses.  Generally small, restricted to wet habitats, display combination of  traits that are characteristic of algae and of vascular plants.
• "Algal traits":
• Small, non woody, non-vascular (no xylem or phloem).
• Mostly restricted to moist environments.
• Produce free-swimming sperm that require water to reach and fertilize egg.
• No differentiation into roots and shoots.
• "Vascular plant" traits:
• Develop structures that resemble roots, stems and leaves.
• Some species have cuticles and stomata.
• Gametes (sperms and eggs) produced in structures which have jacket of protective cells, similar ot those in vascular plants.
• Zygote develops into embryo which is retained and supported by maternal parent.
• Sporic life cycle with multicellular gametophyte and sporophyte generations.
• Bryophyte life cycle is unique among land plants in that the gametophyte is the dominant generation and the sporophyte is small, morphologically reduced and dependent on the gametophyte. The opposite pattern is displayed by vascular plants (sporophyte dominant, gametophyte reduced).
• Evolutionary status of the bryophytes is uncertain.  Some hypotheses:
• Bryophytes are ancestral to the vascular plants.
• Bryophytes and vascular plants have a common ancestor, but bryophytes are not ancestral to the vascular plants.
• Bryophytes are derived from the vascular plants by a process of morphological simplification.
• All bryophytes share a common ancestor and are therefore a natural group, OR mosses and liverworts evolved from Chlorophyta, hornworts from Phaeophyta.
• Poor fossil record doesn't help.  Oldest bryophyte fossils (Devonian, 350 MYA) co-occur with fossils of vascular plants.
• Liverworts: about 9000 species, mostly restricted to moist, shady habitats.
• Gametophyte is the dominant generation.  Thallus with modest amount amount of differentiation, but no vascular tissue (See Figure 22.5).
• Gametes are produced by gametangia.
• Antheridia (singular, antheridium) produce sperms (many each).  Sperms are flagellate, swim through films of water to reach eggs.
• Archegonia (singular archegonium) produce eggs (one each).
• Fertilization occurs in archegonia.  Zygote develops into embryo (diploid sporophyte).
• Sporophyte: most of tissue is sporogenous, develops into sporocytes.  Meiosis produces meiospores which disperse, germinate, develop into gametophytes.
• A note on bryophyte spores: the spore walls are composed of sporopollenin, a very resistant compound, as in pollen grains of vascular plants.
• Mosses: about 14,000 species, broader ecological range than liverworts (e.g., some occur in seasonally dry habitats).
• Gametophyte is the dominant generation.  Morphologically more complex than in the liverworts: differentiation into "stem" (caulids), "leaves" (phyllids) and "roots" (rhizoids).  These structures are not true stems, leaves and roots because they are not vascularized and are produced by the gametophyte, not sporophyte generation.  See Figure 22.18.
• Moss life cycle essentially the same as in the liverworts (sperms produced in antheridia, eggs in archegonia, fertilization takes place in archegonia, where the embryo develops into diploid sporophyte).  See Figure 22.12.
• Sporophyte more complex than in the liverworts, composed of stalk (seta) and capsule (Figure 22.18).  Sporophyte is parasitic on gametophyte in the sense that it lacks chlorophyll and must depend on maternal gametophyte.  The spores develop in the capsule and are released to be dispersed by wind.
• Spores germinate to produce protonema (alga-like filament).  This subsequently differentiates into "leafy" gametophyte.
• Moss physiology
• Translocation tissues - some moss gametophytes have rudimentary xylem like conducting tissues.
• Photosynthesis - most species have very low compensation points, allowing them to survive low light conditions.  Some species can photosynthesize at low light levels and low temperatures under snow banks. All mosses are C3 photosynthesizers.
• Moss ecology
• Sphagnum is an important peat producing species in cold climates (e.g. sub arctic, Ireland and Scotland) and in bogs (there are many bogs in WNY).  Drained peat bogs form "muck" soils in WNY.
• In tundra vegetation, mosses are major plant component. Along with lichens, comprise base of grazing food chain, e.g. for caribou and reindeer.