Land Plants

• Symbiotic associations between fungi

  May have helped the first land plants to obtain nutrients
• Alternation of generations


  A reproductive cycle with two multicellular stages, a haploid gametophyte and a diploid sporophyte
• Nutrients are transferred from the parent plant to the embryo through placental transfer cells
• Land plants


  Plants with terrestrial ancestors, even though some are now aquatic
• Plants supply oxygen and are the ultimate source of most food eaten by land animals
• Green algae called charophytes


  The closest relatives of land plants
• Characteristics shared by land plants and charophytes

  • Rings of cellulose-synthesizing complexes
  • Peroxisome enzymes
  • Structure of flagellated sperm
  • Formation of a phragmoplast
• Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plants
• Land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes
• Sporopollenin


  A durable polymer that prevents exposed zygotes from drying out, also found in plant spore walls
• Land presented challenges: a scarcity of water and lack of structural support
• The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants
• Embryophytes


  Plants with embryos
• Derived traits of plants

  • Alternation of generations and multicellular, dependent embryos
  • Walled spores produced in sporangia
  • Multicellular gametangia
  • Apical meristems
• Alternation of generations


  A reproductive cycle where plants alternate between a haploid gametophyte and a diploid sporophyte stage
• Gametophyte


  The haploid stage that produces haploid gametes by mitosis
• Sporophyte


  The diploid stage that produces haploid spores by meiosis
• Land plants are called embryophytes because of the dependency of the embryo on the parent
• Sporangia


  Organs where the sporophyte produces spores
• Sporocytes


  Diploid cells that undergo meiosis to generate haploid spores
• Spore walls contain sporopollenin, which makes them resistant to harsh environments
• Archegonia


  Female gametangia that produce eggs and are the site of fertilization
• Antheridia


  Male gametangia that produce and release sperm
• Apical meristems


  Regions of continuous growth that sustain plant growth
• Additional derived traits of plants include cuticle, mycorrhizae, and secondary compounds
• Those ancestral species gave rise to a vast diversity of modern plants
• Informal groupings of land plants

  • Nonvascular plants (bryophytes)
  • Seedless vascular plants
  • Seed plants
• Bryophytes are not a monophyletic group; their relationships to each other and to vascular plants are unresolved
• Seedless vascular plants


  • Lycophytes (club mosses and their relatives)
  • Pterophytes (ferns and their relatives)
• Seed


  An embryo and nutrients surrounded by a protective coat
• Seed plants


  • Gymnosperms (the "naked seed" plants, including conifers)
  • Angiosperms (the flowering plants)

  
• Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants: liverworts, hornworts, and mosses
• In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes
• Moss life cycle


  1. Spore germinates into a gametophyte composed of a protonema and gamete-producing gametophore

  2. Mature gametophytes produce flagellated sperm in antheridia and an egg in each archegonium

  3. Sperm swim through a film of water to reach and fertilize the egg

  4. Sporophyte grows out of archegonia, consisting of a foot, seta (stalk), and sporangium (capsule) that discharges spores
• Bryophyte sporophytes are the smallest and simplest sporophytes of all extant plant groups
• Fertilization


  Within archegonium
• Spore germinates


  Into a gametophyte composed of a protonema and gamete-producing gametophore
• Bryophyte sporophytes


  • Grow out of archegonia
  • Smallest and simplest sporophytes of all extant plant groups
  • Consist of a foot, a seta (stalk), and a sporangium, also called a capsule, which discharges spores through a peristome
  • Hornwort and moss sporophytes have stomata for gas exchange; liverworts do not

  
• Marchantia polymorpha,

  • Thalloid liverwort
• Polytrichum commune - hairy-cap mos