Systematics Finals

• Since colonizing land at least 475 million years ago, plants have diversified into roughly 290,000 living species
• 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
• Land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes
• Traits shared by land plants and charophyte green algae

  • DNA comparisons of both nuclear and chloroplast genes
  • Rose-shaped complexes for cellulose synthesis
  • Peroxisome enzymes - minimize loss from photorespiration
  • Structure of flagellated sperm
  • Formation of a phragmoplast - allignment of cytoskeletal elements and Golgi vesicles for cell plate
• Rosette cellulose-synthesizing complexes


  Found only in land plants and charophycean green algae
• Adaptations enabling the move to land

  • In green algae charophytes a layer of a durable polymer called sporopollenin prevents dehydration of exposed zygotes
  • The movement onto land by charophyte ancestors provided advantages: unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogens
  • Land presented challenges: a scarcity of water and lack of structural support
• Three clades are candidates for Plant Kingdom


  • Red algae
  • Chlorophytes
  • Charophytes
• Embryophytes are land plants
• Derived traits of plants

  • A cuticle and secondary compounds evolved in many plant species
  • Symbiotic associations between fungi and the first land plants may have helped plants without true roots to obtain nutrients
• Four key derived traits of plants absent in the green algae charophytes

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


  The multicellular gametophyte is haploid and produces haploid gametes by mitosis. Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis.
• Multicellular dependent embryos


  The diploid embryo is retained within the tissue of the female gametophyte. Nutrients are transferred from parent to embryo through placental transfer cells.
• Land plants


  Called embryophytes because of the dependency of the embryo on the parent
• Walled spores produced in sporangia

  1. Diploid cells called sporocytes undergo meiosis to generate haploid spores

  2. Spore walls contain sporopollenin, which protects against dessication making them resistant to harsh environments
• Multicellular gametangia


  Gametes are produced within 'sex organs' called gametangia. Female gametangia, called archegonia, produce eggs and are the site of fertilization. Male gametangia, called antheridia, are the site of sperm production and release.
• Apical meristems


  Growth regions at plant tips, allowing plants to sustain continual growth in their length. Cells from the apical meristems differentiate into various tissues.
• Ancestral species gave rise to land plants which can be informally grouped based on the presence or absence of vascular tissue
• Nonvascular plants


  Commonly called bryophytes
• Most plants have vascular tissue; these constitute the vascular plants: seedless vascular and seed plants
• Seedless vascular plants can be divided into clades

  • Lycophytes (club mosses and their relatives)
  • Pterophytes (ferns and their relatives)
• Seedless vascular plants are paraphyletic, and are of the same level of biological organization, or grade
• Seed


  An embryo and nutrients surrounded by a protective coat
• Seed plants can be divided into further clades

  • Gymnosperms, the "naked seed" plants including the conifers / cone = sex organ
  • Angiosperms, the flowering plants including monocots and dicots / flower = sex organ
• Nonvascular plants


  Have life cycles dominated by gametophytes
• Bryophytes are nonvascular and represented today by three phyla

  • Liverworts, phylum Hepatophyta
  • Hornworts, phylum Anthocerophyta
  • Mosses, phylum Bryophyta
• Mosses are most closely related to vascular plants
• Gametophytes in bryophytes

  Larger and longer-living than sporophytes. Sporophytes are present only part of the time and dependent on the gametophytes.
• Life cycle of a bryophyte (moss)

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

  2. Rhizoids anchor gametophytes to substrate

  3. The height of gametophytes is constrained by lack of vascular tissues

  4. Mature gametophytes produce flagellated sperm in antheridia and eggs in archegonia
• Gametophytes


  • Larger and longer-living than sporophytes
  • Sporophytes are present only part of the time and dependent on the gametophytes
• Life Cycle of a Bryophyte (Moss)

  1. Spore germinates into gametophyte composed of protonema and gametophore

  2. Rhizoids anchor gametophytes to substrate

  3. Mature gametophytes produce sperm in antheridia and egg in archegonia

  4. Sperm swim through water to fertilize egg
• Protonema


  Part of gametophyte
• Gametophore


  Part of gametophyte
• Antheridia


  Produce sperm
• Archegonia

  Contain egg
• Sporangium


  Capsule containing spores
• Seta


  Stalk supporting sporangium
• Peristome


  Structure around opening of sporangium
• Foot


  Part of sporophyte attached to gametophyte
• Mosses are capable of inhabiting diverse and sometimes extreme environments, but are especially common in moist forests and wetlands