Evolution of Plants


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Summary of Campbell & Reece, 7th Edition of Chapters 29 & 30 in preparation of 2009 International Biology Olympiad in Tsukuba, Japan

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Evolution of Plants

  1. 1. PLANT EVOLUTION Chapters 29 & 30 Protists  Land Plants. Plantae. Draws line @ embryophytes.
  2. 2. GREEN ALGAE [29.1] Closest relationship charophyceans (esp these 2 genera: chara and coleochaete). Plants are multicell photosynth autotroph eukaryo like red, brown and some green algae. Cell walls and chlorophyll a and b overlap Plant & Charophyceans alike: 1. Rosette cellulose-synthesizing complex (prot synth cellulose microfibrils for cell wall cf linear arrangement in noncharophycean algae). 2. Peroxisome enzymes present in Charophyceans peroxisome that minimise organic product loss due to photorespiration. 3. Flagellated sperm. In land plants w flagellated sperm, structure v similar w charophycean sperm. 4. Phragmoplast formation (phragmoplast = cytoskeleton alignment & coalising of golgi-derived vescicles that form cell plate during cytokinesis) Many charophycean species live shallow waters where exposed to drying out. Contain sporopollenin = layer durable polymer prevent drying out  precursor to tough sporopollenin walls encasing plant spores
  3. 3. Terrestrial adaptations [29.2]  Plant differences w charophyceans: 1. apical meristem, sustain growth by localised regios of cell div @ shoot and root tips. Cells diff to various tissues. 2. alternation of generations. Gametophyte (haploid), mitosis (produce gametes), fertilization, zygote, mitosis, sporophyte (spore-producing generation), meiosis, mitosis, gametophyte. 3. sporangia producing walled spores from sporocytes (undergo meiosis). Sporopollenin – chemical adaption. 4. multicell, dependent embryo – has specialised placental transfer cells, sometimes in adj maternal tissue, increase nutrient transfer via elaborate ingrowths of wall surface (PM & cell wall)... why called embryophytes. 5. multicell gametangia, where gametes produced. Female archegonia, - vase shaped organ w single egg retain w/in base of organ. Male antheridia –prod/release sperm into environm.  Dvp of cuticle (polymers of polyesters + waxes)  Produce secondary compounds – secondary mtabolic pthways (side braches of primary mtabolic pthways –those produce lipids, carbohydrates, amino acids– common to all organisms). Aids survival
  4. 4. Phylogeny [29.2] Vascular tissue  vascular plants. Nonvascular plants  bryophytes (liverworts, hornworts, mosses) w/out real roots and leaf system. Seedless vascular plants  lycophytes (club moss, quill worts, spike mosses) & pterophytes (ferns, horsetails). Clade not monophyletic. Vascular seed plants  gymnosperms (naked seeds – seed not enclosed in chambers & angiosperms (all flowering plants – seed dvp inside ovaries) Gymnosperms: Cycadophyta, ginkgophyta, gnetophyta, coniferophyta. Angiosperms: All anthophyta. Monocots form a clade but remaining angiosperms possibly not monophyletic. Most dicots = eudicots “true dicots”. Others divided into lineages: 3 together form Basal angiosperms, another lineage: magnoliids.
  5. 5. Bryophyte Gametophytes [29.3] Terminology: Bryophyta – exclusively mosses. Bryophytes – include all nonvasc plants. Gametophyte dominant = larger, longer living than sporophytes. Germinating moss produce mass of green, branched, 1-cell-thick filmts protonema increase SA for absorb of water + minerals. Produce buds w apical meristem gen gametophore, gamete-produc structure. Gametophore + protonema = body of moss gametophyte. Rhizoids anchor gametophyte. Long tubular cells in worts, filaments in mosses. Not roots, cuz no vasc tissue. Mature gametophytes prod gametes in gametangia covered by protctive tissue. Most bryophytes have sep male/fem gametophytes w antheridia/archegonia produce sperm/egg. Flagellated sperm swim to egg at archegonia base in response to chemical attractants through film of water. Post-fertilization: embryo remain in archegonia, layers of placental transfer cells transport nutrients to embryo while embryo dvp into sporophytes.
  6. 6. Bryophyte Sporophytes [29.3] Bryophyte sporophyte (smallest + simplest of all plant groups) are photosynthetic when young but not independent. Remain attached to gametophyte parent, absorb nutrients. Fertilization  zygote  sporophyte by mitosis, attached by foot to gametophyte  grows long stalky seta that emerges from archegonium. Nutrients conducted from foot to seta to sporangium aka capsule produce spore by meiosis. Immature capsule protected by gametophytic tissue calyptra (shed when capsule mature). Upper capsule feature ring of toothlike structures peristome specialised for gradual spore discharge. Advtg of periodic wind gusts carry spores long distances. Hornworts + moss have stomata, liverworts don’t. Mosses can suffer severe dessication. Forms extensive deposits (peat bogs) of partially decayed organic materal (peat). Inhibit decay.
  7. 7. Seedless Vascular Plants [29.4] Lycophytes (qillworts, spike mosses, club mosses), & Pterophytes (Ferns, horsetail – sometimes arthrophytes due to jointed stems). Sperm are flagellates, swim through water film to reach eggs. Fragile gametophytes. Need damp environments. Dominant (length wise same, complexity wise sporophyte greater) sporophyte not gametophyte-dependent. Branching = more complex body increase efficiency w many sporangia, increase survival. Vascular system = xylem (tracheids lignified, no vessel elemnts – vasc plants sometimes tracheophytes)/phloem (sieve-tube membrs), roots and leaves (microphylls, spine shaped w single vein cf megaphylls, highly branched vasc system), spore-bearing leaves sporophylls on underside have sori (sorus) – clusters of sporangia. Strobili – cones formed by groups of sporophylls Homosporous – one type of sporophyll produce one type spore that dvp into bisexual gametophyte cf Heterosporous has 2 types of sporophylls give 2 diff spores. Megasporangia  megaspores  female gametophytes. Microsporangia in microsporophyll  microspores dvp male gametophytes.
  8. 8. Seedless Vasc Life Cycle [29.4] Spore  Young photosynthetic gametophyte (usually bisexual, though can be from megaspore or microspore) Mechanisms promote cross-fertilization. Bisexual gametophytes have antheridium/archegonium structures that prod sperm and egg. Flagellated sperm swim from antheridia to archegonia  Fertilization  dvp into sporophyte Sporophyte grows from out of archegonium of parent gametophyte. Mature sporophyte have sporophylls with sori on underside. Meiosis occurs in sporangia. Sporangia release spores. Gametophyte shrivels and dies after young sporophyte detaches itself.
  9. 9. Seed Plants [30.1] Emergence/diversification of seed plants. 1. Reduced gametophytes – microscopic in seed plants. Gametophyte dependent on sporophyte for nutrients. Miniaturization allowed spores to be retained w/in sporangia of parental sporophyte  protect gametophores from ext stress. 2. Heterospory (all seed plants, & in some seedless plants) Megasporangia/microsporangia produce mega/microspores giving to fem/male gametophytes. Megasporangium  one single function megaspore. Microsporangium  many function microspores. 3. Ovules (megasporangium, megaspore, integuments) – one (in gymnosperms) or two (in angiosperms) integuments from sporophytic layers envelop/protect megasporangium. In each ovule, female gametophyte dvp from megaspore, produce 1+ egg cells. 4. Pollen grains dvp from microspores, protected by tough sporopollenin coat. Transfer by pollination. Pollen grain germination  pollen tube discharge 2 sperm into ovule through micropyle  ovule dvp into seed (embryo + food suply w/in seed coat from integuments) – resists harsh environment, disperse widely
  10. 10. Gymnosperms [30.2] Progynosperms (heterosporous plants acquiring adaptations characterising seed plants but don’t produce seeds) Naked seeds not enclosed in ovaries but exposed on modified leaves that form cones (strobili) – cone bearers are conifers. Ovulate cone has scales, each has two ovules, each containing a megasporangium. Pollen cone contains many microsporangia (each contain many microsporocytes that will dvp into microspores after meiosis into haploids  pollen grains) held in sporophylls. Pollen grain enters through micropyl, germinates, form pollen tube slowly digesting through megasporangium, meanwhile megasporocyte  meiosis prod 4 haploid, one megaspore survive. W/in megaspore, female gametophyte dvp, contain few archegonia each w egg. Sperm cells would’ve dvp in pollen tube which extends to female gametophyte. Fertilization of nuclei.  only one zygote dvp into embryo usually.
  11. 11. Angiosperms [30.3] Flowering plants – flowers = reproductive organs spec for sexual reproduction. Require pollination. Receptacle, sepals, petals, stamens (filament + anther) are microsporophylls (NB: flower organs are mod leaves), carpels (stigma, style, ovary) = megasporophylls which make megaspores & fem gametophyte. Single/fused carpels sometimes called pistil. Fruit (fleshy or dry) from mature ovary (can incl other flower parts). Seeds dvp from ovules, ovary wall thickens  pericarp. LIFE CYCLE: flower of sporophyte has micro/megasporangia that produce microspores (prod male gametophytes) in pollen grains and megaspores (prod female gametophytes). Pollen grain to stigma. Self/cross-pollination. Pollen grain  2 haploid: tube cell that forms pollen tube and generative cell that divides in tube cell. Tube cell penetrate ovary via micropyle (pore in integuments), discharge 2 sperm cells into embryo sac. One generative cell fertilizes egg  diploid zygote. Other fuse w 2 nuclei of large central cell of female gametophyte (embryo sac) = double fertilization.  zygote dvp into embryo (rudimentary root + cotelydons), packaged w food (endosperm dvp from triple nuclei nucleus of central fem gametophyte cell in seed. Evo-devo hypothesis: ancestor angiosperm sep micro/megasporophylls.
  12. 12. Angiosperms Diversity [30.3] Monocots have one cotyledon in embryo, parallel leaf veins, vascular tissue scattered, root system often fibrous, pollen grain w one opening, floral organs in multiples of three. Dicots have two embryonic cotyledons, netlike leaf veins, ring- arranged vascular tissue, taproot system usually, three opening on pollen grains and floral organs multiples of four or five. Coevolution between angiosperms and animals shown often (mutualistic symbiosis)