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What characteristics or structures are common to ALL plants? • • • • • Chapter 21 The Evolution of Plants Eukaryotic Multicellular Photosynthetic autotrophs Chloroplasts Cell walls made of cellulose To which domain and kingdom do algae belong? But some of these characteristics are also true of other organisms…especially algae…and especially especially green algae…and MOST especially charophyceans. Plant Phylogeny: here’s the “big picture” of all extant plant life Why did land plants evolve? Most land plants have (but charophyceans do not): Charophyceans •freshwater green algae, closest ancestral relative of plants •four characteristics of plants shared only with charophytes (other algae differ): •Rosette cellulose-synthesizing complexes (as opposed to linear) •Peroxisome enzymes to limit loss of organic product due to photorespiration •Structure of flagellated sperm •Formation of a phragmoplast (cell plate) • • • • • Apical meristems Alternation of generations Walled spores produced in sporangia Multicellular gametangia Multicellular, dependent embryos Briefly, about each of these: 1 When your light and CO2 are ABOVE GROUND, and your water and minerals are BELOW GROUND, you need specialized tissues and organs in both locations. Life cycles Apical meristems are areas of concentrated cell division and differentiation (sometimes described as “perpetually embryonic”). Alternation of Generations “sporophytes make spores make gametophytes make gametes” ~unique to plants and some algae (but not charophyceans, so a derived characteristic) Gametes: haploid Gametes: Spores: haploid Spores: reproductive reproductive cells that fuse with another cell cells that do not fuse with another cell Reduction in Size of Gametophyte ~all sexually reproducing organisms have haploid and diploid stages– but only in “alternation of generations” do both the haploid and diploid stages include multicellular stages Multicellular sporophytes possess sporangia which contain diploid sporocytes which produce haploid spores with walls made of sporopollenin. Plant spores are specialized to deal with harsh, drying conditions. (Do you think algae spores have sporopollenin?) C-fern (Ceratopteris richardii) sporangium with spores C-fern spores, showing trilene marks (where three other spores were attached as a tetrad after meiosis) 2 More C-Fern beauty… Hermaphrodite gametophyte with 4 archegonia and 2 antheridia on notch margins Gametophytes have multicellular gametangia (female=archegonia, male=antheridia) which produce haploid gametes (egg and sperm) Male gametophyte with spherical antheridia Antheridia close-up And, note the meristem notch Close-up of sperm release; they begin round and nonmotile. Here, one is swimming away, propelled by a helical structure. Sperm release Plants are classified as “embryophytes” because the embryos are retained in the female parental tissue. Protection of Eggs and Embryos Found in embryo, to enhance the transfer of nutrients via ingrowths in wall and plasma membrane 3 Remember, what we’re looking at are characteristics that land plants have and charophyceans do not– in other words, what are the derived traits that separate them from their ancestors • • • • • Apical meristems Alternation of generations Walled spores produced in sporangia Multicellular gametangia Multicellular, dependent embryos Leaves of Vascular Plants Other Terrestrial Adaptations • Vascular tissue transports water and nutrients to the body of the plant. • Cuticle provides an effective barrier to water loss. • Stomata bordered by guard cells that regulate opening, and thus water loss. Recall the “big picture”… now, classify land plants as nonvascular or vascular BRYOPHYTES (mosses)--nonvascular •Have flagellated sperm, so need to be near water, tend to grow in moist, dark areas •Spores with sporopollenin (resists potential environmental damages) •Lack lignin-fortified tissues (lignin “hardens” the cell walls) to grow taller, so grow in “mats” Nonvascular Plants • Lack specialized means of transporting water and organic nutrients. •Gametophyte is dominant generation • Do not have true roots, stems, and leaves. • Produces eggs in archegonia and flagellated sperm in antheridia. 4 Nonvascular Plants • Hornworts have small sporophytes that carry on photosynthesis. Figure 29.15x1 Hornwort Nonvascular Plants • Liverworts have either flattened thallus or leafy appearance. Nonvascular Plants Moss Life Cycle • Mosses (phylum Bryophyta) usually have a leafy shoot, although some are secondarily flattened. 5 Figure 29.18 A moss sporangium with a “spore-shaker” tip Sphagnum moss, or “peat moss” is an ecologically important bryophyte • Many mosses produce phenolic compounds that absorb radiation • Sphagnum also releases compounds that reduce bacterial activity, so it slows decay • Huge reservoir for carbon (400 billion tons worldwide) for the carbon cycle…and as a fuel source Figure 29.19x A peat moss bog in Norway Next classification: vascular without seeds (seedless), and vascular with seeds SEEDLESS VASCULAR PLANTS Vascular Plants • Xylem conducts water and dissolved minerals up from roots. • Phloem conducts sucrose and other organic compounds throughout the plant. • Lignin strengthens walls of conducting cells in xylem. •Sporophyte is dominant generation Sporphylls= leaves that bear sporangia (clusters in ferns called sori) •Pterophytes (ferns) primary example •Flagellated sperm (so need at least a film of water) •Homosporous= produces one type of spore which results in a bisexual gametophyte; other option is to be heterosporous: megaspores= female, microspores= male) 6 Figure 29.20 Cooksonia, a vascular plant of the Siluria Vascular Plants Vascular Tissue Club Mosses • Most seedless vascular plants are homosporous. – Windblown spores are dispersal agents. • All seed plants are heterosporous and have male and female gametophytes. Seedless Vascular Plants Horsetail • Ferns and Allies – Horsetails – Rhizome produces tall aerial stems. • Contains whorls of slender, green branches. • Small, scalelike leaves also form whorls at the joints. 7 Figure 29.21 Pteridophytes: club "moss" (top left), whisk fern (top right), horsetail (bottom left), fern (bottom right) Ferns • Whisk Ferns – Branched rhizome has rhizoids. – Mutualistic mycorrhizal fungus helps gather nutrients. • Ferns – Large conspicuous fronds. • Divided into leaflets. • Dominant sporophyte produces windblown spores. Figure 29.23 The life cycle of a fern Whisk Ferns and Ferns Figure 29.0 Ferns Avoids self-fertilization by archegonium and antheridium maturing at different times Figure 29.23x1 Life cycle of a fern: mature fern 8 Figure 29.24a Fern sporophyll, a leaf specialized for spore production Figure 29.24b Fern sporophyll, a leaf specialized for spore production Figure 29.24c Fern sporophyll, a leaf specialized for spore production Figure 29.23x3 Life cycle of a fern: sporangium Figure 29.23x5 Life cycle of a fern: germinating Figure 29.23x6 Life cycle of a fern: gametophyte 9 Figure 29.23x7 Life cycle of a fern: archegonia Figure 29.23x8 Life cycle of a fern: sporophytes Figure 30.1 Three variations on gametophyte/sporophyte relationships Two classifications of seed plants All seed plants have: *seeds (obviously) *reduced gametophytes *heterospory *ovules *pollen • Reduction of the gametophyte stage allows for protection of gametophytes, and gametophytes can gain nutrition from the sporophytes Development of the seed –Another reproductive strategy for protecting the embryo and dispersing offspring –has the nutrient source and embryo packaged in a protective coat Evolution of pollen A way to transfer sperm that lack a flagella without a need for water (microspores develop into the male gametophyte pollen grain, which contains the male gamete sperm) 10 “sporophytes make spores make gametophytes make gametes” Seed Plants • Seed plants are the most plentiful plants in the biosphere. – Seed coat and stored food allow an embryo to survive harsh conditions during long period of dormancy. – Heterosporous • Drought-resistant pollen grains. • Ovule develops into seed. • Ovary becomes the fruit. GYMNOSPERMS ~largest division is the conifers Male cone Female cone ~ “naked seeds” because they lack ovaries (have ovules) ~modified leaves = needles An ovule contains a megaspore which produces a female gametophyte (called an embryo sac) that produces female gametes (eggs) Conifers • Conifers, as well as other gymnosperm phyla, bear cones. – Tough, needlelike leaves of pines conserve water with a thick cuticle and recessed stomata. • Confiers • Cycads • Ginkgoes • Gnetophytes Pine Life Cycle • Considered a “soft” wood because it consists primarily of xylem tissue. Pine Life Cycle 11 Figure 30.8bx Sequoias Conifers Figure 30.8c Phylum Coniferophyta: Cypress Figure 30.8x2 Frasier fir Figure 30.10 A closer look at pine cones (Pinus sp.) Figure 30.3 Winged seed of a White Pine (Pinus strobus) 12 Figure 30.10x3 Pine embryo Cycads • Cycads have large, finely divided leaves that grow in clusters at the top of the stem. – Pollen and seed cones on separate plants. • Pollinated by insects. Figure 30.5c Phylum Ginkgophyta: Ginkgo biloba Ginkgoes • Ginkgoes are dioecious, with some trees producing seeds and others producing pollen. Thus only the “female” trees produce fruit. – One surviving species. (Gingko biloba) Angiosperms • Angiosperms are an exceptionally large and successful group of plants – Ovules are always enclosed within diploid tissues. Monocots and Eudicots • Two classes of flowering plants. – Monocotyledones (Monocots) • One cotyledon in seed. • Examples: corn, grasses, lilies – Eudicotyledones (Dicots) • Two cotyledons in seed. • Examples: beans, oak tree, roses • Cotyledons (seed leaf)- an embryonic organ that stores and digests reserve materials 13 Figure 35.1 A comparison Mof monocots and dicots Monocots versus Eudicots • One cotyledon • Flower parts in 3s or multiples of 3 • Usually herbaceous • Usually parallel veins • Scattered vascular bundles in stems • Fibrous root system • Two cotyledons • Flower parts in 4s or 5s or multiples of 4 or 5 • Woody or herbaceous • Usually net venation • Vascular bundles in a ring in stems • Taproot system 14