Ch. 21 Notes
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Date Created: 03/20/16
CH. 21 The Evolution of Plants 21.1: Primary Endosymbiosis Produced the First Photosynthetic Eukaryotes Plantae: group w/ synapomorphy of primary endosymbiosis Ancestor algae of plantae was unicellular, most related to glaucophytes - Thought to be sister group to the rest of plantae - Chloroplasts retain some peptidoglycan between membranes (as in cyanobacteria); peptidoglycan has been lost from remaining photosynthetic eukaryotes Green Algae: represents most aquatic photosynthetic eukaryotic groups in Plantae - Like land plants, they contain chlorophyll a and b & store energy as starch in chloroplasts - Clades: Chlorophytes, Coleochaetophytes, Stoneworts Streptophytes: all other green plants; chlorophytes are the sister group of them Embryophytes: embryo is protected by tissues of parent plant; primary synapamorphy of land plants - Provides water to prevent desiccation Land plants that exist today fall into ten major clades: Vascular Plants (tracheophytes): developed vascular systems w/ fluid-conducting cells, tracheids - 7 clades: Seedless vs. seeded plants Nonvascular Land Plants: lack tracheids, some have conduction cells - 3 clades: Liverworts, hornworts, mosses 21.2: Key Adaptations Permitted Plants to Colonize Land Necessary Changes in Land Plants: - Transport systems for water/nutrients - Structural support - New ways to disperse gametes/progeny - Adapt to dry conditions (to prevent desiccation) Adaptations in Land Plants: - Cuticle: waxy coating slows water loss - Stomata: closable openings that regulate water/gas exchange - Gametangia: organs that enclose gametes & prevent them from drying out - Embyros: protective structure surrounding young plants - Pigments that protect against UV radiation - Thick spore walls: contains polymer to resist decay - Mutually beneficial associations w/ fungi (mycorrhizae): promotes nutrient uptake from soil Alternation of Generations: universal feature of life cycles of land plants - Includes multicellular diploid stage & multicellular haploid stage - Gametes are produced by mitosis; spores by meiosis - Spores à multicellular haploid organisms 1. Diploid zygote undergoes mitosis/cytokinesis à multicellular embryo à Diploid Sporophyte 2. Sporangia cells (of sporophyte) undergo meiosis à Unicellular haploid spores 3. Haploid spores undergo mitosis à Haploid Gametophyte undergoes mitosis à Diploid Zygote In the life cycle of land plants, a multicellular diploid sporophyte produces spores by meiosis, and a multicellular haploid gametophyte produces gametes by mitosis Nonvascular Plants: (liverworts, mosses, hornworts) - Live in moist habitats, have thin cuticles (water environment eliminates problem of desiccation) - Mostly small; no vascular system to transport water à restricted size - Small enough that minerals can be distributed throughout bodies via diffusion/capillary axn - Haploid gametophyte is dominant over diploid sporophyte In nonvascular plants, gametophyte is photosynthetic (nutritionally independent) - Sporophyte is nutritionally dependent on gametophyte & remains attached to it - Gametes are produced in gametangia (specialized sex organs both on individual; antheridia- male & archegonia-female) - When released from antheridium, sperm must swim or be splashed with water to reach egg in archegonium on same or nearby plant; water must be present for reproduction Liverworts: - Green leaf-like gametophytes - No stomata - Sporophyte remains attached to larger gametophyte - Most can reproduce both sexually/asexually Mosses: - Contain stomata (gas exchange/water retention) - Hydroids: cells that die leaving channel through which water can move (similar to tracheids) Hornworts: (look like horns) - Cells contain single large, plate-like chloroplast (others contain small chloroplasts) - Sporophytes lack a stalk enabling it to grow without a limit (others have stalk that stops growing when spore-producing structure matures), but is eventually limited by lack of transport system - Symbiotic relationship promotes growth by giving them access to nitrogen 21.3: Vascular Tissues Led to Rapid Diversification of Land Plants Vascular System - Xylem: conducts water/minerals from soil throughout plant Tracheid: principal water-conducting element in all vascular plants except angiosperms; provide path for water/nutrients & structural support - Phloem: conducts products of photosynthesis from production sites to use/storage sites - Branching sporophyte: produces more spores than unbranched; nutritionally independent of gametophyte when matured - Phloem = Food, Xylem = Water Rhyniophytes: extinct group that is the earliest known vascular plants - Rhizomes: horizontal stems that anchored them in soil due to lack of roots - Rhizoids: water-absorbing unicellular filaments on rhizomes - Contain aerial branches & sporangia at tips of branches - Dichotomous branching pattern: tip of shoot divided into two equivalent new branches, each diverging at same angle from original stem Lycophytes: club mosses and relatives; sister group to remaining vascular plants - Contain true roots/stems that branch dichotomously; in stems it occurs by division of apical cluster of dividing cells - Simpler arrangement of vascular tissue in stems than other vascular plants - Microphylls: simple leaf-like structures arranged spirally on stem - Apical growth - Strobili: cone-like structure of aggregated sporangia; cluster of spore-bearing leaves inserted on an axis - If lacking strobili, sporangia are located on upper surfaces of specialized leaves Monilophytes: clade containing horsetails and ferns; sister group to seed plants - Differentiation between main stem & side branches; contrasts w/ dichotomous branching Horsetails: - contain reduced true leaves that form circles around stem - silica deposits in cell walls - irregularly branching in roots - large sporophyte & small gametophyte independent of each other Ferns: - Most are terrestrial; some live in shallow water - Terrestrial ferns have large leaves w/ branching vascular strants - Some have climbing organs - Alternation of generations: gametophyte is small/short-lived, sporophyte is larger/lives long - Require liquid for transport of male gametes to females - Sori: sporangia located on stalk in clusters on underside of leaves Euphyllophytes: clade containing monilophytes/seed plants - Overtopping: synapomorphy; growth pattern in which one branch grows beyond others - Overtopping à Megaphyll: larger/more complex leaf - Followed by development of photosynthetic tissue between members of overtopped groups of branches Homosporous: Single spore develops into gametophyte which produces both sperm & egg Heterosporous: System w/ two separate types of spores that develops in diff ways - Megaspore: develops into female gametophyte (produces eggs) o Megasporangia: sporophyte produces small amounts of megaspores - Microspore: develops into male gametophyte (produces sperm) o Microsporangia: sporophyte produces large amounts of microspores 21.4: Seeds Protect Plant Embryos Seed Plants: (gymnosperms/angiosperms) - provide secure, lasting dormant stage for embryo (seed) - Seeds can be dormant for many years/centuries - Gametophyte generation is reduced further than in ferns - Haploid gametophyte develops while attached to/nutritionally dependent on diploid sporophyte - Seed plants evolved to be able to fertilize without water, allowing colonization in drier habitats - Heterosporous; one spore becomes microgametophyte (male) and other becomes megagametophyte (female); form separate microsporangia/megasporangia on structures grouped on short stems Pollen Grain: multicellular male gametophyte formed when microspore divides mitotically in spore wall; released from microsporangium to be distributed by wind/animals - Wall around pollen grain contains sporopollenin, protects grain against drying/chemical damage - Megaspores are not shed; develop into female gametophytes (megagametophytes) within megasporangia which are dependent on sporophyte for nutrition - Only one meitotic product in megasporangium survives to be divided mitotically eventually creating multicellular female gametophyte - Megasporangium surrounded by sterile sporophytic structures à integument: protects megasporangium/contents Ovule: composed of megasporangium and integument; develops into seed after fertilization Pollen Tube: pollen grain develops further and elongates/digests its way toward megagametophyte à sperm are released for fertilization à Diploid zygote divides à Embryonic sporophyte à Multicellular seed Pollination: transfer of pollen grain from male anther to female stigma (doesn’t mean fertilization) - Sexual reproduction of plants Seed: contains tissues from three generations - Three Generations: Seed coat develops from tissues of diploid sporophyte parent (integument) for protection Haploid tissue from female gametophyte provides nutrients for developing embryo Embryo is the new diploid sporophyte generation - Seeds remain viable for long periods - Many seeds have adaptations to aid in dispersal - When embryo begins to grow it draws on nutrients stored in seed Secondary Growth: increases diameter of stems/roots by growth of xylem (forms wood) - Younger portion produced by secondary growth is adapted for water transport - Older wood gets clogged with resins and provides support, allowing great heights of plants - Many plant groups have lose woody growth habit Gymnosperms: seed plants that don’t form flowers/fruits - Ovules/seeds aren’t protected by ovary/fruit tissue - Contain only modified tracheids as water-conducting/support cells within xylem - Four groups: Cycads: tropical, earliest diverging clade Ginkgos: common in Mesozoic Gnetophytes: characteristics similar to angiosperms Conifers: cone-bearing plants Cones: contain reproductive structures Megastrobilus: female cone, seeds protected by woody scales (interior) Microstrobilus: male pollen-bearing cone (tips) à microspores à pollen grain 21.5: Flowers and Fruits Increase the Reproductive Success of Angiosperms Angiosperms: reproductive organs are in flowers; seeds enclosed in fruits - Carpel: modified leaf that contains ovules/seeds (pistil) - Female gametophyte more reduced than gymnosperms - Sporophyte generation becomes larger/independent of gametophyte while gametophyte does opposite Angiosperm Synapomorphies: - Flowers - Fruits - Ovules/seeds enclosed in carpel - Highly reduced gametophytes - Germination of pollen on stigma - Double fertilization - Endosperm: nutritive tissue for embryo - Phloem w/ companion cells Flower: attracts pollinators w/ colorful petals, scent, nectar, pollen Reproductive Organs of Flower: (all parts are modified leaves) Stamens: male reproductive organs Filament: stalk-like structure that holds anther Anther: produces pollen Carpel/Pistil: female reproductive organs Stigma: receives pollen grains Ovary: swollen base of carpel w/ ovules (will become fruit) Ovules: contains female gametophyte; becomes the seed Style: tube connecting stigma to ovary Non-Reproductive Organs: - Petals: whorl of brightly colored flower organs that attract pollinators - Corolla: whorl of petals in flower - Sepals: whorl of leaf-like organs outside corolla; protect unopened flower bud Calyx: whorl of sepals in a flower Tepals: when sepals & petals resemble each other Pollen Transportation Wind-Pollination: used by gymnosperms and some flowering plants (grasses/trees) - Flowers are small/grouped together making it not very efficient/wasteful - Smooth shaped pollen grains Animal-Pollination: Herbivores than pollinate - Barbed shaped pollen grains - Bees: most important pollinators, guided by smell/sight Butterflies/moths: guided by sight/smell, proboscis used to get nectar Flies: decomposing smell of flowers attracts them to lay their eggs Beetles: attracted to strong odor, feed on pollen not nectar Birds: bad sense of smell, good sense of color, attracted to flowers with sticky pollen Mammals: pollinate flowers at night - Pollinate for reward of food advertised with colors, nectar/honey guides, aromas Nectar: sugary soln produced in special flower glands (nectaries) - Concentration matches energy requirements of pollinator Pollen: high in protein, eaten by bees/beetles - Flowers can produce two kinds of pollen, a normal kind and a sterile tasty kind for insect Inflorescence: group of flowers; characteristic of plant families Strategies to Avoid Self-Pollination: Perfect Flowers: contain both megasporangia & microsporangia (female/male) on same flower Imperfect: two separate flower types (male/female) 1. Timing: male/female structures mature at different times 2. Morphological: structure/location of male/female organs prevents it (imperfect flower) 3. Biochemical: chemical on surface of pollen and stigma/style prevents pollen tube germination on same flower (incompatible) Monoecious: male/female flowers produced on same plant - Different from perfect flower in which male/female parts are on the same flower Dioecious: male/female flowers produced on different plants Double Fertilization: - Each pollen grain contains two male gametes o One male gamete combines w/ egg à diploid zygote o Other combines w/ other haploid nuclei of female gametophyte à triploid cell - Triploid cell à Endosperm: nourishes embryo during early development - Zygote à embryo w/ embryonic axis & 1-2 cotyledons Cotyledons: seed leaves; absorb/digest endosperm or enlarge becoming photosynthetic - Ovule à seed containing diploid embryo & triploid endosperm Monocots: single embryonic cotyledon (one leaf initially) Eudicots: two embryonic cotyledon (two leaves initially) Types of Fruits Simple Fruit: develops from single carpel or fused carpels Aggregate Fruit: develops from several carpels of a single flower Multiple Fruits: formed from inflorescence Accessory Fruits: derived from parts in addition to carpel/seeds
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