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Chapter 36: Plants Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-1 Origin of plants • Plants colonised land c. 410 million years ago – earliest plants were small – confined to wet margins of wetlands and rivers • • Plants arose from green algae (phylum Chlorophyta) Plants and chlorophytes both have – – – – chlorophylls a and b similar chloroplast structure cellulose in cell walls starch as storage material (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-2 Origin of plants (cont.) • • Closest relatives of plants are Charophytes (order Charales) and Coleochaete Charophytes have characters in common with plants that are not found in other algae – during cell division nucleus is not enclosed in nuclear envelope – cross-wall forms in phragmoplast, a structure containing remnants of mitotic spindle fibres at right angle to new cross-wall (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-3 Origin of plants (cont.) • • • Charophytes, Coleochaete and plants all possess similar asymmetric, motile flagellated cells Coleochaete and plants both possess reproductive structures enclosed in protected cells Charophytes and land plants both possess glycolate oxidase and share similarities in 5 S ribosomal RNA Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-4 Adaptations to land • Transition from water to land required structural adaptations to – – – – maintain water balance extract water and nutrients from soil transport water and nutrients around plant support plant Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-5 Features of plants • Cuticle – waterproof layer of insoluble polymers and waxes that covers above-ground parts of plants • Sporopollenin – tough polymer of carotenoids protecting spores and pollen • Stomata – pores on surface of plant leaves and stems allowing gas exchange (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-6 Features of plants (cont.) • Vascular supply and lignin – transport system allowing movement of water and nutrients (xylem) and manufactured carbohydrates (phloem) – lignin in vessel walls prevents collapse • Stems, roots and leaves – division of labour Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-7 Reproduction • • Plant life cycles involve alternating sexual and asexual generations Haploid gametophyte generation produces male and female gametes by mitosis – gametes protected in reproductive organs (male antheridia, female archegonia) • Resulting zygote grows into diploid sporophyte generation, which produces haploid spores by meiosis – zygote retained in gametophyte Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-8 Heterospory • • Some ferns and clubmosses and all seed plants produce two types of spores in sporophyte Megaspores in megasporangia – form female gametophytes producing egg cells in archegonia • Microspores in microsporangia – form male gametophytes producing sperm cells in antheridia • Heterospory allowed seed plants to develop easilytransported pollen Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-9 Non-vascular plants • Phylum Hepatophyta (liverworts) • Phylum Anthocerophyta (hornworts) • Phylum Bryophyta (mosses) • Vascular system absent • Lignin absent • Gametophyte generation dominant Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-10 Gametophytes • • Dominant generation in non-vascular plants Gametophytes of non-vascular plants are leafy (mosses) or thalloid (liverworts, hornworts) – attached to substrate by rhizoids • Egg and sperm produced in archegonia and antheridia respectively • Flagellated sperm swim to archegonia and fertilised egg – sperm require water (rain, dew) for locomotion Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-11 Sporophytes • • After fertilisation, sporophyte develops on gametophyte Sporangium (spore capsule) on stalk (seta) – derives nutrition from gametophyte • When spores are ready to be shed, spore capsule open, exposing spores – hygroscopic elaters in liverworts deform to flick spores from capsule – moss peristome changes shape, releasing spores Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-12 Fig. 36.9: Life cycle of a moss (phylum Bryophyta) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-13 Spore-producing vascular plants • Phylum Lycophyta (clubmosses, quillworts) • Phylum Psilophyta (Psilotum, Tmesipteris) • Phylum Sphenophyta (Equisetum) • Phylum Filicophyta (ferns) • Vascular system (xylem and phloem) present • Lignin present in xylem • Sporophyte generation dominant Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-14 Lycophytes • Lycopodium, Selaginella, Isoetes – fossil lycophytes formed extensive forests, but modern species are small • • Sporophyte generation dominant Selaginella and Isoetes have heterosporous life cycle – mega- and microgametophytes develop within spore – megagametophyte develops into archegonia with eggs – microgametophyte develops into single antheridium with flagellated sperm – exposed when spore wall splits Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-15 Fig. 36.19: Heterosporous life cycle of Selaginella (phylum Lycophyta) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-16 Filicophytes • • • Diverse plants with terrestrial, epiphytic and aquatic species Fern leaves (fronds) are characteristically divided into pinnae or pinnules Sporophyte generation dominant – sporangia are clustered in sori and may be covered by a protected membrane (indusium) – small gametophyte bears archegonia and antheridia – water is required for fertilisation Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-17 Fig. 36.23: Life cycle of homosporous fern Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-18 Seed plants • Gymnosperms • Phylum Coniferophyta (conifers) • Phylum Cycadophyta (cycads) • Phylum Ginkgophyta (ginkgos) • Phylum Gnetophyta (gnetophytes) • Angiosperms • Phylum Magnoliophyta (flowering plants) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-19 Ovules • Megagametophyte develops within megasporangium – megasporangium protected by cellular casings or integuments – ovule is megagametophyte + megasporangium + integuments • Megasporangium produces four megaspores by meiosis – three degenerate, leaving one to grow into female gametophyte Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-20 Seeds • Plant embryos are surrounded by nutritive tissue • Enclosed in a protective case (seed case or testa) • Embryo remains dormant until it is shed from parent plant and is dispersed to suitable habitat Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-21 Pollen • Microspores develop into microgametophytes, which produce sperm • Microgametophytes of seed plants develop as pollen, which can be transported by wind, water or animals • Enclosing sperm in pollen means that seed plants are not dependent on water as a medium allowing sperm access to eggs Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-22 Secondary growth • Vascular cambium produces woody tissue – large quantities of secondary xylem (wood) are produced in the stem and roots adding girth – smaller quantities of secondary phloem (bark) are added to the outside of the plant • • Cambium generates new phloem and xylem Old vessels are filled with waste products Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-23 Cycads • • • • Ancient seed plants with a fossil record extending back over 250 million years Unbranched or sparsely branching trunk with pinnate leaves Specialised coralloid roots housing nitrogen-fixing bacteria Male and female plants produce cones bearing sporangia and ovules respectively Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-24 Conifers • • • Conifers are the most diverse group of nonflowering seed plants Branching trunk composed mostly of tracheids (hence ‘softwood’) Pollen and ovules are in separate cones – pollen is dispersed by wind – produces a pollen tube that conducts non-flagellated sperm to archegonia Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-25 Magnoliophytes Flowering plants (angiosperms) are divided into two groups: • Monocotyledons (class Liliopsida) – – – – • seeds have one cotyledon flower parts in threes or multiples leaves with parallel veins vascular bundles in ground tissue in stem Dicotyledons (class Magnoliopsida) – – – – seeds have one cotyledon flower parts in fours or fives or multiples leaves with netted veins vascular bundles in a ring in stem Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-26 Flower structure • Flowers consist of four whorls of elements – calyx of leaf-like sepals that protect flower – corolla of petals – stamens (each stamen consists of an anther and filament) – carpels (each carpel consists of a basal ovary containing ovules, style and terminal stigma) • • The superior ovaries of hypogynous flowers are attached to or above the receptacle The inferior ovaries of epigynous flowers are within the receptacle Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-27 Origin of flowers • Classical theory – flowers as modified leaves – stamens and carpels specialised leaf-like appendages (sporophylls) being sporangia • Genetic evidence supports sepals as modified leaves • Some petals may be modified leaves, others sterile stamens • Stamens may also represent a reduced branching system with terminal microsporangia rather than leaves Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-28 Evolution of flowers • Carpels are the most distinctive reproductive structures of angiosperms • Enclosure of ovules by carpels resulted in modification of reproduction • Carpels provide protection and formed specialised fruits to assist in seed dispersal • Pollen must grow a pollen tube through the stigma to deliver sperm to the embryo sac – creates possibility of selection by female choice (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-29 Evolution of flowers (cont.) • Flowers of magnolia family possess primitive characteristics – bisexual, numerous stamens and carpels, pollinated by beetles • • Variety of other flower forms contemporaneous with early magnolias DNA evidence suggests that early flowers were small, with few parts in threes (trimerous flowers) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-30 Reproduction in flowering plants • Pollen lands on stigma and develops into the microgametophyte – two non-flagellated sperm cells and one cell associated with pollen tube growth • In ovule, functional megaspore develops into megagametophyte – embryo sac of eight nuclei in seven cells – one cell has two polar nuclei • Double fertilisation – one sperm fuses with egg to form zygote – other fuses with two polar nuclei to form triploid endosperm nucleus, which then gives rise to nutritive endosperm Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-31 Fig. 36.40: Reproductive cycle of flowering plants (angiosperms) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-32 Pollinators • Plants produce flowers to attract pollinating animals and maximise the chances of successful pollination – most animal-pollinated flowering plants produce showy clusters of flowers (inflorescence) – scent attracts pollinators (some night-flowering species have strong scents) – red coloration attracts birds, yellow and blue flowers attract bees some patterns reflect UV light – tubular shape of some flowers ensures that animals have to brush against anthers to get nectar – nectar provides a high-energy lure Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-33 Pollination by deceit • Some species of orchid mimic the shape and colouring of female insects – a few species also mimic female pheromones • Male insects attempting to mate with the flowers pick up pollen • Pollen is brushed off onto stigma of the next flower visited Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-34 Fruits • • Fruits protect seeds and aid dispersal by fruiteating animals Simple fruits – derived from ovary of a single flower with one or more carpels (grape, apple, peas) • Aggregate fruits – derived from clusters of carpels in a single flower (raspberries, blackberries) • Multiple fruits – derived from clusters of carpels from several flowers on an inflorescence (pineapples) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 36-35