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29 Chapter 29 - Sadava The Evolution of Seed Plants Late in Devonian, some plants developed secondary growth – Thickened woody stems of xylem – The first species with secondary growth were progymnosperms seedless vascular plants, now extinct http://www.futura-sciences.com/uploads/tx_oxcsfutura/comprendre/d/images/600/fig_1213.jpg Earliest seed plants from Devonian – These seed ferns were also woody – Clades of seed ferns are known only as fossils – Two of the clades are basal to surviving seed plants http://www.palaeos.com/Plants/Lists/Glossary/Images/SeedFern.jpg Surviving seed plants fall into two groups: – Gymnosperms: conifers and cycads – Angiosperms: flowering plants Gametophyte generation is reduced even further than it is in ferns – Haploid gametophyte develops partly or entirely attached to sporophyte Vascular, Nonvascular seedless – Ferns – Mosses Vascular, seed plants Seed plants are heterosporous – Produce two types of spores • One becomes female gametophyte, one becomes male gametophyte • Seed plants form separate megasporangia (female) and microsporangia (male) http://bio1151.nicerweb.com/doc/c lass/bio1151/Locked/media/ch30/ 30_03OvuleToSeedA.jpg The Seed Plants - Megasporangium • Megasporangium • Contains the megagametophyte • develops into an egg that is eventually fertilized next generation (sporophyte) • Surrounded by integument made up of sporophyte structures • Megasporangium and integument together form ovule • Will become seed after fertilization http://herbarium.usu.edu/teaching/4420/seed.GIF Seed Plants - Microsporangium In microsporangium, microspores divide mitotically to produce the male gametophyte, or pollen grain – Walls of pollen grains contain sporopollenin, the most resistant biological compound known protects pollen grain from chemicals and dehydration Pollination – When pollen grain lands near a female gametophyte – Pollen tubes are produced that digests way through sporophyte tissue to megagametophyte – Sperm are released from the tube, and fertilization results in a diploid zygote. http://pix.botany.org/Setabot/abot-90-5_700.jpg After Pollination Resulting diploid zygote divides to produce an embryonic sporophyte – Growth is then suspended – Embryo enters a dormant stage, with the end product being a multicellular SEED Pumpkin seeds http://nosheteria.com/uploaded_images/PumpkinSeeds.jpg Seed Seed contain tissues from three generations • Seed coat (from integument) and megasporangium develop from ovule tissues of diploid sporophyte parent • 2n outside • Within megasporangium is haploid female gametophyte tissue, contains nutrients for next generation • 1n • Third generation, the embryo, is the new diploid sporophyte is contained in the center of seed package. • 2n inside – new generation Seeds • Seeds are well-protected resting stages. • May remain viable for many years, germinating when conditions are favorable. • Seed coat protects from drying out as well as predators. • Many seeds have adaptations for dispersal. Besides seeds, secondary growth also contributes to success of seed plants – Wood: proliferated xylem, gives support and allows plants to grow above their competitors for sunlight http://cache.eb.com/eb/image?id=72251&rendTypeId=35 http://mayhem-chaos.net/photoblog/images/pine_tree_med.jpg Embryophytes Vascular Plants Euphyllophytes (“True Leaves”) Seed Plants Gymnosperms – Gymnosperm • “naked-seeded” • ovules and seeds are not protected by ovary or fruit tissue Pinus longaeva, Bristlecone pine, cone and needles http://www.conifers.org/pi/pin/lambertiana5.jpg Embryophytes Vascular Plants Euphyllophytes (“True Leaves”) Seed Plants Gymnosperms Four major groups of living gymnosperms: • Cycads: Cycadophyta http://www.biologyreference.com/images/biol_02_img0213.jpg Embryophytes Vascular Plants Euphyllophytes (“True Leaves”) Seed Plants Gymnosperms Four major groups of living gymnosperms: • Ginkgos: Ginkgophyta—one living species, Ginkgo biloba Branch of male Ginkgo biloba with clusters of pollen-producing microsporophylls Branch of female Ginkgo biloba with leaves and ovules Embryophytes Vascular Plants Euphyllophytes (“True Leaves”) Seed Plants Gymnosperms Four major groups of living gymnosperms: • Gnetophytes: Gnetophyta Ephedra viridis (Mormon tea or joint fir) with cones; Arizona Embryophytes Vascular Plants Euphyllophytes (“True Leaves”) Seed Plants Gymnosperms Four major groups of living gymnosperms: • Conifers: Coniferophyta— the cone bearers http://www.nasa.gov/centers/goddard/images/content/156030main_Conifers_JPG.jpg • All living gymnosperms except gnetophytes have only tracheids for water conduction and support Tracheids and fiber tracheids • Gymnosperms were dominant during Mesozoic • Tallest gymnosperms are California coastal redwoods • over 100 m • Angiosperms have vessel elements and fibers alongside of tracheids http://www.uri.edu/cels/bio/plant_anatomy/images/47.gif Gymnosperms were dominant during the Mesozoic, until about 65 Mya. – Today, conifers still dominate many forests, especially at high latitudes and altitudes. – The oldest living organism on Earth is a bristlecone pine that germinated about 4,800 years ago. http://media.rd.com/rd/images/rdc/family-travel/poi/CA-poiancient-bristlecone-pine-forest-af.jpg Gymnosperms Male and female cones contain the reproductive structures of conifers. – Megastrobilus (female, seed-bearing Cone) • Modified stem, bearing a tight cluster of woody scales (reduced branches) • Produces seeds – Microstrobilus (male, pollen-bearing cone) • “Cone-like” structure • Scales are modified leaves; not woody • Microsporangia produces microspores pollen grains (microgametophyte) Gymnosperms Pine life cycle: – Wind carries pollen grains from microstrobilus to megastrobilus The Gymnosperms: Naked Seeds • Pollen grains enter ovule through a small opening in the integument at tip of the ovule called the micropyle • 2 sperm enter • 1 fertilizes • 1 degenerates Pollen grains. Note the swollen bladders which helps them float in air currents. www.botany.hawaii.edu/faculty/webb/BOT201/Conifers Gymnosperms Most conifer ovules are born on the upper surfaces of the cone scales Stone pine cone with pine nuts - note two nuts (seeds) that develop from the ovule under each cone scale Gymnosperms • Some pine cone scales (e.g., longleaf and slash pines) can only be opened by fire to release the seeds • helps insure new growth after fires. • Some conifers have soft, fleshy, fruit-like tissue around seeds (e.g., juniper and yew “berries”) • Animals may eat these and then disperse the seeds in their feces. • Not true fruits Embryophytes Vascular Plants Euphyllophytes (“True Leaves”) Seed Plants Angiosperms • Angiosperms: • “enclosed seed” • Oldest fossils are Jurassic, 150 my old • Explosive species radiation angiosperms became dominant during Tertiary in only 60 million years • Over 250,000 species exist today http://gpc.edu/~pgore/myphotos/fossils/angiosperm-leaf.jpg Extreme of evolutionary trends in vascular plants: – Sporophyte generation becomes larger, gametophyte become smaller – Female gametophyte even more reduced — usually only seven cells Angiosperms • Synapomorphies (shared, derived traits) in angiosperms: • Double fertilization • Triploid endosperm – nutritive tissue in seeds • Ovules and seeds enclosed in a carpel • Flowers • Fruits • Xylem with vessel elements and fibers • Phloem with companion cells Phalaenopsis Orchid http://www.gallerym.com/images/work/big/schatz_howard_Botanica-%20Orchid%20Phala%20001_L.jpg Angiosperms Double fertilization: – Microgametophyte has two male gametes • One nucleus combines with egg • Second nucleus combines with two haploid nuclei of female gametophyte to form triploid nucleus endosperm (3n) –Endosperm nourishes developing sporophyte (embryo) http://www.lclark.edu/~seavey/images%20/carpel.jpg Angiosperms Angiosperm (“enclosed seed”) – Ovules and seeds are enclosed in a modified leaf called a carpel • Carpels provide protection • May interact with pollen to prevent selfpollination – Angiosperms also produce flowers and fruits – Fiber cells: important in supporting plant body (associated with xylem) http://student.nu.ac.th/u46410387/fiber.jpeg Angiosperms Flowers – Stamens bear microsporangia • Male • consist of filament and anther Angiosperms Flowers – Carpels bear megasporangia • Female • One or more carpels form the pistil — stigma, style, ovary and ovule In this example, the pistil is a single carpel Angiosperms • Flowers often have other specialized leaves that often play a role in attracting pollinators • Petals – inner whorl (collectively, the corolla) • Sepals – outer whorl (collectively, the calyx) • Calyx protects immature flower in bud before it opens Angiosperms – Perfect and Imperfect Flowers Types of flowers: – Perfect flowers • have both mega- and microsporangia – Imperfect flowers • either mega or microsporangia, but not both http://leon.ifas.ufl.edu/images – Monoecious: “one-housed” • male and female flowers occur on the same plant or in perfect flowers – Dioecious: “two-housed” • male and female flowers on different plants • separate sexes • Have to have imperfect flowers • Perfect flowers: • favors self-pollination, but usually disadvantageous • many mechanisms have evolved to circumvent this problem • bush monkeyflower is constructed so that two different hummingbirds must participate in pollination Mimulus aurantiacus, bush monkeyflower http://www.dkimages.com/discover/previews/931/50375608.JPG Most angiosperms are animal-pollinated by insects, birds, and bats – Many flowers entice pollinators with nectar and pollen – Plants and their pollinators have coevolved – Some relationships are very specific—e.g., one species of moth pollinates one species of yucca plant. http://bobklips.com/BOBS_WEBSITE/ILF-YUCCFILAlaying.jpg Figure 29.14 The Life Cycle of an Angiosperm Angiosperms - Fruit Ovary and seeds develop into fruits – Fruit protects seed and aids in dispersal, (e.g., can become attached to or eaten by animals) http://www.hickerphoto.com/data/media/152/kiwi-fruit_3629.jpg Angiosperms - Fruit Types of fruits: – Simple fruits develop from one carpel • Plums, cherries – Aggregate fruits develop from several carpels on same flower • raspberries – Multiple fruits form from a cluster of flowers • pineapples – Accessory fruits develop from parts other than carpels • Apples, pears, strawberries http://www.youtube.com/watch?v=bwCpQflmQG4 Angiosperms Angiosperm life cycle: – Zygote develops into an embryo • consists of embryonic axis (will become stem and root), and 1 or 2 cotyledons — seed leaves • Cotyledons absorb and digest endosperm, some become photosynthetic cotyledons www.botany.hawaii.edu/faculty/webb/Bot201/Angiosperm/MagnoliophytaLab99 Embryophytes Vascular Plants Euphyllophytes (“True Leaves”) Seed Plants Angiosperms Most angiosperms are in two major clades: – Monocots • One cotyledon • Grasses, cattails, orchids, palms – Eudicots (dicots) • Two cotyledons • Majority of familiar flowering plants, most herbs, trees, shrubs – Other clades include star anise and relatives, water lilies, and magnoliids Monocots and Eudicots Are Not the Only Surviving Angiosperms Water lilies Amborella trichopoda Black pepper Star anise Dutchmean’s pipe Avocado Magnoliid clade Monocots Lilies Wheat Date palm Eudicots Prickly pear cactus Catclaw Brier Dogwood How Do Plants Support Our World? Plants contribute to ecosystem services: processes by which environment maintains resources that benefit humans – Plants are primary producers: photosynthesis traps energy and carbon, making them available for their own needs and for herbivores and omnivores that consume them, and for the whole food chain – Plants produce O2 and remove CO2 from the atmosphere – Contribute to soil formation and soil fertility – Roots hold soil in place, preventing erosion – Moderate local climate by increasing humidity, providing shade, and blocking wind. Seed plants are our primary food source – Twelve are most important: rice, coconut, wheat, corn, potato, sweet potato, cassava, sugarcane, sugar beet, soybean, common bean, banana http://3.bp.blogspot.com/_vhhw-dmXBE/SEKkOrSMjVI/AAAAAAAAAtw/E3uz7yigtU/s400/biopact_cassava_biofuels.jpg Cassava root is an important food in Africa http://scienceafrica.com/cms/wp-content/uploads/2010/06/cassava-ethanol.jpg – Half the world’s population gets most of its food energy from rice (Oryza sativa) • Rice been cultivated for more than 8,000 years http://www.equita.qc.ca/images/produits/gn_008.jpg Many medicines come from seed plants – Medicines are found by screening large numbers of plants, or screening large numbers of chemical compounds http://www.mountainvalleycenter.com/store/images/cherheal.JPG