Chapter 29 - Sadava The Evolution of Seed Plants
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Transcript of Chapter 29 - Sadava The Evolution of Seed Plants
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Chapter 29 - SadavaThe Evolution of Seed Plants
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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
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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
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Surviving seed plants fall into two groups:– Gymnosperms: conifers and cycads– Angiosperms: flowering plants
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Gametophyte generation is reduced even further than it is in ferns– Haploid gametophyte develops partly or
entirely attached to sporophyte
Nonvascular – Mosses
Vascular, seedless – Ferns
Vascular, seed plants
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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/class/bio1151/Locked/media/ch30/30_03OvuleToSeedA.jpg
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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
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Seed Plants - MicrosporangiumIn 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
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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
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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
http://nosheteria.com/uploaded_images/PumpkinSeeds.jpg
Pumpkin seeds
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SeedSeed 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
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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.
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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
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EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms
– Gymnosperm• “naked-seeded” • ovules and seeds are not protected by
ovary or fruit tissue
http://www.conifers.org/pi/pin/lambertiana5.jpg
Pinus longaeva, Bristlecone pine, cone and needles
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Four major groups of living gymnosperms:
• Cycads: Cycadophyta
http://www.biologyreference.com/images/biol_02_img0213.jpg
EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms
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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
EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms
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Four major groups of living gymnosperms:
• Gnetophytes: Gnetophyta
Ephedra viridis (Mormon tea or joint fir) with cones; Arizona
EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms
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Four major groups of living gymnosperms:
• Conifers: Coniferophyta— the cone bearers
http://www.nasa.gov/centers/goddard/images/content/156030main_Conifers_JPG.jpg
EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsGymnosperms
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• All living gymnosperms except gnetophytes have only tracheids for water conduction and support• Gymnosperms were
dominant during Mesozoic • Tallest gymnosperms are
California coastal redwoods
• over 100 m
• Angiosperms have vessel elements and fibers alongside of tracheids
Tracheids and fiber tracheids
http://www.uri.edu/cels/bio/plant_anatomy/images/47.gif
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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-poi-ancient-bristlecone-pine-forest-af.jpg
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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
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Pine life cycle:– Wind carries pollen grains from
microstrobilus to megastrobilus
Gymnosperms
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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
www.botany.hawaii.edu/faculty/webb/BOT201/Conifers
Pollen grains. Note the swollen bladders which helps them float in air currents.
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Most conifer ovules are born on the upper surfaces of the cone scalesStone pine cone with pine nuts - note two nuts (seeds) that develop from the ovule under each cone scale
Gymnosperms
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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
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EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsAngiosperms
• 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
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Extreme of evolutionary trends in vascular plants:– Sporophyte generation becomes larger,
gametophyte become smaller– Female gametophyte even more reduced
— usually only seven cells
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• 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
http://www.gallerym.com/images/work/big/schatz_howard_Botanica-%20Orchid%20Phala%20001_L.jpg
Phalaenopsis Orchid
Angiosperms
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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
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Angiosperm (“enclosed seed”)– Ovules and seeds are
enclosed in a modified leaf called a carpel• Carpels provide
protection• May interact with pollen
to prevent self-pollination
– Angiosperms also produce flowers and fruits
Angiosperms
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– Fiber cells: important in supporting plant body (associated with xylem)
http://student.nu.ac.th/u46410387/fiber.jpeg
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Flowers– Stamens bear
microsporangia• Male
• consist of filament and anther
Angiosperms
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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
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• 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
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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
Angiosperms – Perfect and Imperfect Flowers
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– 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
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• 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
http://www.dkimages.com/discover/previews/931/50375608.JPG
Mimulus aurantiacus, bush monkeyflower
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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
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Figure 29.14 The Life Cycle of an Angiosperm
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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
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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
Angiosperms - Fruit
http://www.youtube.com/watch?v=bwCpQflmQG4
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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
www.botany.hawaii.edu/faculty/webb/Bot201/Angiosperm/MagnoliophytaLab99
cotyledons
embryonic axis
Angiosperms
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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
EmbryophytesVascular PlantsEuphyllophytes (“True Leaves”)Seed PlantsAngiosperms
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Monocots and Eudicots Are Not the Only Surviving Angiosperms
Water lilies
Star anise
Avocado
Magnoliid clade
Black pepper Dutchmean’s pipe
Amborella trichopoda
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Date palm
Lilies
Wheat
Monocots
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Prickly pear cactus
Dogwood
Catclaw Brier
Eudicots
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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.
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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://scienceafrica.com/cms/wp-content/uploads/2010/06/cassava-ethanol.jpg
http://3.bp.blogspot.com/_vhhw--dmXBE/SEKkOrSMjVI/AAAAAAAAAtw/E-3uz7yigtU/s400/
biopact_cassava_biofuels.jpg
Cassava root is an important
food in Africa
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– 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
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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
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