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Seed Plants Chapter 31 1 The Evolution of Seed Plants • Seed plants first appeared 305–465 MYA – Evolved from spore-bearing plants known as progymnosperms 2 http://www.devoniantimes.org/who/pages/archaeopteris.html The Evolution of Seed Plants • Success attributed to evolution of seed – Protects and provides food for embryo – Dormancy allows the “clock to be stopped” to survive harsh periods before germinating – Later development of fruits enhanced seed dispersal 3 http://www.sweetacrebirdfarm.com/fruits.htm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stored food Integument (seed coat) Embryo • Seed right: © Biology Media/Photo Researchers, Inc. 300 m – Embryo protected by integument • An extra layer or two of sporophyte tissue • Hardens into seed coat – Megasporangium divides meiotically inside ovule to produce haploid megaspore – Megaspore produces egg that combines with sperm to form zygote – Also contains food supply for embryo 4 Seed plants produce 2 kinds of gametophytes • Male (micro-) gametophytes – Pollen grains – Dispersed by wind or a pollinator – No need for water • Female (mega-) gametophytes – Develop within an ovule – Enclosed within diploid sporophyte tissue in angiosperms 5 Five Phyla of Extant Seed Plants • • • • Coniferophyta Cycadophyta Gnetophyta Ginkophyta • Anthophyta Gymnosperms Angiosperms 6 7 – – – – Coniferophytes Cycadophytes Gnetophytes Ginkgophytes • All lack flowers & fruits of the angiosperms • All have ovule exposed on a scale 8 http://www.gc.maricopa.edu/biology/lsola/bio182/plantselftests/nonflwr/conif105.htm Angiosperms • Plants with “naked seeds” • There are four living groups Gymnosperms Gymnosperms Ferns and Allies Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. http://en.wikipedia.org/wiki/Sequoia_sempervirens Conifers (phylum Coniferophyta) • Most familiar gymnosperm phylum • Pines, spruces, firs, cedars, and others – Coastal redwood – Tallest living vascular plant – Bristlecone pine – Oldest living tree 9 http://www.arhomeandgarden.org/plantoftheweek/articles/Bristlecone_Pine.htm http://borealforestinnamerica.blogspot.com/ Conifers (phylum Coniferophyta) • Found in colder and sometimes drier regions of the world • Conifers are sources of important products – Timber, paper, resin, and taxol (anti-cancer) 10 Boreal forest in Saskatchewan, Canada Taxol was first isolated from the bark of the Pacific yew tree, from paclitaxel produced by endophytic fungi in the bark http://www.projectnoah.org/spottings/17406090 • Pines – More than 100 species, all in the Northern hemisphere – Produce tough needlelike leaves in clusters – Leaves have thick cuticle and recessed stomata to retard water loss – Leaves have canals with resin to deter insect and fungal attacks Florida slash pine (Pinus elliottii var. densa) 11 http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/Secretion/Secretion-2.htm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pollen Microspores Air bladder Pollination Microspore mother cell Pollen tube Scale Sperm Pollen tube Megaspore mother cell Megaspore n 2n Pollenbearing cone Ovulate (seed-bearing) cone (15 months after pollination) Zygote Sporophyte MITOSIS Seedling MITOSIS Pine seed Section of seed (second year), showing embryo embedded in megagametophyte 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cycads (phylum Cycadophyta) • Slow-growing gymnosperms of tropical and subtropical regions • Sporophytes resemble palm trees • Female cones can weigh 45 kg • Have largest sperm cells of all organisms! a. © Luca Invernizzi Tetto/agefotostock Female cone 17 http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/cycadpg?taxname=Encephalartos+manikensis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gnetophytes (phylum Gnetophyta) • Only gymnosperms with vessels in their xylem • Contain three (unusual) genera Welwitschia of Africa b. © Juan Carlos Muñoz/agefotostock – Welwitschia – Ephedra – Gnetum 18 Ephedra viridis (Mormon Tea) of American Southwest http://www.swcoloradowildflowers.com/Yellow%20Enlarged%20Photo%20Pages/ephedra.htm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ginkgophytes (phylum Ginkgophyta) • Only one living species remains – Ginkgo biloba • Flagellated sperm • Dioecious – Male and female reproductive structures form on different trees c. © Robert Gustafson/Visuals Unlimited Flagellated sperm of Ginkgo 19 http://kwanten.home.xs4all.nl/ovule.htm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Angiosperms Ferns and Allies • Flowering plants • Ovules are enclosed/ protected in diploid tissue at the time of pollination • Carpel, a modified leaf that covers seeds, develops into fruit (modified ovary wall) Gymnosperms Angiosperms Cross-section of pea pod (carpel) showing ovule (where seed develops) inside the fruit 20 http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/Angiosperm/placentation.htm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fruits Angiosperm origins are a mystery – Origins as early as 145–208 MYA – Oldest known angiosperm in the fossil record is Archaefructus Recent evidence from fossil pollen pushes angiosperm appearance back 100 million years to ~245 mya, alongside the earliest dinosaurs http://www.livescience.com/40088-flowers-existed-with-dinosaurs.html Paired stamens © David Dilcher and Ge Sun 22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gymnosperms Ginkgo Gnetophytes Conifers Angiosperms Cycads Archaefructus Amborella Water lillies Staranis (extinct) Eudicots Magnoliids Monocots Archaefructus may be the sister clade to all other angiosperms 23 Closest living relative to the original angiosperm is Amborella Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Horizontal gene transfer explains the presence of moss mitochondrial genes in the genome of Amborella – Mosses may have been epiphytes on Amborella ancestor, increasing likelihood of gene transfer Carpel Petal Sepal Courtesy of Sandra Floyd Moss epiphyte growing on leaf of Amborella trichopoda in New Caledonia 24 http://www.pnas.org/content/101/51/17747.figures-only a. Mitochondrial Genes A–C b. Moss species 4 – gene C Amborella – gene C (HGT) Moss species 3 – gene B Moss species 2 – gene B Amborella – gene B (HGT) Moss species 1 – gene A Amborella gene A (HGT) Charales Mosses Ferns Pines Flowering plants Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. http://fruitandnuteducation.ucdavis.edu/generaltopics/AnatomyPollination/VegetativeFloral_Development/ Flowers house the gametophyte generation • Flower morphology – Modified stems bearing modified leaves – Primordium develops into a bud at the end of a stalk called the pedicel – Pedicel expands at the tip to form a receptacle, to which other parts attach – Flower parts are organized in circles called whorls Stigma Style Carpel Ovule Ovary wall a. Anther Stamen Filament Petal Sepal Ovary Receptacle Pedicel 26 http://ww2.biol.sc.edu/~krizeklab/krizek.html • Flower whorls – Outermost whorl – sepals – Second whorl – petals – Third whorl – stamens (androecium) • Pollen is the male gametophyte • Each stamen has a pollenbearing anther and a filament (stalk) – Innermost whorl – gynoecium • Consists of one or more carpels • House the female gametophyte 27 https://s10.lite.msu.edu/res/msu/botonl/b_online/library/knee/hcs300/angio1.htm Almond flower development Click on image for YouTube video http://www.youtube.com/watch?v=DijGvPMuHhI 28 http://homemadenaturalsoaps.blogspot.com/2011/0 6/sweet-almond-oil-as-name-suggests-is.html Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Anther Filament Stigma Petal Sepal Style Carpel Ovule Ovary wall Stamen Ovary Receptacle Pedicel a. • Carpel has 3 major regions – Ovary – swollen base containing ovules • Later develops into a fruit – Stigma – tip where pollen lands – Style – neck or stalk 29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MITOSIS Megaspore (n) Generative cell Tube nucleus Polar nuclei Egg Ovule Megaspore mother cell (2n) Pollen tube Sperm Pollen (n) Tube nucleus Formation of pollen tube (n) Anther Stigma Style Anther (2n) Microspore mother cells (2n) 2n Ovary Adult sporophyte with flower (2n) Young sporophyte (2n) Cotyledons Polar nuclei Endosperm Embryo (2n) Endosperm (3n) Seed coat Zygote DOUBLE FERTILIZATION Seed (2n) Sperm Egg 32 http://leavingbio.net/the%20structure%20and%20functions%20of%20flowers.htm • Pollination – Mechanical transfer of pollen from anther to stigma – May or may not be followed by fertilization – Pollen grain grows pollen tube • Guided to embryo sac by pheromones from synergids • Pollen tube enter embryo sac via micropyle – One of the two pollen grain cells lags behind • This generative cell divides to produce two sperm cells • No flagella on sperm See YouTube video of pollen tube development 34 • Double fertilization – One sperm unites with egg to form diploid zygote • New sporophyte – Other sperm unites with the two polar nuclei to form the triploid endosperm • Provides nutrients to embryo 35 http://vitae-scientia.tumblr.com/ Seeds • In many angiosperms, embryo development is arrested soon after meristems & cotyledons (embryonic leaves) differentiate • Integuments develop into a relatively impermeable seed coat – Encloses the seed with its dormant embryo and stored food • May remain dormant for many years – Germinate when conditions are favorable Shoot apical meristem Seed coat (integument) Procambiu m Root apical meristem Root cap Endosper m Cotyledo ns 36 http://www.cactus-art.biz/note-book/Dictionary/Dictionary_S/dictionary_seed.htm http://www.allposters.com/-sp/Sectioned-Seed-of-a-Runner-Bean-Phaseolus-Coccineus-Showingthe-Seed-Coat-Embryo-and-Cotyledons-Posters_i6010357_.htm • Seeds are an important adaptation 1. Maintain dormancy under unfavorable conditions 2. Protect young plant when it is most vulnerable 3. Provide food for the embryo until it can produce its own food 4. Facilitate dispersal of the embryo Runner bean seed (Phaseolus coccineus) Seed dispersal in white fir (Abies concolor) 37 http://commons.wikimedia.org/wiki/File:Abies_concolor_seed-dispersal.jpg • Once a seed coat forms, most of the embryo’s metabolic activities cease • Germination cannot take place until water and oxygen reach the embryo • Seeds of some plants have been known to remain viable for thousands of years 38 http://www.ask.com/explore/what-seed-germination • Specific adaptations ensure that seeds will germinate only under appropriate conditions – Some seeds lie within tough cones that do not open until exposed to fire Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. a: © Ed Reschke; b: © David Sieren/Visuals Unlimited 39 http://biology-pictures.blogspot.com/ Fruits • Most simply defined as mature ovaries (carpels) • During seed formation, the flower ovary begins to develop into fruit • It is possible for fruits to develop without seed development Sterile cultivated bananas with reduced seeds – Bananas are propagated asexually 40 http://b4fa.org/biosciences-and-agriculture/plantbreeding/where-do-our-crops-come-from-banana/ • The ovary wall is termed the pericarp – With 3 layers: • Exocarp – skin or rind • Mesocarp – flesh or pulp • Endocarp – surrounds seeds (pit) – Their fate determines fruit type 41 http://vitae-scientia.tumblr.com/ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. True Berries Outer pericarp The entire pericarp is fleshy, although there may be a thin skin. Berries have multiple seeds in either one or more ovaries. The tomato flower had four carpels that fused. Each carpel contains multiple ovules that develop into seeds. Fused carpels Seed © Kingsley Stern 43 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Legumes Split along two carpel edges (sutures) with seeds attached to edges; peas, beans. Unlike fleshy fruits, the three tissue layers of the ovary do not thicken extensively. The entire pericarp is dry at maturity. Stigma Pericarp Seed Style © Kingsley Stern 44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Drupes Pericarp Single seed Exocarp (skin) enclosed Mesocarp in a hard pit; Endocarp (pit) peaches, plums, cherries. Each layer of the pericarp has a different structure and function, with Seed the endocarp forming the pit Samaras Not split and with a wing formed from the outer tissues; maples, elms, ashes. Pericarp Seed (left): © Kingsley Stern; (right): Courtesy of Robert A. Schisling 45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Aggregate Fruits Derived from many Sepals of a ovaries of a single single flower flower; strawberries, blackberries. Unlike tomato, Ovary these ovaries Seed are not fused and covered by a continuous pericarp. Multiple Fruits Individual flowers form fruits around a single stem. The fruits fuse as seen with pineapple. Main stem Pericarp of individual flower (left): Courtesy of Robert A. Schisling; (right): © Charles D. Winters/Photo Researchers, Inc. 46 Fruit Dispersal • Occurs through a wide array of methods a. Ingestion and transportation by birds or other vertebrates b. Hitching a ride with hooked spines on birds and mammals c. Blowing in the wind d. Floating and drifting on water e. Burial in caches by herbivores Cedar Waxwing (Bombycilla cedrorum) 47 http://blog.duncraft.com/2011/08/26/birds-migration-and-berries/ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. a: © Edward S. Ross; b: © Nigel Cattlin/Visuals Unlimited b. Hitching a ride with hooked spines on birds and mammals 48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. c. d. c: © Phil Ashley/Getty Images; d: © John Kaprielian/Photo Researchers, Inc. c. Blowing in the wind d. Floating and drifting on water 49 http://en.wikipedia.org/wiki/Wikipedia:Wikipedia_Signpost/2011-12-26/WikiProject_report Eastern chipmunk (Tamias striatus) carrying nuts for caching e. Burial in caches by herbivores 50 Western scrub-jays (Aphelocoma californica) caching seed http://www.courierpress.com/news/2012/dec/29/blue-jays-cashing-in-on-caches-to-survive/