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Chapter 26 lecture presentation
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- 2. The Evolution ofSeed Plants • Seed plants first appeared 305–465 mya • Success attributed to evolution of seed – Seed includes embryo, food for the embryo, and a seed coat – Allows the “clock to be stopped” to survive harsh periods before germinating – “Naked” seeds first, then later development of fruits enhanced dispersal 2
- 3. • Seed plantsproduce 2 kinds of gametophytes • Male gametophytes – Pollen grains – Dispersed by wind or a pollinator – No need for water • Female gametophytes – Develop within an ovule – Enclosed within diploid sporophyte tissue in angiosperms 3
- 4. Five Phyla ofExtant Seed Plants • Coniferophyta • Cycadophyta • Gnetophyta • Ginkophyta • Anthophyta 4
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- 6. Gymnosperms • Plants with“naked seeds” • There are four living groups • Coniferophytes • Cycadophytes • Gnetophytes • Ginkgophytes • All lack flowers and fruits of angiosperms • All have the ovule exposed on a “scale” 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Angiosperms Gymnosperms FernsandAllies
- 7. Conifers (phylum Coniferophyta) •Most diverse and familiar gymnosperm phylum • Pines, spruces, firs, cedars, and others • Found in colder and sometimes drier regions of the world • Conifers are sources of important products – Timber, paper, resin, and taxol (anti-cancer) 7
- 8. • Pines – Morethan 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 8
- 9. • Pine reproduction •Male gametophytes (pollen grains) – Develop from microspores in male cones by meiosis • Female pine cones form on the upper branches of the same tree – Female cones are larger, and have woody scales – Two ovules develop on each scale – Each contains a megasporangium • Each will become a female gametophyte 9
- 10. 10 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Microspore mother cell Microspores Pollen Air bladder Pollination Pollen tube Sperm MITOSIS Zygote Section of seed (second year), showing embryo embedded in megagametophyte MITOSIS Pine seed MITOSIS Seedling Sporophyte Pollen- bearing cone Scale Megaspore mother cell Ovulate (seed-bearing) cone Megaspore Pollen tube n 2n (15 months after pollination) MEIOSIS FERTILIZATION
- 11. • Female conesusually take 2 or more seasons to mature • During the first spring, pollen grains drift down between open scales – Pollen grains drawn down into micropyle – Scales close • A year later, female gametophyte matures – Pollen tube is digesting its way through – Mature male gametophyte has 2 sperm • 15 months after pollination, pollen tube reaches archegonium and discharges contents – One sperm unites with egg = zygote – Other sperm degenerates 11
- 12. 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! 12 a. © Luca Invernizzi Tetto/agefotostock Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 13. Gnetophytes (phylum Gnetophyta) •Only gymnosperms with vessels in their xylem • Contain three (unusual) genera – Welwitschia – Ephedra – Gnetum 13 b. © Juan Carlos Muñoz/agefotostock Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 14. Ginkgophytes (phylum Ginkgophyta) •Only one living species remains – Ginkgo biloba • Flagellated sperm • Dioecious – Male and female reproductive structures form on different trees 14 c. © Robert Gustafson/Visuals Unlimited Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. c.c.
- 15. Angiosperms • Flowering plants •Ovules contain eggs, and become seeds after egg fertilization • Carpel, a modified leaf that covers seeds, develops into fruit 15 Ferns and Allies Angiosperms Gymnosperms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 16. 16 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. (bottom right): © Goodshoot/Alamy RF Fusion of leaf margins Modified leaf with ovules Cross section Ovules (seeds) Carpel (fruit) Folding of leaf protects ovules Ovules
- 17. Angiosperm origins are amystery – Origins as early as 145–208 MYA – Oldest known angiosperm in the fossil record is Archaefructus 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fruits © David Dilcher and Ge Sun Paired stamens
- 18. 18 Archaefructus may bethe sister clade to all other angiosperms Gymnosperms Angiosperms Ginkgo Gnetophytes Conifers Cycads Archaefructus (extinct) Amborella Water lillies MonocotsMagnoliidsEudicotsStaranis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 19. Flowers contain the gametophytegeneration • Flower morphology – All flower parts have evolved either from stems or leaves – Flower parts are organized in circles called whorls 19
- 20. • Flower whorls –Outermost whorl – sepals – Second whorl – petals – Third whorl – stamens (androecium) • Each stamen has an anther (where the pollen is) and a filament (stalk) • Pollen is the male gametophyte – Fourth whorl - gynoecium • Consists of one or more carpels • Contain the female gametophyte 20
- 21. • Carpel has3 major regions – Ovary – contains the ovules • The ovary later develops into a fruit, and the ovules inside become the seeds – Stigma – tip where pollen lands – Style – neck or stalk 21 Carpel Stigma Style Ovule Ovary wall Stamen Anther Filament Petal Sepal Ovary a. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 22. Megaspore (n) Generative cell Tube nucleus Pollen tube Sperm Tube nucleus Style Pollen(n) Formation of pollen tube (n) Polar nuclei Sperm Egg Polar nuclei Egg MITOSIS MITOSIS Zygote Embryo (2n) Cotyledons Seed (2n) Endosperm (3n) Endosperm Anther Ovary Stigma Anther (2n) Ovule 2n DOUBLE FERTILIZATION MITOSIS G ERM INATIO N Microspore mother cells (2n) Megaspore mother cell (2n) Adult sporophyte with flower (2n) Young sporophyte (2n) Seed coat MITOSIS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 22
- 23. • Pollination – Mechanicaltransfer of pollen from anther to stigma – May or may not be followed by fertilization – Pollen grains develop a pollen tube that is guided to the embryo sac – The “generative cell” divides to produce two sperm cells • No flagella on sperm 23
- 24. • Double fertilization –One sperm unites with egg to form the diploid zygote (which becomes the new sporophyte embryo) – Other sperm unites with the two polar nuclei to form the triploid endosperm • Provides nutrients to new sporophyte embryo • Seed may remain dormant for many years – Germinate (grow by mitosis) when conditions are favorable 24
- 25. Seeds • In manyangiosperms, development of the embryo stops soon after meristems and cotyledons differentiate • Integuments develop into a relatively impermeable seed coat • Encloses the seed with its dormant embryo and stored food 25
- 26. 26 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Shoot apical meristem Root apical meristem Root cap Endosperm Procambium Seed coat (integument) Cotyledons
- 27. • Seeds arean important adaptation 1. They maintain dormancy under unfavorable conditions 2. They protect the young plant when it is most vulnerable 3. They provide food for the embryo until it can produce its own food 4. They facilitate dispersal of the embryo 27
- 28. • Once aseed 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 28
- 29. • Specific adaptationsensure that seeds will germinate only under appropriate conditions – Some seeds lie within tough cones that do not open until exposed to fire 29a. b. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a: © Ed Reschke; b: © David Sieren/Visuals Unlimited
- 30. Fruits • Most aresimply defined as “mature ovaries” (carpels) • At the same time seeds are forming, the flower ovary begins to develop into a “fruit” • However, it is also possible for fruits to develop without seed development – Bananas are propagated asexually • Not all fruits are something you would want to eat! 30
- 31. 31 True Berries The entirepericarp 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. Outer pericarp SeedFused carpels
- 32. 32 Drupes Seed Pericarp Exocarp (skin) Mesocarp Endocarp (pit) Singleseed enclosed in a hard pit; peaches, plums, cherries. Each layer of the pericarp has a different structure and function, with the endocarp forming the pit Samaras Pericarp Not split and with a wing formed from the outer tissues; maples, elms, ashes. Seed (left): © Kingsley Stern; (right): Courtesy of Robert A. Schisling Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 33. 33 Aggregate Fruits Sepals ofa single flower Ovary Seed Derived from many ovaries of a single flower; strawberries, blackberries. Unlike tomato, these ovaries 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. Pericarp of individual flower Main stem (left): Courtesy of Robert A. Schisling; (right): © Charles D. Winters/Photo Researchers, Inc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 34. Fruit Dispersal • Occursthrough a wide array of methods – Ingestion and transportation by birds or other vertebrates – Hitching a ride with hooked spines on birds and mammals – Burial in caches by herbivores – Blowing in the wind – Floating and drifting on water 34
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