Plant Evolution

1. Plant Evolution Chapter 21

2. The Plant Kingdom • Nearly all are multicelled • Vast majority are photoautotrophs • Energy from sun • Carbon dioxide from air • Minerals dissolved in water

3. Setting the Stage for Plants • Earth’s atmosphere was originally oxygen free • Ultraviolet radiation bombarded the surface • Photosynthetic cells produced oxygen and allowed formation of a protective ozone layer

4. Invading the Land • Cyanobacteria were probably the first to spread into and up freshwater streams • Later, green algae and fungi made the journey together • Every plant is descended from species of green algae

5. Adaptations to Land • Root systems • Shoot systems • Vascular tissues • Waxy cuticle for water conservation

6. Evolutionary Trend in Plant Life Cycles • Algae and bryophytes put most energy into making gametophytes • Land plants put energy into structures that produce spores and retain, nourish, and protect gametes

7. Evolutionary Trend

8. Milestones in Plant Evolution charophytes bryophytes lycophytes horsetails ferns cycads ginkgos conifers gnetophytes flowering plants seed plants plants with true leaves vascular plants land plants (closely related groups)

9. Pollen • Pollen grains are sperm-bearing male gametophytes that develop from microspores • Allows transfer of sperm to egg without water • Can drift on air currents, or be carried by pollinators

10. Seeds • Ovules are female reproductive structures that become seeds • Consist of: • Female gametophyte with egg cell • Nutrient-rich tissue • Jacket of cell layers that will form seed coat

11. Nonvascular Plants • Bryophytes • Include 24,000 species of: Liverworts Hornworts Mosses

12. Bryophytes • Small, nonvascular, nonwooody • Gametophyte dominates life cycle; has leaflike, stemlike, and rootlike parts • Usually live in wet habitats • Flagellated sperm require water to reach eggs

13. Moss Life Cycle Development of mature sporophyte (still attached to gametophyte) Zygote Diploid Stage Fertilization Meiosis Haploid Stage Spores released male gametophyte tip Sperm Male gametophyte female gametophyte tip Female gametophyte Egg

14. Peat Mosses • 350 species • Sphagnum is an example • Grow in acidic bogs; important ecosystems of cold and temperate regions • Peat can be harvested and burned as fuel

15. Marchantia: A Liverwort • Reproduces asexually by way of gemmae cups • Sexual reproduction • Gametophytes are male or female • Gametes are produced on elevated structures

16. Vascular Plants • Majority of plants • Have internal tissues that carry water and solutes • Two groups • Seedless vascular plants • Seed-bearing vascular plants

17. Seedless Vascular Plants • Arose during the Devonian • Produce spores but no seeds • Main groups: Lycophytes Horsetails Ferns

18. Seedless Vascular Plants • Like bryophytes: • Live in wet, humid places • Require water for fertilization • Unlike bryophytes: • Sporophyte is free-living and has vascular tissues

19. Lycophytes and Horsetails • Tree-sized lycophytes lived in Carboniferous swamp forests – 1,100 modern forms are smaller and include club mosses • Also as tall as trees, sphenophytes grew in Carboniferous swamp forests – 30 smaller species exist today, known as the “horsetails”

20. Ferns • 12,000 species, mostly tropical • Most common sporophyte structure • Perennial underground stem (rhizome) • Roots and fronds arise from rhizome • Young fronds are coiled “fiddleheads” • Mature fronds divided into leaflets • Spores form on lower surface of some fronds

21. Fern Life Cycle The sporophyte (still attached to the gametophyte) grows, develops Sori rhizome zygote Diploid Stage fertilization meiosis Haploid Stage Spores are released Spores develop egg-producing structure mature gametophyte (underside) egg sperm-producing structure sperm Spore germinates gametophyte

22. Ancient Carbon Treasures • 300 mya – during Carboniferous • Mild climate and swamp forests • Plants with lignin-reinforced tissues and well-developed root and shoot systems had competitive edge • Rising and falling sea levels led to submerged and compressed forests – the source of coal

23. Rise of Seed-Bearing Plants • Seeds appeared about 360 million years ago • Seed ferns and gymnosperms were dominant at first • Angiosperms arose later

24. Seed-Bearing Vascular Plants • Gymnosperms arose first • Cycads • Ginkgos • Gnetophytes • Conifers • Angiosperms arose later • Monocots • Dicots

25. Seed-Bearing Plants • Microspores that give rise to pollen grains • Megaspores inside ovules • More water-conserving than seedless vascular plants

26. Special Traits of Seed-Bearing Plants • Pollen grains • Arise from megaspores • Develop into male gametophytes • Can be transported without water • Seeds • Embryo sporophyte inside nutritive tissues and a protective coat • Can withstand hostile conditions

27. Gymnosperms • Plants with “naked seeds” • Seeds don’t form inside an ovary • Four groups Conifers Ginkgos Cycads Gnetophytes

28. Conifer Characteristics • Widest known, largest number of living species • Woody trees or shrubs • Most are evergreen • Bear seeds on exposed cone scales • Most produce woody cones

29. Conifer Distribution • Conifers dominated during Mesozoic • Reproduce more slowly than angiosperms; at competitive disadvantage in many habitats • Still dominate in far North, at higher elevations, and in certain parts of Southern Hemisphere

30. Cycads • Most diverse during age of dinosaurs • Only 130 living species • Two species of Zamia are native to U.S. • Palmlike appearance • Pollen-bearing and seed-bearing cones on different plants

31. Ginkgos • Diverse during age of dinosaurs • Only surviving species, Ginkgo biloba, is native to China • Deciduous tree with fan-shaped leaves • Trees are male or female • Female trees produce seeds covered with a fleshy, foul-smelling coat

32. 3 Genera of Gnetophytes • Gnetum (tropical climbing vines) • Ephedra (joint fir, Mormon tea) • Welwitschia mirabilis

33. Pine Cones • Woody scales of a “pine cone” are the parts of where megaspores formed and developed into female gametophytes • Male cones, where microspores and pollen are produced, are not woody

34. PineLifeCycle Female cone Sporophyte Ovule Male cone Pollen sac Seed Fertilization Meiosis Egg View inside ovule Microspores Megaspores Pollen tube

35. Angiosperms • Flowering plants • Dominant land plants (260,000 species) • Defining feature: Ovules and (after fertilization) seeds are enclosed in an ovary • Two classes: Monocots and dicots

36. Double Fertilization • Distinctive feature of angiosperms • Male gametocyte delivers two sperm to an ovule • One fertilizes egg; other fertilizes a cell that gives rise to endosperm that supports embryo

37. Flower Structure

38. Flowering Plant Life Cycle Diploid Double fertilization Meiosis Meiosis Haploid Mitosis without cytoplasmic division Microspores Pollination Two sperm enter ovule Female gametophyte

39. Flowering Plant Diversity • Almost 90 percent of living plants are angiosperms • Three major groups: • Magnoliids – 9,200 species, such as pepper plants • Eudicots – 170,000 species, including daisies • Monocots – 80,000 species, including grasses

40. Flowering Plant Evolution water lilies star anise Amborella eudicots magnoliids monocots basal groups

41. Deforestation • Result of demand for wood as fuel and lumber; cultivation of land for agriculture • Greatest occurrence in Brazil, Indonesia, Columbia, and Mexico