Meet the Plants

1. Meet the Plants Plant features: Multicellular eukaryotes Photosynthetic autotrophs (a few are parasitic heterotrophs) Mostly terrestrial (some aquatic) - requires new modes of nutrition, support, protection & reproduction Multicellular gametangia, stomates Characteristic cellular features: Chloroplasts with thylakoid membranes and chlorophyll (other eukaryotic organelles, including mitochondria, also present) Starch as a storage compound (in plastids) Cellulose walls (structural strength) Cuticle (waxy coating) on the outside

2. Bryophytes & Seedless Vascular Plants Plants share several key features with their likely sister group, the charophyceans(green algae) A common ancestor of modern charophyceans and plants moved from an aquatic habitat onto land - this involves changes in size, gas exchange, protection for fragile embryos, protection from desication, ways for sperm to reach egg, mechanisms of dispersal (getting around)... etc

4. Kingdom Plantae 1st appeared about 475 million years ago, • along with terrestrial fungi • most plants retain a symbiotic association with fungi • Stages in Land Plant Evolution: • 1) Early land plants (~400 MYA)  bryophytes and seedless vascular plants • 2) Diversification (~390 MYA)  abundance of seedless vascular plants • 1) Seeds (~ 300 MYA)  Gymnosperms (naked-seeded plants) • 2) Flowers and fruits (~ 130 MYA)  Angiosperms (flowering plants)

5. 3 contrasting opinions of what constitutes the “plant kingdom” Standard definition: true plants have multicellular embryos

6. 3 contrasting opinions of what constitutes the “plant kingdom” should we include their closest algal relatives, the charophyceans, who share so many features with land plants?

7. 3 contrasting opinions of what constitutes the “plant kingdom” should we include green algae (chlorophytes), who share the same photosynthetic machinery?

8. Unlike green algae, terrestrial plants have different cell and tissue types Coleochaete orbicularis, a disc-shaped charophycean showing simple structure Polypodium sp., a terrestrial plant with greater cell & tissue complexity

9. Evidence for common ancestorof green algae & plants Features shared by plants and charophyceans: Primary endosymbiosis (double membrane around chloroplast) Photosynthetic pigments (chlorophyll a, b and carotenoids) Starch storage in plastids Production of cell walls from cellulose Cell plate formation during cell division Sporic life cycle New features in terrestrial plants: Specialized tissues + organs (roots, stems, leaves) derived from the apical meristem, the growing tip of a shoot Alternation of multicellular generations Multicellular gametangia Walled spores produced in sporangia Multicellular, dependent embryos

10. Plants organs derived from apical meristem Above ground Below ground

11. Alternation of Generations gametophyte (N) sporophyte (2N) All plants have multicellular haploid stages, and multicellular diploid stages, which produce each other

12. Walled spores spores = haploid cells that can grow into the gametophyte by mitosis protective outer layer allows spores to resist dry conditions Sporophyte (2N) stage has multicellular structures called sporangia that produce the haploid spores through meiosis

13. Multicellular Gametangia Archegonium: vase-shaped female organ holds single egg cell Antheridium: male organ releases sperm Gametophyte (N) stage has multicellular structures called gametangia that produce the haploid gametes

14. Multicellular, Dependent Embryos Placental transfer cells, analogous to mammalian placenta Multicellular embryos develop from zygote inside tissue of female parent, which provide embryo with nutrients during development

15. Non-vascular plants (7%) mosses, liverworts, hornworts - no roots or true leaves Vascular plants (93%) have a system of tubes that transport water + nutrients throughout the plant

16. Seedless vascular plants ferns + horsetails Seed plants gymnosperms + angiosperms

17. Trends in Plant Evolution: Increasing adaptation to a terrestrial environment Progressive reduction of gametophyte, increasing dominance of sporophyte Increased protection (esp. of reproductive parts) Increased height, structural strength, and more complex transport tissues Campbell & Reece 2002

18. # of living species in different plant groups 250,000 100 6500 12,000 100 1 1000 550 12,000 Key innovations are special features that allow one group to speciate

19. Early land plants (~438-408 MYA, Paleozoic Era) • Bryophytes and 1st vascular plants appeared at same time Bryophytes features (e.g. moss) Short Lack elaborate vascular tissue No true leaves, roots, or stems. Dominant gametophyte Flagellated (=swimming) sperm

20. Bryophytes (plants lacking vascular tissues) They are all short -- why? Typically associated with moist habitats Most parts only 1 cell thick

21. Bryophyte life cycle gametophytes are anchored into soil by threadlike protonema and single-celled rhizoids sporophytes = brown stalks growing out of the female gametophyte

22. Bryophyte life cycle 1) spores can disperse by wind – why is dispersal important? - allows colonization of new (maybe better) habitats - allows “escape” for offspring if local conditions turn bad - prevents inbreeding 2) sperm have to swim to reach egg – this requires environmental water - limits where bryophytes can grow - think about how this limited the ability of bryophytes to take over the world of dry land, compared to plant groups that evolved later - on the plus side: no need for pollinators, just rain or splashes 3) what features of the bryophyte body also limit: - the size of these plants? - the environment in which they can live?

23. Sphagnum sp. Forms extensive peat bogs in northern latitudes (arctic regions) around world

24. Seedless vascular plants appeared ~ 420 MYA Seedless vascular plant features (e.g., ferns) - Dominant sporophyte - Most lack true roots, stems, and leaves - Primitive vascular tissues Diversification culminated in Carboniferous Period (~300-350 MYA), when our modern-day coal deposits were first formed damp conditions favored the growth of huge forests of seedless vascular plants, limiting opportunities for seed plants until a global drying out

25. Primitive vascular plants had branching sporophytes that were independent of the gametophyte for growth Enable multiple sporangia per individual Fossil Cooksonia, a seedless plant with water-conducting tissue in stems, but no true roots or leaves

26. Seedless Vascular Plants – have primitive vascular tissues Phylum Pterophyta Lycopodium sp. Psilotum sp. Phylum Lycophyta Equisetum sp. (horsetail) Polypodium sp. (fern)

27. Fern Life cycle

28. Complex tissues and organs 1st appeared in seedless vascular plants xylem - tubes of deadcells w/ lignin-reinforced cell walls, used for transporting water + dissolved minerals phloem - living cells that transport sugars + other organic nutrients roots - organ for uptake of nutrients from soil... - leaves- increase surface area for photosynthesis

29. Microphylls, single vein Megaphylls, w/ complex branching - found in Lycophytes, the vascular system club mosses

30. Major evolutionary development: modified leaves called sporophylls on which sporangiagrow Fern sporophylls bear many sori, small clusters of sporangia on the underside of the sporophyll (look for these in lab) In groups we will see later, sporophylls diversified in shape and structure; stopped looking like leaves in gymnosperms  in angiosperms 