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Seed Plants & Gymnosperms

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  1. Seed Plants & Gymnosperms Spring 2014

  2. Outline • Review of land plant phylogeny • Characters of seed plants • Gymnosperm phylogeny & diversity • Cycads • Gingko • Conifers • Gnetophytes

  3. Review of land plant phylogeny Green plants (viridophytes) Land plants (embryophytes) Vascular plants (tracheophytes) Seed plants (spermatophytes) Gymnosperms Angiosperms

  4. Fig. 5.1 Evolution of wood before the evolution of seeds.

  5. Fossil lignophytes Progymnosperm (wood, no seeds) Seed fern (wood, seeds)

  6. Seed fern fossils from Pella, Iowa

  7. Fig. 5.1 Evolution of wood (>380 mya) before the evolution of seeds (>360 mya).

  8. Characters of seed plants • Eustele (ancestral) • Axillary branching • Wood (ancestral) • Cambia (vascular cambium, cork cambium) • Seed • Heterospory • Megaspore reduction/retention • Integument/micropyle • Nutritive tissue • Male gametophyte • Pollen grain • Pollen tube

  9. Characters of seed plants: Eustele eustele = primary stem vasculature comprising a single ring of vascular bundles

  10. Characters of seed plants: axillary branching Bud/branch traces Leaf traces

  11. Characters of seed plants: cambia cambia: vascular cambium (wood) & cork cambium (periderm) Fig. 5.3

  12. Fig. 5.4 X-section of woody stem

  13. Characters of seed plants: seed • Heterospory • Megaspore reduction/retention • Integument/micropyle • Nutritive tissue

  14. Life cycle of most seed-free plants • homospory Multicellular Sporophyte < embryo cells in sporangium [with sporangia] Zygote < 2n < alternation of generations SYNGAMY MEIOSIS < n < Spores Gametes [egg + sperm] Multicellular Gametophyte < < < [with gametangia: archegonia + antheridia]

  15. < < Life cycle of seed plants • heterospory Multicellular Sporophyte < embryo megasporangia Zygote microsporangia < 2n < alternation of generations SYNGAMY MEIOSIS < n microspores < sperm Male Gametophyte egg < megaspores [antheridia] < < Female Gametophyte [archegonia]

  16. Evolution of the seed • Megaspore reduction: • -reduction to 1 megaspore reduction to 1 megaspore Fig. 5.6 #3

  17. Evolution of the seed • Megaspore retention: • -the one megaspore is retained within • megasporangium, not released Fig. 5.6 #4

  18. Evolution of the seed Evolution of integument/micropyle from sterile sporophyte tissue Fig. 5.6 #5

  19. Evolution of the seed • pollination droplet: • -secreted by young ovule through micropyle • -water + sugars, amino acids (megasporangium) • -adhering pollen grains pulled inside! Fig. 5.10

  20. Evolution of the seed -nutritive tissue from the female gametophyte -integument becomes the seed coat Fig. 5.10

  21. Characters of seed plants • male gametophyte • pollen grain = extremely reduced male gametophyte, a few cells • pollen tube – formed by the pollen, grows though sporophytic tissue to deliver sperm cells to egg (in ovule) Pine pollen

  22. Characters of seed plants: seed Adaptive advantages of the seed: • protection (seed coat) • dispersal unit of sexual reproduction • dormancy mechanisms • nutritive tissue – provides energy for young • seedling, aiding in establishment

  23. Two major groups of seed plants: • Gymnosperms—not sure of the early evolutionary history of gymnosperms; could be monophyletic or could be paraphyletic • Angiosperms—monophyly supported by many characters including the carpel

  24. Gymnosperm Phylogeny Cycads Pines Gingko Gnetales Cypresses et al. pollen tube compound ♀ cones (strobili) simple leaves 1° DNA data

  25. Gymnosperm diversity -ca. 15 families, 75-80 genera, ca. 1,000 species -4 or 5 main lineages -all woody -mostly without effective vegetative reproduction -only tracheids in the xylem (except for gnetophytes, which also have vessels) -naked seeds -relatively slow sexual reproduction -worldwide but dominant in many colder or arctic regions -include the tallest, the most massive, and the longest living individual plants

  26. Gymnosperm wood: tracheids only

  27. Major groups of gymnosperms • Cycads • Gingko • Conifers • Gnetophytes

  28. Major groups of gymnosperms • Cycadophyta – Cycads • squat, unbranched trunk (little wood), • usually pinnately compound leaves • loss of axillary branching • dioecious: male and female plants • male and female strobili (cones) • motile, multiflagellate sperm! • (ancestral) • coralloid roots with nitrogen-fixing • cyanobacteria Zamia female strobilus Cycas male strobilus

  29. Major groups of gymnosperms • Cycadophyta – Cycads • ca. 11 genera (130 spp.) • now restricted distribution • seeds with bright fleshy seed coat--dispersed by plant-eating dinos!

  30. A native U.S. cycad: Zamia floridana

  31. Gymnosperm Phylogeny Cycads Pines Gingko Gnetales Cypresses et al. pollen tube compound ♀ cones (strobili) simple leaves 1° DNA data

  32. Major groups of gymnosperms • Ginkgophytes – Ginkgo • extensive fossil record but…only • 1 living species: Ginkgo biloba! • highly branched tree with • well developed wood • deciduous, fan-shaped leaves with • dichotomous venation • dioecious: male and female trees • -male: “cone” with lateral stalks bearing • microsporangia • -female: no cone, axis with 2 ovules • (outer integument layer fleshy) • motile sperm (ancestral)

  33. Major groups of gymnosperms Ginkgophytes – Ginkgo

  34. Fig. 5.1

  35. Major groups of gymnosperms • Coniferophyta – Conifers • ca. 700 spp. • once dominant worldwide, displaced by angios • shrubs or small trees, highly branched with • well developed wood • leaves simple, often needle-like or awl-shaped • -pines: in fascicles • non-motile sperm (pollen tube needed) • female (seed-bearing) cones in most • include both traditional conifers but now also the Gnetales (gnetophytes)

  36. Major groups of gymnosperms • Coniferophyta – Conifers • pollen cone or male cone • -microsporangia & modified leaves • seed cone or female cone • -axis with modified leaves (bracts, usually reduced), each subtending seed-bearing scale (modified branch system) • -woody or leathery or fleshy male female

  37. Major groups of gymnosperms • Coniferophyta – Conifers • seed cone and pine nuts Stone Pine nuts [w U.S.] Korean pine nuts

  38. Evolution of the compound conifer female cone Fig. 5.19

  39. Gymnosperm Phylogeny Cycads Pines Gingko Gnetales Cypresses et al. pollen tube compound ♀ cones (strobili) simple leaves 1° DNA data

  40. Pollen usually with 2 appendages Leaves linear to needle-like Resin canals in wood & leaves Pinaceae Ovules 2, inverted Winged seeds

  41. Pseudotsuga (Douglas fir) Abies (fir) Picea (spruce) Larix (larch)

  42. Pinus (pines) -needles in bundles -cone scales thickened at the tip and often armed with a prickle

  43. Gymnosperm Phylogeny Cycads Pines Gingko Gnetales Cypresses et al. pollen tube compound ♀ cones (strobili) simple leaves 1° DNA data

  44. Pollen without appendages Leaves scale-like to linear Cone scales fused to bracts Microsporangia 2-10 per microsporophyll & ovules 1-20 per cone scale Cupressaceae

  45. Juniperus (juniper) Taxodium (bald cypress) Chamaecyparis

  46. Sequoia sempervirens (redwood) Sequoiadendron giganteum (giant sequoia)

  47. Taxaceae Ovules solitary, cones lacking Seeds with a fleshy, brightly colored aril

  48. Podocarpus Araucariaceae

  49. Gymnosperm Phylogeny Cycads Pines Gingko Gnetales Cypresses et al. pollen tube compound ♀ cones (strobili) simple leaves 1° DNA data

  50. Major groups of gymnosperms • Gnetophytes or Gnetales • 3 extant genera: Ephedra (65 spp.); Gnetum (28 • spp.); Welwitschia mirabilis • related to angiosperms? • recent molecular data: a gymnosperm group • defined by many characters, e.g.: • -opposite leaves, similar pollen • -vessel structure (independent of angiosperms) • -nonmotile sperm (independent) • -double fertilization (independent of angiosperms) • -some with insect pollination