Plant Diversity II

1. Plant Diversity II Level 1 Biological Diversity Jim Provan Campbell: Chapter 30

2. Reproductive adaptations of seed plants • Three life cycle modifications led to the success of terrestrial plants: • Reduction of the gametophyte: retained in the moist reproductive tissue of the sporphyte • Origin of the seed: • Zygotes developed into embryos packaged with a food supply within a protected seed coat • Seeds replaced spores as the main means of dispersal • Evolution of pollen: plants were no longer tied to water for fertilisation

3. Sporophyte (2n) Gametophyte (n) Sporophyte (2n) Sporophyte (2n) Gametophyte (n) Gametophyte (n) Sporophyte dependent on gametophyte (e.g. bryophytes) Large sporophyte and small independent gametophyte (e.g. ferns) Reduced gametophyte dependent on sporophyte (seed plants) Reduction of gametophytes in seed plants

4. In seed plants, the seed replaced the spore as the main means of dispersing offspring • Relatively harsh terrestrial environment: • Bryophytes and seedless vascular plants release spores • Seeds are more hardy because of their multicellular nature • Seed is a sporophyte embryo and a food supply surrounded by a protective coat • All seed plants are heterosporous • Development of seed associated with megasporangia: • Seed plant megasporangia are fleshy structure called nucelli • Additional tissues (integuments) surround megasporangium • Resulting structure is called an ovule • Female gametophyte develops in wall of megaspore, is fertilised (embryo) and resulting ovule develops into a seed

5. Integuments (2n) Seed coat (2n) derived from integuments Female gametophyte (n) Nucellus (megasporangium) (2n) Spore case (n) Food supply (derived from female gametophyte tissue Megaspore (n) Egg nucleus (n) Embryo (2n) new sporophyte Pollen tube (n) Discharged sperm nucleus (n) Micropyle From ovule to seed

6. Pollen became the vehicle for sperm cells in seed plants • Microspores develop into pollen grains which mature to form the male gametophytes of seed plants: • Pollen grains coated with a resistant polymer, sporopollenin • Can be carried away by wind or animals (e.g. bees) following release from microsporangia • A pollen grain near an ovule will extend a tube and discharge sperm cells into the female gametophyte within the ovule: • In some gymnosperms, sperm are flagellated (ancestral) • Other gymnosperms (including conifers) and angiosperms do not have flagellated sperm cells

7. Gymnosperms • Descended from Devonian progymnosperms: • Seedless • Seeds evolved late Devonian • Climatic changes during Permian led to lycopods, horsetails and ferns being replaced by conifers and cycads • Lack enclosed chambers (ovaries) in which seeds develop

8. Cycads Gingko Conifers Gnetophytes Four divisions of extant gymnosperms

9. Conifers are the largest division of gymnosperms • Mostly evergreens e.g. pines, firs, spruces, larches, yews, cypresses etc. • Include the tallest, largest and oldest living organisms • Needle-shaped leaves adapted to dry conditions: • Thick cuticle covers leaf • Stomata in pits, reducing water loss • Megaphylls cf. other leaves

10. The life cycle of a pine

11. Angiosperms

12. Angiosperms (flowering plants) • Flowering plants are the most widespread and diverse (250,000 species) • Only one division (Anthophyta), with two classes: • Monocotyledons • Dicotyledons • Less dependent on wind pollination - use insects and animals

13. Evolution of vascular tissue in angiosperms • Conifers have water-conducting cells called tracheids • Angiosperms have vesselelements: • More specialised for transport • Less specialised for support • Xylem reinforced by fibres: • Specialised for support - thick lignified wall • Evolved in conifers (conifers lack vessel elements)

14. The flower is the defining reproductive adaptation of angiosperms • Sepals: sterile, enclose bud • Petals: sterile, attract pollinators • Stamen: produces pollen • Carpel: evolved from seed-bearing leaf that became rolled into a tube • Stigma: sticky structure that receives pollen • Ovary: protects ovules, which develop into seeds after fertilisation

15. Fruits help disperse the seeds of angiosperms • Fruits are ripened ovaries that protect dormant seed and aids in its dispersal • Modifications of fruits that aid dispersal include: • Seeds within fruits that are shaped like kites or propellors (e.g. maple) • Burr-like fruit that cling to animal fur • Edible fruit - tough seeds pass through digestive tract

16. Anther MITOSIS Ovule Germinating seed Stigma Pollen tube Ovary Food supply Seed Pollen tube MITOSIS Style Pollen tube Life cycle of an angiosperm

17. Angiosperms and animals shaped one another’s evolution • Coevolution: reciprocal evolutionary responses among two or more interacting species • Coevolution led to diversity of flowers: • Flower-specific pollinators • Usually adapted for types of pollinators • Attraction of ripening fruits: • Soft, fragrant and sugary • Attractive change of colour

18. Colonisation of land by plants Diversification of vascular plants 25 20 15 10 5 Plants transformed the atmosphere and the climate • Plants decreased atmospheric carbon dioxide, resulting in global cooling • Cooler environment made terrestrial life more habitable for other organisms