Evolution of Seed Plants: Diverse and Adaptive Structures for Survival

1. Chapter 30Plant Diversity II: Evolution of Seed Plants AP Biology

3. Seed Plants • Sporophyte dominant • Being diploid allows carrying recessive alleles from one generation to the other • Eventually, these recessive alleles may be selected for • All heterosporous • Megasporangia • Nucellus; is solid & fleshy part of the ovule located in the ovary of the flower • Produces megaspores (egg cells) that produce female gametophyte • Female gametophyte consists of a group of cells called the embryo sac • Microsporangia • Polen sacs of an anther • Contain microsporocytes (2n) that produce microspores (n) that produce the male gametophyte • Microspores develop into pollen grains when they divide by mitosis and are covered with cell wall sporopollenin • Pollen grains will germinate and form male gametophyte

4. Seed Plants, cont. • Embryo – multicellular sporophyte that will grow into new plant • Encased in protective seed coat (derived from outer structure called integuments) • Supplied with its own source of food (endosperm or cotyledons) • Seeds allow embryo to be moved away from parent through wind, water, and animals • Truly land plants – not tied to water for pollination; gametophytes or embryos do not require as much water for support

5. Seed coat (derived from integument) Integument Female gametophyte (n) Sporewall Egg nucleus (n) Food supply (female gametophyte tissue) (n) Male gametophyte (within germinating pollen grain) (n) Discharged sperm nucleus (n) Megasporangium (2n) Embryo (2n) (new sporophyte) Pollen grain (n) Megaspore (n) Micropyle Unfertilized ovule Fertilized ovule Gymnosperm seed

6. Seeded Vascular Plants • Gymnosperms – “naked seed” • Lack enclosed chambers (ovaries) in which seeds develop • Seeds form naked on surface of modified leaf (called a scale) • Economically important as a lumber source • About 900 species

7. Division: Ginkgophya (Ginkgo) • Single extant species – Ginkgo biloba • Used in Chinese herbal medicines • Grows wild in China • May play a role in a cure for Alzheimer’s, memory loss, increased circulation • Dioescious (“two houses”) – produces pollen and seeds on separate trees • Male and female trees • Male tree creates small pollen cone • Female seed covered by fleshy coat (not a fruit) that smells BAD when rotting

8. Division: Cycadophyta (Cycads) • Approximately 100 species; some threatened by habitat loss • Palm-like appearance (ex: Sago palm) • Beetles and bees used for pollination – plant generates heat and odors to attract them • Seeds are quite large, often brightly colored (red, purple, or yellow) • Roots possess coralloid roots (coral-like in appearance); contain nitrogen fixing bacteria • Large, slow growing Seeds of Puerto Rican cycad, Zamia portoricensis

9. Division: Gnetophyta • One species in US (Ephedra) • Gnetum and Welwitschia other genera • Found in southwest US, South America, Africa

10. Division: Coniferophyta (Conifer) • Cone bearers – often called evergreens since they retain their needle-shaped leaves • Most have woody cones; yews and junipers have fleshy, berry-like tissues (called aril) • Oldest living organisms (Bristle-cone pine) • Tallest organisms (Coast Redwood)

11. Division: Coniferophyta (Conifer), cont. • Needles have thick cuticle, sunken stomata, mesophyll without air spaces and veins only in center of the leaf (why might this be valuable?) • Tracheids (xylem) transport water • Larger in summer than in winter • Have pits at ends to facilitate water movement up tissue • Have secondary cell walls • Economically important for wood, pulp, turpentine, resins, and ornamentals • Monoecious (“one house”) – male and female cones (actually sporophyte tissue) appear on same tree

12. Division: Coniferophyta (Conifer) Male Cones • Male cone (staminate or pollen cone) produces microspores that develop into male gametophytes • Each cone made up of a number of small sporophylls (scales) • Microsporangia contain microsporocytes that undergo meiosis to produce haploid microspores • Each microspore divides a number of times to form a viable pollen grain (immature male gametophyte)

13. Division: Coniferophyta (Conifer) Female Cones • Female cones (ovule or seed cones) produce megaspores that develop into female gametophytes • The megasporangium and the outer integuments form the ovule • Megasporocyte divides meiotically to form 4 haploid megaspores (only one develops into female gametophyte – other degenerate) • Within gametophyte tissue, archegonia are formed, containing egg cell

14. Conifer Life Cycle

15. Angiosperms – “Vessel Seed” • Division: Anthophyta • Flowering plants • Reproductive structure of angiosperms – a cluster of highly-specialized leaves which participate in reproduction • Diversity of species – partially attributed to coevolution of plants with animal pollinators (specific pollinators = edible fruits) • Nearly all our food comes from flowering plants • Grains, beans, nuts, fruits, vegetables, herbs, spices, tea, coffee, chocolate, wine, beer, cola

16. Division Anthophyta, cont. • Two classes: • Monocotyledones (monocots) • Ex: lily, orchid, grass, palm, yucca and grains such as corn, oats, rice, rye, and wheat • Dicotyledones (dicots) • Ex: sunflower, bean, snapdragon, carrot, rose

18. Division Anthophyta, cont. • Have vessel elements (xylem) for water transport instead of tracheids • Vessel elements have perforated or absent end plates to speed transport • Reinforced with fiber cells • Both tracheids and vessel elements are dead cells • Heterosporous

19. Division Anthophyta, cont. • Gametophyte greatly reduced • Male gametophytes (pollen) have only two nuclei – tube nucleus and generative nucleus • Female gametophyte is reduced to only 7 cells and 8 nuclei • Sperm and egg nuclei fuse to form 2n zygote that forms embryo • Other sperm nucleus fuses with polar nuclei in center to form a 3n endosperm • Two fertilizations take place  double fertilization

20. Double Fertilization

21. Division Anthophyta, cont. • After fertilization, the ovule (female gametophyte) develops into a seed • The endosperm is either retained in the seed or absorbed by developing cotyledons • As the seed develops, the ovary begins to mature around the seed to form fruit

22. Angiosperm Flower Anatomy

23. Angiosperm Life Cycle