Chapter 30

• Seed plants

  Originated about 360 million years ago
• Seeds


  • Changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems
  • Can disperse over long distances by wind or other means
• Common to all seed plants

  • Reduced gametophytes
  • Heterospory
  • Ovules
  • Pollen
• Reduced gametophytes


  The gametophytes of seed plants are microscopic<|>They develop within the walls of spores that are retained within tissues of the parent sporophyte<|>This arrangement protects the developing gametophyte from environmental stress and enables it to obtain nutrients from the sporophyte
• Homosporous


  Ferns and other close relatives produce one kind of spore, which usually produces a bisexual gametophyte
• Heterosporous


  Seed plants produce two types of spores, which develop into either male or female gametophytes<|>Megasporangia produce megaspores that give rise to female gametophytes and develop on modified leaves called megasporophylls<|>Microsporangia produce microspores that give rise to male gametophytes and develop on microsporophylls
• Ovule


  Consists of a megasporangium, megaspore, and one or more protective integuments

  Gymnosperm megasporangia have one integument

  Angiosperm megasporangia usually have two integuments
• Pollen grain


  A microspore develops into a pollen grain that consists of a male gametophyte enclosed within the pollen wall
• Pollination


  The transfer of pollen to the part of a seed plant containing the ovules
• Pollen tube


  A germinated pollen grain produces a pollen tube that carries sperm into the female gametophyte
• Pollen


  • Eliminates the need for a film of water and can be dispersed great distances by air or animals
• Seed


  A sporophyte embryo, along with its food supply, packaged in a protective coat

  May remain dormant for days to years, until conditions are favorable for germination

  Have a supply of stored food

  May be transported long distances by wind or animals
• Gymnosperms


  Means "naked seeds"

  The seeds are exposed on sporophylls that form cones

  Angiosperm seeds are found in fruits, which are mature ovaries

  Most gymnosperms are cone-bearing plants called conifers
• Origins of characteristics found in living seed plants date back to the late Devonian period


  380 million years ago
• A 360-million-year-old fossil from the genus Elkinsia provides the earliest evidence of seed plants

  360 million years ago
• Gymnosperms appear early in the fossil record


  305 million years ago
• Conditions became drier at the end of the Carboniferous, favoring gymnosperms over the previously dominant seedless vascular plants
• Gymnosperms dominated terrestrial ecosystems during the Mesozoic era

  252 to 66 million years ago
• Angiosperms began to replace gymnosperms near the end of the Mesozoic era and now dominate most terrestrial ecosystems, though gymnosperms remain an important part of Earth's flora
• The gymnosperms consist of four phyla

  • Cycadophyta (cycads)
  • Ginkgophyta (one living species: Ginkgo biloba)
  • Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)
  • Coniferophyta (conifers: pine, fir, and redwood)
• Cycadophyta


  • Individuals have large cones and palmlike leaves
  • Unlike most seed plants, cycads have flagellated sperm
  • These thrived during the Mesozoic, but most of the few surviving species are endangered
• Ginkgophyta


  • This phylum consists of a single living species, Ginkgo biloba
  • Like the cycads, this group also has flagellated sperm
  • It has a high tolerance to air pollution and is a popular ornamental tree
• Gnetophyta


  • This phylum comprises three genera: Gnetum, Ephedra, and Welwitschia
  • Species vary in appearance, and some are tropical, whereas others live in deserts
• Coniferophyta


  • This phylum is the largest of the gymnosperm phyla
  • Most species have woody cones, but a few have fleshy cones
  • Most conifers are evergreens and can carry out photosynthesis all year round
• The Life Cycle of a Pine

  1. Pollen cones are small and consist microsporophylls that bear microsporangia

  2. Cells called microsporocytes undergo meiosis to produce haploid microspores inside the microsporangia

  3. Each microspore develops into a pollen grain

  4. Ovulate cones are larger and consist of megasporophylls bearing megasporangia (ovule)

  5. Within each ovule, megasporocytes undergo meiosis to produce haploid megaspores

  6. Megaspores develop into female gametophytes, which are retained within the megasporangia

  7. Ovulate cone scales separate at maturity and seeds are dispersed by wind
• Fossil evidence suggests that angiosperms originated

  140 million years ago
• Angiosperms diversified over

  20- to 30-million-year period
• The earliest pollen fossils with angiosperm characteristics are

  130 million years old
• Early fossils of larger flowering plant structures are about

  125 million years old
• Angiosperms


  All angiosperms are classified in a single phylum, Anthophyta

  They are the most widespread and diverse of all plants<|>Angiosperms are seed plants with reproductive structures called flowers and fruits
• Flower


  An angiosperm structure specialized for sexual reproduction<|>Many species are pollinated by insects or animals, while some species are wind-pollinated
• Floral organs

  • Sepals, which enclose the flower
  • Petals, which are often brightly colored to attract pollinators; wind-pollinated flowers generally lack brightly colored parts
  • Stamens, the male reproductive organs
  • Carpels, the female reproductive organs
• Stamen


  Consists of a stalk called a filament, with a sac called an anther<|>Microspores, which are produced in the anthers, develop into pollen grains containing the male gametophytes
• Carpel


  Consists of an ovary at the base of a style leading up to a sticky stigma, where pollen is received<|>The ovary contains the female gametophyte(s) within the ovule(s)<|>Fertilized ovules develop into seeds
• Pistil


  Can be used to refer to a single carpel or two or more fused carpels
• Fruit


  Formed when the ovary wall thickens and matures<|>Protects seeds and aids in their dispersal<|>Mature fruits can be either fleshy or dry<|>Various fruit adaptations help disperse seeds
• The Angiosperm Life Cycle

  1. Gametophyte development – ovules from megaspores and pollen from microspores

  2. Sperm delivery by pollen tubes

  3. Double fertilization – fertilization of the egg and endosperm

  4. Fruit and seed development
• Embryo sac


  The female gametophyte develops within an ovule contained within an ovary at the base of a stigma
• Micropyle


  The pore through which the pollen tube enters the ovule
• Double fertilization


  Occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule<|>One sperm fertilizes the egg, while the other combines with two nuclei in the central cell of the female gametophyte and initiates development of food-storing endosperm