Seed Plant Evolution

• Seeds changed plant evolution, allowing seed plants to dominate terrestrial ecosystems.
• A seed consists of an embryo and nutrients protected by a coat.
• Seed plant gametophytes develop in the walls of spores within parental sporophyte tissue.
• Reduced gametophytes, heterospory, ovules, and pollen are common to all seed plants.

Heterospory

• Megasporangia produce megaspores that yield female gametophytes.
• MIcrosporangia produce microspores that yield male gametophytes

Ovules

• An ovule contains a megasporangium, megaspore, and protective integuments.
• A fertilized ovule develops into a seed.

Evolutionary Advantages of Seeds

• A seed develops from the whole ovule.
• Seeds consist of a sporophyte embryo, food supply, and a protective coat.
• Seeds can remain dormant for extended periods until conditions favor germination.
• Seeds can be transported long distances via wind or animals.

Gymnosperms

• Gymnosperms bear "naked" seeds exposed on modified leaves or cones, not enclosed by ovaries
• The four gymnosperm phyla are:
• Cycadophyta (cycads)
• Ginkgophyta (Ginkgo biloba, one living species)
• Gnetophyta (Gnetum, Ephedra, Welwitschia)
• Coniferophyta (conifers)
• Seed plants are divided into gymnosperms and angiosperms.
• Gymnosperms appeared early in fossil records and dominated the Mesozoic terrestrial ecosystem
• Gymnosperms are better suited to drier conditions compared to nonvascular plants.
• Cone-bearing gymnosperms called conifers dominate the northern latitudes today.
• Ginkgo biloba tolerates air pollution and is a popular ornamental tree.
• Coniferophyta makes up the largest gymnosperm phylum.
• Most conifers are evergreen, photosynthesizing year-round.

Angiosperms

• Angiosperms are seed plants with reproductive structures called flowers and fruits.
• Angiosperms are the most diverse and widespread of all plants and classified under a single phylum: Anthophyta
• A flower is a specialized shoot for sexual reproduction with modified leaves and includes Sepals, Petals, Stamens, and Carpels
• A carpel consists of an ovary which contains ovules and a style which holds up a stigma, where pollen is received
• A fruit consists of a mature ovary, which can include flower parts.
• Fruits protect seeds and aid in seed dispersal, and can be fleshy or dry.
• Fruit adaptations aid in seed dispersal by wind, water, or animals.
• Male gametophytes are pollen grain and are produced by the microsporangia of anthers
• Female gametophyte (embryo sac) develops within an ovule contained in an ovary at the base of the stigma
• Most flowers use cross-pollination between different plants of the same species.
• A pollen grain that lands on a stigma germinates, and the pollen tube grows down to the ovary.
• Sperm enters the ovule through the micropyle pore.
• Double fertilization occurs when the pollen tube releases two sperm into the female gametophyte within an ovule.
• One sperm fertilizes the egg, forming a zygote.
• The other sperm combines with two nuclei, initiating the development of food-storing endosperm, which nourishes the developing embryo.
• Within a seed, the embryo consists of a root and cotyledons.
• Angiosperm and gymnosperm ancestors diverged about 305 million years ago.
• Angiosperms may be related to Bennettitales, which are extinct seed plants with flowerlike structures
• Amborella and water lilies are descendants of ancient angiosperm lineages.
• The two main angiosperm groups include monocots (one cotyledon) and eudicots (two cotyledons).
• Over one-quarter of angiosperm species are monocots, and over two-thirds are eudicots.
• Pollination of flowers and seed transport by animals is an important relationship in terrestrial ecosystems.
• Clades with bilaterally symmetrical flowers tend to have more species than those with radially symmetrical flowers.
• Bilateral symmetry impacts pollinator movement, reduces gene flow

Humans Need for Seed Plants

• Seed plants are important for human survival.
• Plants are sources of food, fuel, wood products, and medicine.
• Angiosperms provide most of human food; just six crops account for 80% of consumed calories
• Modern crops are products of recent genetic change through artificial selection.
• Seed plants provide secondary compound for wood and medicines.
• Habitat destruction is causing extinction of many plant species.
• Loss of plant habitat is also causing loss of animals which rely on the plants for support.
• If habitat loss continues at current rates, 50% of Earth's species will become extinct within 100-200 years.