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The Evolution of Seed Plants: Advantages, Diversity, and Feeding the World

Discover how seeds changed the course of plant evolution and enabled them to become the dominant producers in terrestrial ecosystems. Explore the common characteristics of seed plants and the advantages of reduced gametophytes. Learn about the evolution of seeds, ovules, and pollen, as well as the four phyla of gymnosperms.

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The Evolution of Seed Plants: Advantages, Diversity, and Feeding the World

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  1. Biology 102 Week 4 Plant Diversity II: The Evolution of Seed Plants Fungi

  2. Overview: Feeding the World • Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems

  3. Concept 30.1: The reduced gametophytes of seed plants are protected in ovules and pollen grains • In addition to seeds, the following are common to all seed plants: • Reduced gametophytes • Heterospory • Ovules • Pollen

  4. Advantages of Reduced Gametophytes • The gametophytes of seed plants develop within the walls of spores retained within tissues of the parent sporophyte

  5. LE 30-2 Sporophyte (2n) Sporophyte (2n) Gametophyte (n) Gametophyte (n) Sporophyte dependent on gametophyte (mosses and other bryophytes) Large sporophyte and small, independent game-tophyte (ferns and other seedless vascular plants) Microscopic female gametophytes (n) in ovulate cones (dependent) Sporophyte (2n), the flowering plant (independent) Microscopic male gametophytes (n) in inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Microscopic female gametophytes (n) in inside these parts of flowers (dependent) Sporophyte (2n), (independent) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms)

  6. Heterospory: The Rule Among Seed Plants • Seed plants evolved from plants with megasporangia, which produce megaspores that give rise to female gametophytes • Seed plants evolved from plants with microsporangia, which produce microspores that give rise to male gametophytes

  7. Ovules and Production of Eggs • An ovule consists of a megasporangium, megaspore, and one or more protective integuments • Gymnosperm megaspores have one integument • Angiosperm megaspores usually have two integuments

  8. LE 30-3 Seed coat (derived from integument) Integument Female gametophyte (n) Spore wall 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

  9. Pollen and Production of Sperm • Microspores develop into pollen grains, which contain the male gametophytes • Pollination is the transfer of pollen to the part of a seed plant containing the ovules • Pollen can be dispersed by air or animals, eliminating the water requirement for fertilization • If a pollen grain germinates, it gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

  10. The Evolutionary Advantage of Seeds • A seed develops from the whole ovule • A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat

  11. Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones • The gymnosperms include four phyla: • Cycadophyta (cycads) • Gingkophyta (one living species: Ginkgo biloba) • Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia) • Coniferophyta (conifers, such as pine, fir, and redwood)

  12. LE 30-4aa Cycas revoluta

  13. LE 30-4ab

  14. LE 30-4ac

  15. LE 30-4ad Gnetum. This genus includes about 35 species of tropical trees, shrubs, and vines, mainly native to Africa and Asia. Their leaves look similar to those of flowering plants, and their seeds look somewhat like fruits.

  16. LE 30-4ae Ephedra. This genus includes about 40 species that inhabit arid regions throughout the world. Known in North America as “Mormon tea,” these desert shrubs produce the compound ephedrine, commonly used as a decongestant.

  17. LE 30-4af Welwitschia. This genus consists of one species Welwitschia mirabilis, a plant that lives only in the deserts of southwestern Africa. Its strap like leaves are among the largest known.

  18. LE 30-4ag Ovulate cones

  19. LE 30-4ba Douglas fir. “Doug fir” (Pseudotsuga menziesii) provides more timber than any other North American tree species. Some uses include house framing, plywood, pulpwood for paper, railroad ties, and boxes and crates.

  20. LE 30-4bb Pacific yew. The bark of Pacific yew (Taxa brevifolia) is a source of taxol, a compound used to treat women with ovarian cancer. The leaves of a European yew species produce a similar compound, which can be harvested without destroying the plants. Pharmaceutical companies are now refining techniques for synthesizing drugs with taxol-like properties.

  21. LE 30-4bc Bristlecone pine. This species (Pinus longaeva), which is found in the White Mountains of California, includes some of the oldest living organisms, reaching ages of more than 4,600 years. One tree (not shown here) is called Methuselah because it may be the world’s oldest living tree. In order to protect the tree, scientists keep its location a secret.

  22. LE 30-4bd Sequoia. This giant sequoia (Sequoiadendron giganteum), in California’s Sequoia National Park weighs about 2,500 metric tons, equivalent to about 24 blue whales (the largest animals), or 40,000 people. Giant sequoias are the largest living organisms and also some of the most ancient, with some estimated to be between 1,800 and 2,700 years old. Their cousins, the coast redwoods (Sequoia sempervirens), grow to heights of more than 110 meters (taller than the Statue of Liberty) and are found only in a narrow coastal strip of northern California.

  23. LE 30-4be Common juniper. The “berries” of the common juniper (Juniperus communis), are actually ovule-producing cones consisting of fleshy sporophylls.

  24. LE 30-4bf Wollemia pine. Survivors of a confer group once known only from fossils, living Wollemia pines (Wollemia nobilis), were discovered in 1994 in a national park only 150 kilometers from Sydney, Australia. The species consists of just 40 known individuals two small groves. The inset photo compares the leaves of this “living fossil” with actual fossils.

  25. Gymnosperm Evolution • Fossil evidence reveals that by the late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants

  26. Gymnosperms appear early in the fossil record and dominated the Mesozoic terrestrial ecosystems • Living seed plants can be divided into two clades: gymnosperms and angiosperms

  27. A Closer Look at the Life Cycle of a Pine • Key features of the gymnosperm life cycle: • Dominance of the sporophyte generation • Development of seeds from fertilized ovules • The transfer of sperm to ovules by pollen • The life cycle of a pine is an example Animation: Pine Life Cycle

  28. LE 30-6_3 Key Haploid (n) Diploid (2n) Ovule Ovulate cone Megasporocyte (2n) Integument Longitudinal section of ovulate cone Micropyle Pollen cone Megasporangium Mature sporophyte (2n) Microsporocytes (2n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS MEIOSIS Surviving megaspore (n) Longitudinal section of pollen cone Sporophyll Microsporangium Seedling Germinating pollen grain Archegonium Integument Egg (n) Female gametophyte Seeds on surface of ovulate scale Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) Seed coat (derived from parent sporophyte) (2n) Discharged sperm nucleus (n) Pollen tube Embryo (new sporophyte) (2n) FERTILIZATION Egg nucleus (n)

  29. Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits • Angiosperms are flowering plants • These seed plants have reproductive structures called flowers and fruits • They are the most widespread and diverse of all plants

  30. Characteristics of Angiosperms • All angiosperms are classified in a single phylum, Anthophyta • The name comes from the Greek anthos, flower

  31. Flowers • The flower is an angiosperm structure specialized for sexual reproduction • A flower is a specialized shoot with up to four types of modified leaves: • Sepals, which enclose the flower • Petals, which are brightly colored and attract pollinators • Stamens, which produce pollen • Carpels, which produce ovules

  32. LE 30-7 Stigma Carpel Stamen Anther Style Filament Ovary Petal Sepal Ovule Receptacle

  33. Video: Flower Blooming (time lapse)

  34. Fruits • A fruit typically consists of a mature ovary but can also include other flower parts • Fruits protect seeds and aid in their dispersal • Mature fruits can be either fleshy or dry Animation: Fruit Development

  35. LE 30-8 Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp Tomato, a fleshy fruit with soft outer and inner layers of pericarp Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp Walnut, a dry fruit that remains closed at maturity Milkweed, a dry fruit that splits open at maturity

  36. Various fruit adaptations help disperse seeds • Seeds can be carried by wind, water, or animals to new locations

  37. LE 30-9 Wings enable maple fruits to be easily carried by the wind. Seeds within berries and other edible fruits are often dispersed in animal feces. The barbs of cockleburs facilitate seed dispersal by allowing these fruits to hitchhike on animals.

  38. The Angiosperm Life Cycle • In the angiosperm life cycle, double fertilization occurs when a 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 • The endosperm nourishes the developing embryo

  39. LE 30-10a Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on Sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary MEIOSIS Megasporangium (n) Surviving megaspore (n) Antipodal cells Female gametophyte (embryo sac) Pollen tube Polar nuclei Synergids Eggs (n) Sperm (n)

  40. LE 30-10b Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary Pollen grains MEIOSIS Stigma Pollen tube Megasporangium (n) Sperm Surviving megaspore (n) Pollen tube Style Antipodal cells Female gametophyte (embryo sac) Pollen tube Polar nuclei Synergids Eggs (n) Sperm (n) Eggs nucleus (n) Discharged sperm nuclei (n)

  41. LE 30-10c Key Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary Pollen grains Germinating seed MEIOSIS Stigma Pollen tube Megasporangium (n) Sperm Embryo (2n) Surviving megaspore (n) Endosperm (food supply) (3n) Seed Pollen tube Seed coat (2n) Style Antipodal cells Female gametophyte (embryo sac) Pollen tube Polar nuclei Synergids Eggs (n) Sperm (n) Zygote (2n) Nucleus of developing endosperm (3n) Eggs nucleus (n) FERTILIZATION Discharged sperm nuclei (n)

  42. Animation: Plant Fertilization Animation: Seed Development Video: Flowering Plant Life Cycle (time lapse)

  43. Angiosperm Evolution • Clarifying the origin and diversification of angiosperms poses fascinating challenges to evolutionary biologists • Angiosperms originated at least 140 million years ago • During the late Mesozoic, the major branches of the clade diverged from their common ancestor

  44. Fossil Angiosperms • Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits • Archaefructus sinensis, for example, has anthers and seeds but lacks petals and sepals

  45. LE 30-11 Carpel Stamen 5 cm Archaefructus sinensis, a 125-million-year-old fossil Artist’s reconstruction of Archaefructus sinensis

  46. An “Evo-Devo” Hypothesis of Flower Origins • Scientist Michael Frohlich hypothesized how pollen-producing and ovule-producing structures were combined into a single flower • He proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures

  47. Angiosperm Diversity • The two main groups of angiosperms are monocots and eudicots • Basal angiosperms are less derived and include the flowering plants belonging to the oldest lineages • Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots

  48. LE 30-12aa BASAL ANGIOSPERMS Amborella trichopoda Star anise (Illicium floridanum) Water lily (Nymphaea “Rene Gerald”)

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