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Angiosperms: Phylum Anthophyta, the flowering plants Figure 29.7 Land plant evolution. 1. Overview of seed plant evolution 2. Traits of flowering plants a) Flowers b) Fruits/Seeds c) Monocots vrs. dicots 3. The angiosperm life cycle 4. Pollen and seed (fruit) dispersal Three variations on gametophyte/sporophyte relationships. Figure 30.3 Review: Development of a seed from an unfertilized ovule. The three most important new adaptations to land found in the seed plants are all shown in this diagram: In angiosperms, female gamteophyte is reduced even further than in gymnosperms. 1. Very small gametophytes that are nourished by and protected inside the parental sporophyte (reduced even further in angiosperms) 2. Pollen grains, which provide protection and dispersal for the male gametophyte (often animal-dispersed in angiosperms) 3. The seed, which protects and disperses the new sporophyte embryo (angiosperms: develop in flowers, dispersed in fruits) Hypothetical phylogeny of the seed plants • Angiosperms – Are commonly known as flowering plants – Are seed plants that produce the reproductive structures called flowers and fruits – Are the most widespread and diverse of all plants 1 Angiosperm evolution • First angiopserms appeared during Mesozoic, ~ 140 mya • By the end of the mesozoic (~65 mya), angiosperms dominated many landscapes • Coevolution between plants and animals very important – Herbivores – Pollinators – Seed (often via fruit) dispersers • In one view, the most primitive angiosperm resembled a Magnolia Magnolias Amborella • The most primitive angiosperm has moderate sized, perfect flowers, • Parts are spirally arranged and have a moderate number of parts. • Floral evolution moves towards unisexual flowers, larger AND smaller flower sizes, other perianth arrangements, and larger AND smaller numbers of flower parts. Hypothesis for the evolutionary origin of the carpel from a reproductive leaf (sporophyll) • had large, showy, unspecialized, spirally arranged flowers. • No fusion of parts • little differentiation of parts. The ‘Mostly Male’ hypothesis: • Angiosperms evolved from male gymnosperms • Pollen and ovule producing structures combined into a single flower: mutation • Mutation: ovules developed on some microsporophylls = carpels • Evidence: Flower development genes are similar to pollen producing gymnosperm genes • Position of ovules can be easily changed with mutations Angiosperms: Phylum Anthophyta, the flowering plants 1. Overview of seed plant evolution 2. Traits of flowering plants 3. The angiosperm life cycle 4. Pollen and seed (fruit) dispersal 2 Traits of angiosperms • Vascular seed plants that produce flowers and fruits • Extremely diverse: ~250,000 known species (compare to 720 gymnosperm spp.) • Xylem tissue not only has tracheids, but also fibers and vessel elements (except Amborella) • Life cycle includes “double fertilization” Flowers • A flower is a specialized shoot with modified leaves – Sepals, which enclose the flower – Petals, which are brightly colored and attract pollinators – Stamens, which produce pollen – Carpels, which produce ovules Carpel Stigma Anther Stamen Style Ovary Filament Petal Sepal Figure 30.7 Receptacle Ovule Fruits • Fruits – Typically consist of a mature ovary (a) Tomato, a fleshy fruit with soft outer and inner layers of pericarp (b) Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp • Can be carried by wind, water, or animals to new locations, enhancing seed dispersal (a) Wings enable maple fruits to be easily carried by the wind. (b) Seeds within berries and other edible fruits are often dispersed in animal feces. (c) Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp Figure 30.8a–e (d) Milkweed, a dry fruit that splits open at maturity (e) Walnut, a dry fruit that remains closed at maturity Figure 36.2 An overview of transport in whole plants (Layer 4) Two kinds of vascular tissue facilitate flow through plants: 1. Xylem, which conducts water and mineral nutrients 2. Phloem, which conducts sugars (c) The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals. Figure 30.9a–c Cells in the xylem tissue of angiosperms In gymnosperms, the only cell type found in xylem tissue is tracheids. Angiosperms (except Amborella) have both tracheids and vessel elements for conduction, as well as fiber cells for support. 3 Figure 38.6 Growth of the pollen tube and double fertilization Figure 38.6 Growth of the pollen tube and double fertilization Figure 38.6 Growth of the pollen tube and double fertilization Angiosperm seeds consist of diploid and triploid tissues Embryo: Diploid (from fertilized egg) Food Supply: Triploid Endosperm (from polar nuclei and second sperm) Seed Coat: Diploid (from ovule wall) Angiosperm seeds consist of diploid and triploid tissues Cotyledons (“seed leaves”) Why double fertilization? • According to one hypothesis, double fertilization synchronizes development of food supply (endosperm) with development of the embryo: if no fertilization of the egg occurs, no resources are wasted on building the food supply 4 Figure 30.7 The structure of a flower. Flower = modified reproductive shoot with 4 circles (“whorls”) of modified leaves When brightly colored, aid in More flower terminology All petals together = “corolla” attracting animal pollinators “Male” parts, produce microspores, which develop into pollen grains All stamens together = “androecium” “Female” parts, produce megaspores & thus female gametophytes, and ultimately ovules and seeds How does this differ from the reproductive structures of the other seed plants, the gymnosperms? All carpels together = “pistil” or “gynoecium” Ovary wall (pericarp) grows into fruit Protects flower before it opens Ovules found in cavity = “locule” All sepals together = “calyx” Trillium: a complete flower (has all 4 basic floral organs) Flower terminology: symmetry Grass flower: incomplete (lacks petals) Bisexual (perfect) flowers have both stamens and carpels, like this Lily Note: most grasses wind-pollinated Why might petals be unimportant for grasses? 5 Begonia, a monoecious species with unisexual (imperfect) flowers. (staminate flowers (left), carpellate flowers (right). Begonia, a monoecious species with unisexual (imperfect) flowers. (staminate flowers (left), carpellate flowers (right). In monoecious species, both sexes of flowers are found on a single plant. Begonia, a monoecious species with unisexual (imperfect) flowers. (staminate flowers (left), carpellate flowers (right). Sagittaria: a dioecious species, with staminate flowers (left), and carpellate flowers (right) growing on separate plants. In monoecious species, both sexes of flowers are found on a single plant. Why are the carpels and stamens similarly colored yellow? Grape hyacinth has flowers clustered in inflorescences. A solitary flower grows on a stalk called a peduncle Individual flowers in an inflorescence grow on pedicels The whole inflorescence is attached to a peduncle 6 Pyrethrum, a composite flower (a special kind of inflorescence). Notice the two types of flowers, disc flowers (center) and ray flowers (at edge, each with a petal). Hypogynous Epigynous Perigynous Figure 30.12 Representatives of major angiosperm clades Figure 30.12 Representatives of major angiosperm clades •Flowering plants used to be divided into two groups: monocots and dicots •Flowering plants used to be divided into two groups: monocots and dicots •We now know that there are a number of different lineages that have the dicot morphology •We now know that there are a number of different lineages that have the dicot morphology •Monocots are a monophyletic group •Monocots are a monophyletic group Figure 30.12 Monocots vs. Dicots Figure 30.12 Monocots vs. Dicots 7 Monocots vs. non-monocots (“dicots” = eudicots and other lineages) • Monocots (incl. lilies, orchids, yuccas, palms, grasses) usually have parallel veins in leaves, a single cotyledon, fibrous root systems, floral parts in multiples of three, and complexly arranged vascular bundles in stem Even more flower terminology • #-merous: having parts # of each kind. E.g. 3merous = flower parts in 3’s. • a-: lacking. E.g. apetalous = no petals. • Floral part arrangement: • Dicots usually have net-like venation, two cotyledons, a taproot system, floral parts in multiples of four or five, and vascular bundles arranged in a ring in stem – Whorled: parts in a circle, on a single plane – Spiral: parts spiral around floral axis or receptacle Figure 38.2 Simplified overview of angiosperm life cycle Review of flower terminology • • • • 4 floral organs: sepal, petal, stamen, carpel radial vs. bilateral symmetry complete vs. incomplete flowers bisexual (perfect) vs. unisexual (imperfect) flowers • monoecious vs. dioecious plants • inflorescences and composite flowers • ovary position Relationship between a pea flower and a fruit (pea pod). Fruit = mature ovary. Fruits protect seeds and aid in their dispersal. Endosperm-derived foods 8 Figure 38.7 The development of a dicot plant embryo Why double fertilization? • According to one hypothesis, double fertilization synchronizes development of food supply (endosperm) with development of the embryo: if no fertilization of the egg occurs, no resources are wasted on building the food supply Figure 38.7 The development of a dicot plant embryo Figure 38.7 The development of a dicot plant embryo Figure 38.7 The development of a dicot plant embryo Figure 30.10 Detailed life cycle of an angiosperm 9 Seed adaptations for survival and germination Figure 38. 8 Review: Three types of seed structure • Many seeds exhibit dormancy, a temporary condition of low metabolism and no growth or development. Some seeds can survive like this for decades or more. What are the potential benefits of dormancy? • Dormancy in some seeds is simply broken by favorable environmental conditions, but others only germinate after specific cues • What would you expect the cues to be for seeds living in deserts, fire-prone habitats (such as California chaparral), or at high latitudes? How about for seeds borne in berries eaten by mammals? Figure 38. 8 Review: Three types of seed structure The four steps of seed germination: 1. imbibition of water, 2. enzyme digestion of stored food, 3. embryo begins growth and radicle is pushed through the seed coat, and 4. shoot tip grows toward soil surface. Figure 38. 8 Review: Three types of seed structure Figure 38.10 Two ways that young shoots break through the soil surface. Germination of a barley seed is shown below. 10