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1 Lecture 7 Outline (Ch. 38 – although some material is Ch. 30 in our text, pp 625-628) I. Flower Structures II. Flower Development IV. Pollination V. Gametophyte Production VI. Fertilization VII. Germination VIII. Preparation for next lecture 2 Angiosperm Overview Stamen Anther Stigma Carpel Style Filament A flower is a specialized shoot with up to 4 rings of modified leaves (sporophylls) Ovary Sepal Petal Receptacle (a) Structure of an idealized flower – not all flowers have all parts! 3 Angiosperm Overview Growth of flower (determinate) • Environmental signals eg. Day length, temperature • Internal signals eg. hormones When and how are flowers produced? Floral meristem identity genes Vegetative growth (indeterminate) 4 Model for Flowering (actually, end of ch. 35 in this text) • Flowering: adult meristem becoming a floral meristem – Activate or repress floral meristem identity genes • Cues lead to activation of floral organ identity genes – These define the four concentric whorls • Sepal, petal, stamen, and carpel ABC Model • • • • 3 classes of floral organ identity genes Specify 4 organ types Classes A and C mutually inhibitory When any one class is missing, aberrant floral organs occur in predictable positions 6 ABC Model 1. 2. 3. 4. Class A genes alone – Sepals Class A and B genes together – Petals Class B and C genes together – Stamens Class C genes alone – Carpels Mutant flower – some floral organs missing 7 Angiosperm Pollination Pollen = male gametophyte Ovule(s) = female gametophyte • Fertilization brings female and male gametophytes together • Fertilization (syngamy) is preceded by pollination, the placing of pollen on the stigma of the carpel One of my favorite pollinator systems: http://www.youtube.com/watch?v=-h8I3cqpgnA Think about how the mode of pollination compares with the number of pollen grains distributed, and how this compares with attracting specific pollinators! 8 Abiotic Pollination by Wind Hazel staminate flowers (stamens only) Hazel carpellate flower (carpels only) 9 Pollination by Bees Common dandelion under normal light Common dandelion under ultraviolet light 10 Pollination by Moths and Butterflies Anther Stigma Moth on yucca flower 11 Pollination by Flies Fly egg Blowfly on carrion flower 12 Pollination by Birds Hummingbird drinking nectar of poro flower 13 Pollination by Bats Long-nosed bat feeding on cactus flower at night 14 Angiosperm Lifecycle Germinated pollen grain (n) (male gametophyte) Anther Ovary Pollen tube Ovule Embryo sac (n) (female gametophyte) FERTILIZATION Egg (n) Sperm (n) Key Zygote (2n) Mature sporophyte plant (2n) Haploid (n) Diploid (2n) Germinating seed Seed Seed Embryo (2n) (sporophyte) (b) Simplified angiosperm life cycle Simple fruit 15 Angiosperm Gametophytes Female gametophytes: 100 m Haploid (n) Diploid (2n) Megasporangium Megasporocyte Integuments Micropyle Surviving megaspore Antipodal cells (3) Polar nuclei (2) Egg (1) Synergids (2) Female gametophyte (embryo sac) In megasporangium in an ovule in the Ovule ovary of the carpel MEIOSIS meiosis megaspores develop mitosis mature gametophyte MITOSIS Ovule Cells: 7 cells, 8 nuclei All in embryo sac Ovule is now the Integuments female gametophyte plus integuments Key to labels Embryo sac (LM) 16 Angiosperm Gametophytes Male gametophytes: Microsporangium (pollen sac) Microsporocyte MEIOSIS Microspores (4) Each of 4 microspores MITOSIS Male gametophyte (in pollen grain) Generative cell (will form 2 sperm) Nucleus of tube cell 20 m In microsporangium in an anther of the stamen meiosis produces 4 microspores Each microspore becomes a pollen grain mitosis mature gametophyte Cells: 2 cells generative cell (will form 2 sperm) inside the tube cell All in pollen grain Key to labels 75 m (LM) Ragweed pollen grain (colorized SEM) Haploid (n) Diploid (2n) 17 Angiosperm Pollination Fertilization • The pollen grain produces a pollen tube that extends down the style toward the embryo sac • Two sperm are released and effect a double fertilization, resulting in a diploid zygote and a triploid (3n) endosperm 18 Double Fertilization One sperm fuses with the egg – diploid (zygote) One sperm fuses with the two polar nuclei – triploid (endosperm) 19 Angiosperm Seed Formation • Ovule becomes a seed - embryo and supply of nutrients • embryo has apical meristems and one or two cotyledons • Mitosis of triploid endosperm gives rise to nutrient-rich mass Ovule Endosperm nucleus Integuments Zygote Cotyledons Plant embryo Zygote Seed coat Endosperm 20 The Mature Seed • The embryo and its food supply enclosed by a hard, protective seed coat Seed coat Epicotyl Hypocotyl Radicle Cotyledons • The seed enters a state of dormancy (a) Common garden bean, a eudicot with thick cotyledons • In dicots, the embryo has two cotyledons (seed leaves) Scutellum (cotyledon) Endosperm Epicotyl Hypocotyl • A monocot embryo has one cotyledon Radicle (c) Maize, a monocot 21 The Ovary ... • develops into a fruit adapted for seed dispersal • a fruit is a mature ovary that protects the enclosed seeds and aids in their dispersal via wind, water, or animals 22 Dispersal by Water Coconut 23 Dispersal by Wind Winged seed of Asian climbing gourd Dandelion “parachute” Winged fruit of maple Tumbleweed 24 Dispersal by Animals Barbed fruit Seeds carried to ant nest Seeds in feces Seeds buried in caches 25 Seed Germination (bean) Germination: seeds imbibe water – expands rupturing its coat, triggers metabolic changes causing embryo to resume growth Radicle (root) first to emerge Foliage leaves Next, the embryonic shoot Cotyledon breaks through the soil Epicotyl surface Hypocotyl Cotyledon Cotyledon Hypocotyl Radicle Seed coat 26 Things To Do After Lecture 7… Reading and Preparation: 1. Re-read today’s lecture, highlight all vocabulary you do not understand, and look up terms. 2. Ch. 38 Self-Quiz: # 1-4 (correct answers in back of book) 3. Read chapter 38, focus on material covered in lecture (terms, concepts, and figures!) 4. Skim next lecture. “HOMEWORK” (NOT COLLECTED – but things to think about for studying): 1. Compare and contrast methods of pollination and methods of seed dispersal used by angiosperms. 2. Explain the difference between pollination and fertilization. 3. Diagram the parts of an idealized flower with labels. 4. Describe the ABC model of flower development.
