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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. Life Cycle VI. Gametophyte Production VII. Fertilization VIII. Germination IX. Preparation for next lecture 2 Angiosperm Overview Stamen Anther Stigma Carpel Style A flower is a specialized shoot with up to 4 rings of modified leaves (sporophylls) Filament 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 ABC Model 8 Based on the ABC model for flower development, if ‘A’ class genes are missing, what develops? A. B. C. D. sepal, sepal, stamen, carpel petal, petal, stamen, carpel stamen, petal, petal, stamen carpel, stamen, stamen, carpel Looking at the images below, which class of genes is deficient in the mutant? 1. 2. 3. 4. 5. Class A Class B Class C Classes A & B Classes B & C Male structure Pollen = male gametophyte Ovule(s) = female gametophyte Female structure 11 Angiosperm Pollination • 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! 12 Abiotic Pollination by Wind Hazel staminate flowers (stamens only) Hazel carpellate flower (carpels only) 13 Pollination by Bees Common dandelion under normal light Common dandelion under ultraviolet light 14 Pollination by Moths and Butterflies Anther Stigma Moth on yucca flower 15 Pollination by Flies Fly egg Blowfly on carrion flower 16 Pollination by Birds Hummingbird drinking nectar of poro flower 17 Pollination by Bats Long-nosed bat feeding on cactus flower at night 18 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 19 Angiosperm Gametophytes • Develop in anthers, ovaries • Pollen: from microspores inside the anther • Within an ovule, a haploid megaspore divides by mitosis - forms the embryo sac, the female gametophyte 20 Angiosperm Gametophytes • Female gametophytes: In a megasporangium in an ovule in the ovary of the carpel meiosis megaspore mother cells gives rise to megaspores mitosis mature gametophyte • Cells: 7 cells and 8 nuclei (3 antipodal; 1 endosperm mother cell with 2 nuclei, 1 egg and 2 synergids) All in embryo sac Ovule is now the female gametophyte plus integuments 21 Angiosperm Gametophytes 22 Angiosperm Gametophytes • Male gametophytes: In a microsporangium in an anther of the stamen meiosis microspore mother cells give rise to 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 23 The megaspore mother cell gives rise to: 1. 2. 3. 4. 5. pollen petals egg cells seeds ovaries 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 25 Double Fertilization One sperm fuses with the egg – diploid (zygote) One sperm fuses with the two polar nuclei – triploid (endosperm) 26 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 27 Double fertilization refers to: 1. Two sperm fuse with the egg cell 2. Two sperm fuse with the polar nuclei 3. One sperm fuses with the egg, one with the polar nuclei 4. One sperm fuses with the endosperm, one with the tube cell 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 29 Dispersal by Water Coconut 30 Dispersal by Wind Winged seed of Asian climbing gourd Dandelion “parachute” Winged fruit of maple Tumbleweed 31 Dispersal by Animals Barbed fruit Seeds carried to ant nest Seeds in feces Seeds buried in caches 32 The Mature Seed Seed coat Epicotyl Hypocotyl • The embryo and its food supply enclosed by a hard, protective seed coat Radicle Cotyledons (a) Common garden bean, a eudicot with thick cotyledons Seed coat Endosperm • The seed enters a state of dormancy • In dicots, the embryo has two cotyledons (seed leaves) Cotyledons Epicotyl Hypocotyl Radicle (b) Castor bean, a eudicot with thin cotyledons • A monocot embryo has one cotyledon Scutellum (cotyledon) Pericarp fused with seed coat Coleoptile Endosperm Epicotyl Hypocotyl Coleorhiza Radicle 33 (c) Maize, a monocot Evolutionary Adaptations ... • the process of germination increases the probability that seedlings will survive • Germination begins when seeds imbibe water – this expands the seed, rupturing its coat, and triggers metabolic changes that cause the embryo to resume growth • The embryonic root, or radicle, is the first structure to emerge from the germinating seed • Next, the embryonic shoot breaks through the soil surface 34 Seed Germination (bean) (a) Common garden bean Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Radicle Seed coat 35 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.