Transcript seed coat
1 Lecture 8 Outline (Ch. 42) I. Flower Structures II. Life Cycle III. Gametophyte Production IV. Flower Development IV. Pollination V. Fertilization VI. Germination 2 Angiosperm Overview Stamen Anther Stigma Carpel Style Filament Ovary Sepal Petal Receptacle (a) Structure of an idealized flower 3 Angiosperm Overview • In alternation of generations in angiosperms, the dominant stage is the diploid sporophyte • Spores develop inside the flower into tiny, haploid gametophytes: – the male pollen grain and the female embryo sac 4 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 5 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 6 Model for Flowering • Flowering leads to an adult meristem becoming a floral meristem – Activate or repress the inhibition of floral meristem identity genes • 2 key genes: LFY and AP1 – Turn on floral organ identity genes – Define the four concentric whorls • Sepal, petal, stamen, and carpel 7 Model for Flowering 8 ABC Model • 3 classes of floral organ identity genes specify 4 organ types 1. Class A genes alone – Sepals 2. Class A and B genes together – Petals 3. Class B and C genes together – Stamens 4. Class C genes alone – Carpels • When any one class is missing, aberrant floral organs occur in predictable positions 9 ABC Model 10 ABC Model 11 Male structure Female structure 12 Angiosperm Pollination • brings female and male gametophytes together • Fertilization (syngamy) is preceded by pollination, the placing of pollen on the stigma of the carpel 13 Abiotic Pollination by Wind Hazel staminate flowers (stamens only) Hazel carpellate flower (carpels only) 14 Pollination by Bees Common dandelion under normal light Common dandelion under ultraviolet light 15 Pollination by Moths and Butterflies Anther Stigma Moth on yucca flower 16 Pollination by Flies Fly egg Blowfly on carrion flower 17 Pollination by Birds Hummingbird drinking nectar of poro flower 18 Pollination by Bats Long-nosed bat feeding on cactus flower at night 19 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 20 Angiosperm Fertilization 21 Angiosperm Seed Formation • develops into a seed containing a sporophyte embryo and a supply of nutrients • The zygote gives rise to an embryo with apical meristems and one or two cotyledons • Mitosis of the triploid (3n) endosperm gives rise to a multicellular, nutrient-rich mass that feeds the developing embryo and later (in some plants) the young seedling 22 Angiosperm Seed Formation 23 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 24 Dispersal by Water Coconut 25 Dispersal by Wind Winged seed of Asian climbing gourd Dandelion “parachute” Winged fruit of maple Tumbleweed 26 Dispersal by Animals Barbed fruit Seeds carried to ant nest Seeds in feces Seeds buried in caches 27 The Mature Seed • The embryo and its food supply are enclosed by a hard, protective seed coat • The seed enters a state of dormancy • In some eudicots, such as the common garden bean, the embryo consists of the embryonic axis attached to two thick cotyledons (seed leaves) • A monocot embryo has one cotyledon 28 Seed coat Epicotyl Hypocotyl Angiosperm Seeds Radicle Cotyledons (a) Common garden bean, a eudicot with thick cotyledons Seed coat Endosperm Cotyledons Epicotyl Hypocotyl Radicle (b) Castor bean, a eudicot with thin cotyledons Scutellum (cotyledon) Pericarp fused with seed coat Coleoptile Endosperm Epicotyl Hypocotyl Coleorhiza Radicle 29 (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 30 Seed Germination (bean) (a) Common garden bean Foliage leaves Cotyledon Epicotyl Hypocotyl Cotyledon Cotyledon Hypocotyl Hypocotyl Radicle Seed coat 31 Vegetative Reproduction & Agriculture • Humans have devised various methods for asexual propagation of angiosperms • Cuttings can be taken from many kinds of plants – They are asexually reproduced from plant fragments • Grafting is a modification of vegetative reproduction from cuttings – A twig or bud from one plant can be grafted onto a plant of a closely related species or a different variety of the same species 32 Self-Check Part of plant Function Later becomes… Examples: Ovary Contains eggs (ovules) Flesh of fruit Apples, strawberries, coconut meat Pollen Ovule Integuments Cotyledons Endosperm 33 Lecture 8 Summary 1. 2. 3. Parts of a flower (Ch. 42) Gamete Formation (Ch. 42) Flowering and flower development (Ch. 42) ABC Model 4. Pollination (Ch. 42) Modes Events 5. Fertilization (Ch. 42) - Steps: what happens to pollen and in ovules 6. Fruit/Seed (Ch. 42) Development Germination