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Plant Reproduction AP Biology Unit 5 Alternation of Generations • All land plants can exist in two different multicellular forms – Sporophyte (2n) – Gametophyte (n) • In a life cycle, plant generations alternate between sporophyte and gametophyte • Each form gives rise to the other form Sporophyte vs. Gametophyte • Sporophyte (2n) – Diploid, multicellular form of a plant – Forms haploid spores through meiosis • Gametophyte (n) – Haploid, multicellular form of a plant – Forms gametes through mitosis Alternation of Generation How is this different from other organisms? • In other organisms (like humans) the haploids are unicellular (sperm and egg) • In plants both forms (sporophyte and gametophyte) can be multicellular Evolution of Plant Reproduction • Each type of plant has a dominant form (sporophyte or gametophyte) • Dominant form = the “plant” we think of • As plants have evolved, there have been changes to relationship between sporophyte and gametophyte Gametophyte/Sporophyte Relationships • 3 different possibilities – Larger gametophyte, sporophyte is dependent on it – Larger sporophyte, gametophyte is independent – Larger sporophyte, gametophyte is dependent Question… • What trend do you see between the evolution of plants (from nonvascular vascular; nonseed seed plants) and the alternation of generations? – As plants have evolved, the gametophyte has become more reduced, sporophyte has become more dominant Reproductive Organs • Archegonium – Female sex organ produces egg • Antheridium – Male sex organ produces sperm Vascular Plants: Homosporous vs. Heterosporous • Vascular plants have sporophylls – modified leaves that produce sporangia (which become spores) • Homosporous plants – Most ferns • Heterosporous plants – All seed plants (angiosperms and gymnosperms) and some seedless plants Water vs. Wind • Mosses and Ferns require water to help the sperm reach the egg. • Seed plants no longer require water to reproduce – Pollen (male gametophyte) can be carried by wind, animals, etc. Gymnosperm Life Cycle • Sporophyte is the dominant form • Heterosporous • Have “naked” seeds – nothing around the seed • Cones are either male (contain pollen) or female (contain ovule) Flower Anatomy • The flower is the reproductive organ of an angiosperm • Female part = Carpel – Consists of the stigma, style, and ovary • Male part = Stamen – Consists of anther and filament Angiosperms • Dominant form is the sporophyte • Heterosporous – Male gametophyte = pollen grain – Female gametophyte = embryo sac Female Gametophyte • The megaspore will go through MITOSIS to create a series of different cells in the female gametophyte • Cells involved in fertilization are – Egg (n)– haploid – Polar nuclei (2n) – 2 nuclei in one cell When the pollen grain lands on the stigma, it will extend a pollen tube down the style to get to the egg Specific cells in the pollen tube go through mitosis to form 2 sperm Sperm will enter Double Fertilization • Angiosperm fertilization actually consists of 2 separate fertilization events = double fertilization • Both sperm in the pollen tube are used – One sperm fertilizes the egg diploid zygote (2n) forms – The other sperm fertilizes the polar nuclei (2n) forms triploid endosperm (3n) Double Fertilization • Double fertilization is unique to angiosperms • Through this process, resources are used most efficiently since endosperm can only develop in a fertilized ovule. • Endosperm = starchy material that serves as nutrition for the growing embryo Other Post-Fertilization events • Embryo develops (diploid) • Seed coat forms • Ovary develops into fruit around the seed Germination and Seedling Formation • Seeds are dormant until they begin germination • When conditions are right, seeds will begin to germinate to form a seedling. • Germination starts with imbibition—when water enters the seed and triggers enzymes that are needed for growth. Seed Benefits • What are the evolutionary benefits of seeds? – Dispersal- embryo can travel far from parent – Protection- seed coat protects embryo – Dormancy- can wait until conditions are favorable to germinate – Storage- food for the embryo can be stored inside seed (ex. endosperm) Monocots vs. Eudicots • Most angiosperms can be classified as either Monocots or Eudicots