Transcript Seed - DavisonScience
Evolution of the seed Adaptations of seed plants • Seeds • Reduced gametophyte stage – seed plants reduced to microscopic (spores develop in sporangia of sporophyte) • Heterospory – Megaspores – make female gametophyte – Microsporangia – male gametophyte • Ovules = megaspore & integuments (protect) • Pollen = micro (male gametophyte) & sporopollenin Microsporangium: on anther Megasporangium • Ovary produces the ovule • Ovule parts: – INTEGUMENT – MACROSPORAGIA – FEMALE GAMETOPHYTE (egg) Seed • Seed : – Embryo: immature plant – Endosperm:food supply – Seed coat of integument • Ovule develops into seed • Cotyledons: 1 or 2 seed leaves • • • • Seed parts Embryo Endosperm Seed coat Seed • Double Fertilization – The union of two sperm cells with different nuclei of the embryo sac. • Endosperm – Food storing tissue of the seed. http://www.learner.org/channel/courses/essential/life/imag es/show4.open_seed.jpg Seed to Seedling • Imbibition is that state when the seed wakes up from dormancy and starts the intake of water. • It causes the seed to expand and ruptures, releasing the shoot, the cotyledons and the stalk. • It is the first sign of life after the dormant state. Seed to Seedling http://students.usm.maine.edu/deidre.rice/_ILLUS_ILT_T630888A.GIF Spore survival • Prior to seeds: spore was the only protected stage for plants. – Spore is single cell – Could survive: cold hot dry – Could be dispersed to another environment Spore Release Then, seeds • Seeds are multicellular • Has protective integument coat from ovule • Can be dormant for days months years Seed Release Vascular Seed plants • Gymnosperms: fir, pine • Angiosperm: flowering plants Pine tree • The pine tree is the sporophyte (2N) • Its sporangia are located on the pine cone • Heterosporous – Small pollen cones: carried by wind and critters – Large ovule cones Each tree has both types of cones Female - Male female male Angiosperm • Flowering plants: vascular and seeds • Most diverse and widespread plants • Flower: specialized for reproduction – Carried by wind: like gymnosperm – Carried by insects: more directed pollination • Fruit: protect seed and aid in dispersal – From ovary. Ovary thickens after pollination. Flower Structure • Four main organs: sepals, petals, stamens, and carpels • Stamens and carpels are reproductive • Carpel: • Stamen: filament, anther – Anther: stalk where pollen made • Sepals and petals: no repro organs – Stigma: collects pollen – Style: to ovary with 1 or more ovules – ovary – Pistil: multiple fused carpels http://images.google.com/imgres?imgurl=http://andromeda.cavehill.uwi.edu/ Gametophyte Development and Pollination • Sporangia- structure on – anther and – ovules where spores are produced • Pollen grains are made up of mature male gametophytes that are enclosed by a spore wall – Found in the microsporangia, or pollen sacs Hormones • Auxin – Stem elongation, root growth, develop fruit • Cytokinins: affect, cell division, cell differentiation, and apical dominance. • Gibberellins – Seed and bud germination • Abscisic acid – Inhibit root growth, close stomata • Ethylene – Fruit ripening Mechanisms that Prevent SelfFertilization • Sexual reproduction ensures that there will be genetic diversity among offspring • “Selfing” refers to self-fertilization in plants • Ensures that seed will develop Pollination Vs. Fertilization • Pollination is the transfer of pollen from anther to stigma (of other plant) – Results in formation of a pollen tube – Purpose is to grow down to ovary and release sperm within embryo sac – This fertilizes the egg – Embryo -> seed -> fruit containing seed – The fruit disperses seeds which germinate and develop into seedlings Development • Microsporocytes form four haploid microspores – These develop into haploid male gametophytes – The microspore goes through mitosis and cytokinesis – This results in a generative cell and tube cell which make up the pollen grain • Megasporocyte grows resulting in four haploid megaspores – May form a multicellular female gametophyte • Contains 3 cells: 1 egg and 2 synegrids • Synegrids attract and guide pollen tubes to the embryo sac • there are also two nuclei at the other end of the cell that are not separate and share cytoplasm embryo sacs • These result in two integuments that form a seed • Ensuring that the egg and sperm cells come from different parents is a mechanism that inhibits self fertilization • Dioecious plants cannot self fertilize because they have either staminate or carpellate flowers Avoid self fertilization • Flowers with functional stamen and carpels have organs that mature at different rates – An animal pollinator would not transfer pollen from the anther to a stigma of the same flower Self Incompatibility • The ability of a plant to reject its own pollen • Also, in some cases, the pollen of similar plants • If pollen were to land on the stigma of a flower on the same plant a biochemical would prevent the pollen from developing and fertilizing an egg Special Circumstances • Plant breeders will hybridize different crop varieties to combine the best traits and to get sufficient results • This can result in inbreeding • May cause plants that are able to self fertilize to lose that characteristic.