evolution of the seed. adaptations of seed plants seeds reduced gametophyte stage – also seen in...
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Evolution of the seed
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Adaptations of seed plants
• Seeds• Reduced gametophyte stage
– Also seen in seedless vascular– 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
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Microsporangium: on anther
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Megasporangium
• Ovary produces the ovule• Ovule parts:
– INTEGUMENT– MACROSPORAGIA– FEMALE GAMETOPHYTE (egg)
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Seed
• Seed :– Embryo: immature plant– Endosperm:food supply – Seed coat of integument
• Ovule develops into seed• Cotyledons: 1 or 2 seed leaves
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• Seed parts• Embryo• Endosperm• Seed coat
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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/images/show4.open_seed.jpg
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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.
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Seed to Seedlinghttp://students.usm.maine.edu/deidre.rice/_ILLUS_ILT_T630888A.GIF
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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
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Spore Release
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Then, seeds
• Seeds are multicellular• Has protective integument coat from ovule• Can be dormant for days months years
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Seed Release
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Vascular Seed plants
• Gymnosperms: fir, pine• Angiosperm: flowering plants
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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
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Female - Male
female male
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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.
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Flower Structure
• Four main organs: sepals, petals, stamens, and carpels
• Stamens and carpels are reproductive
• Carpel: – Stigma: collects pollen– Style: to ovary with 1 or
more ovules – ovary– Pistil: multiple fused carpels
• Stamen: filament, anther– Anther: stalk where pollen
made
• Sepals and petals: no repro organs
http://images.google.com/imgres?imgurl=http://andromeda.cavehill.uwi.edu/
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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
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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
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Mechanisms that Prevent Self-Fertilization
• Sexual reproduction ensures that there will be genetic diversity among offspring
• “Selfing” refers to self-fertilization in plants• Ensures that seed will develop
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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
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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
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• 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
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• 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
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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
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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
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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.