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Transcript
VASCULAR SEED PLANTS GYMNOSPERMS GYMNOSPERMS Although Earth’s ancient forest were dominated by spore-producing vascular plants, early seed bearing plants also evolved. 280 mya Earth’s climate changed Long periods of drought and freezing weather caused many spore bearing plants to become extinct. A few seed-bearing plants had adaptations to survive WHAT ARE GYMNOSPERMS? Gymnosperm = vascular plants that produce seeds on the scales of woody cones Seeds are not protected by a fruit They do not produce flowers 4 Divisions that bear naked seeds: 1. Cycadophyta 2. Ginkophyta 3. Gnetophyta 4. Coniferophyta REPRODUCTION: Gymnosperm produce spores Microspores = produced in the male cone and give rise to the male gametophyte Eventually develop into pollen grains Megaspores = produced in female cone Give rise to female gametophyte Each contained in an ovule and produces archegonia with egg cells Pollen is carried by the wind to the ovule The gametophyte contained in the pollen grain produces a pollen tube This grows into the archegonium and provides a path for the sperm to reach the egg. Occurs without water After fertilization, the zygote develops into an embryo Embryo = an organism at an early stage of growth and development In plants, the embryo is the young, diploid sporophyte of a plant A embryo can have many cotyledons Cotyledons = food storage organs in a plant embryo that becomes the plants first leaves. As the embryo develops, the tissues of the ovule form the food supply and seed coat of the pine seed. Advantages of Seeds Over Spores: A seed consists of an embryo with a food supply enclosed in a tough protective coat The seed contains a supply of food to nourish the young plant in early growth stages. This food is used until the first leaves develop (photosynthesis) Food supply is stored in cotyledons Embryo is protected by a tough seed coat Seeds are also adapted for easy dispersal Young don’t have to compete with parent: sunlight, water, nutrients, space Advantage: Fertilization does not require water Spore producing plants need a film of water for sperm to swim In gymnosperms, male gametophyte develops inside a “pollen grain” Pollen grain = includes sperm cells, nutrients, and a protective outer covering Female gametophyte develops inside a ovule Ovule = contains megaspore cell, one or 2 layers of tissue and a protective covering Sperm is carried by the wind instead of water GINKOPHYTA Most numerous during the Jurassic Period 200 mya Most died out about 65 mya (with dinosaurs) Only one living species Ginko biloba Tree size Sacred in China and Japan Asian temple gardens seem to have prevented the tree from becoming extinct Male and Female cones on separate plants Male = pollen in strobilus like cones Grow near bases of leaf clusters Female = bears seeds Orange-yellow “cherries” Seed coat has an unpleasant smell Most gardeners plant the male CONIFEROPHYTA Conifers Largest and most diverse divisions of gymnosperms Trees and Shrubs Needle or scalelike leaves Most produce seeds in woody cones Pine, Spruce, Juniper, Cedar, Redwood, Sequoia Fist emerged around 280 mya During Jurassic (150 mya) prominent forest Remain prominent today Adapted to cold climates Form large forests Dry climates Under dry freezing conditions, roots cannot absorb water from soil Needlelike leaves are covered with a thick cuticle Cuticle = protective, waxy layer (prevent water lose) Have sunken stomata (openings for gas exchange) Help retain water Bark also helps reduce water loss by forming a protective covering over the stem EVERGREENS: = retain their leaves all year Most are evergreens Individual leaves drop off as age or damaged Tree never loses all its leaves at once Pine needles remain 2-40 years Benefit = begin photosynthesis in early spring as soon as warms up Usually have heavy coating of cutin Often found where warm growing season is short Keeping leaves gives a head start on growth They are abundant where nutrients are scarce. Thus, eliminate the need to grow a whole new set of leaves each year Branches and needles are flexible Allow snow and ice to slide off No buildup = no breakage DECIDUOUS: A few conifers are deciduous (Bald Cypress tree) = lose all their leaves at same time Adaptation for reducing water loss when water is unavailable during winter Plants lose most of water through the leaves Very little is lost through the bark or roots Cannot photosynthesize and must remain dormant during winter HOW GROW SO TALL? Tissues that make up much of the trunk of a conifer “wood” is composed of thick-walled, nonliving cells = Tracheids Tracheids = form the xylem and vascular systems Provides support and transport water and dissolved minerals from the roots to all other parts of the plant Life Cycle of a Pine 1. Adult develops male and female cones on separate branches Male is smaller 2. Female develops 2 ovules on upper surface of each cone scale Each contains haploid megaspores 3. Male cone produces microspores = pollen grains Pollen grains have hard water resistant coverings 4. As the female produces the egg = cone closed 5. Pollination = wind borne pollen falls near the opening of the ovule on female cone Each male pollen forms a pollen tube (penetrates tissue of female gametophyte) 6. Sperm fertilizes the egg 7. Zygote develops into an embryo and a mature seed is produced 8. Female cone opens and releases seeds 9. Seed germinates into a new seedling CONIFERS ARE NAMED FOR CONES: Reproductive structures produced in cones Male and Female cones Male = pollen grain Female = eggs fertilization seed Male = small and easy to overlook Usually drop off the tree soon after pollen is released Female cones are much larger Stay on the trees until seeds are mature (few months to 2 years) Most conifers have male and female cones on different branches of the same tree.