alternation of generations

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  • 1. Alternation ofGenerationsLife Cycles of Plants

2. Fill in the Blank Diploid (2n) individuals called ______________ andhaploid (n) individuals called ____________generate each other in the life cycle Male and female gametophytes produce____________ by _____________ Fertilization results in a ________ ___________ The zygotes undergoes __________and developsinto the diploid ________________ The sporophyte produces haploid ____________by _________ A spore develops by mitosis into a _____________ 3. Were you right? Diploid (2n) individuals called sporophytes andhaploid (n) individuals called gametophytes generateeach other in the life cycle Male and female gametophytes produce gametes bymitosis Fertilization results in a diploid zygote The zygotes undergoes mitosis and develops into thediploid sporophyte The sporophyte produces haploid spores by meiosis A spore develops by mitosis into a gametophyte 4. Evolution An important distinction between bryophytes and seedlessvascular plants is a gametophyte-dominated life cycle forbryophytes and a sporophyte-dominant life cycle forseedless vascular plants. Continuing that trend, the gametophytes of seed plants areeven more reduced than those of seedless vascular plants In seeds plants, the delicate female gametophyte and youngembryos are protected from many environmental stressesbecause they are retained within the moist sporangia of theparental sporophyte. The gametophytes of seed plants obtain nutrients from theirparents, while those of seedless vascular plants are free-living and fend for themselves. 5. Mosses Most mosses we see are gametophytes After fertilization, zygote remains in the gametangium There it divides by mitosis, and develops into asporophyte Each sporophyte remains attached to a gametophyte Meiosis occurs in the sporangia at the tips of thesporophyte stalks Haploid spores resulting from meiosis are released Spores undergo mitosis and develop intogametophytes 6. Ferns Dominant sporophyte generation Fern gametophytes have distinctive heartlike shapeand are small After fertilization, the zygote remains on thegametophyte where it develops into the sporophyte Cells in sporangia undergo meiosis, producinghaploid spores Spores develop into gametophytes by mitosis 7. Seed Plants About 95% of all plants, including all seed plants,have a dominant sporophyte generation in their lifecycle The evolution of pollen, produced by thesporophyte, was a key step in the adaptation ofseed plants to dry land 8. Gymnosperms A pine tree is a sporophyte The gametophyte generation consists ofmicroscopic stages that grow inside the trees cones Cones hold all of a conifer trees reproductivestructures: Diploid sporangia- produce haploid spores by meiosis Haploid female and male gametophytes Gametes Zygotes A pine tree bears two types of cones 9. Cones Female cone has many hard, radiating scales, eachbearing a pair of ovules; ovule starts out as asporangium Male cones are generally much smaller, soft, andshort-lived Each scale on a male cone produces many sporangia, eachof which makes numerous spores Male gametophytes, or pollen grains, develop from thespores When male cones are mature, the scales open andrelease a cloud of pollen 10. Pollination Pollination occurs when a pollen grain lands onand enters an ovule After pollination, meiosis occurs in the ovule, and ahaploid spore cell begins developing into thefemale gametophyte A tiny tube grow out of the pollen grain andeventually a sperm into the egg Fertilization does not occur until more than a year after pollination 11. Seed Following fertilization, the zygote develops into asporophyte embryo, and the whole ovule transformsinto the seed The seed contains the embryos food supply and hasa tough seed coat In a typical pine, seeds are shed from the conesabout 2 years after pollination The seed falls to the ground, or is dispersed by windor animals When conditions are favorable, it germinates Eventually, embryo grows into a tree 12. Angiosperms and Flowers The flower is the trademark of angiosperms Basic anatomy of a flower: Sepals- modified leaves; usually green; enclose theflower before it opens Petals- usually important in attracting animal pollinators Stamens- the flowers male parts; may be few or many Anther- sac on stamen in which pollen grains develop Carpel- female; consists of a stalk an ovary at the baseand a sticky tip known as the stigma Stigma traps pollen Ovary is a protective chamber containing one or moreovules, in which the eggs develop 13. Plant we see is sporophyte and tiny gametophytelives on it An angiosperm has its gametophytes in its flowersand its seeds packaged inside fruits Meiosis occurring in the anthers of the flower leadsto the male gametophytes, or pollen grains Meiosis in the ovules leads to the femalegametophytes, each of which produces an egg 14. Pollination occurs when a pollen grain, carried bythe wind or an animal, lands on the stigma A tube grows from the pollen grain to an egg, and asperm ferilizes the egg creating a zygote A seed develops from each ovule As the seeds develop, the ovarys wall thickens,forming the fruit that enclose the seeds The seed germinates and the embryo grows into amature sporophyte


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