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Chapter 30 Plant Diversity 2 The Evolution of Seed Plants Concept 30 1 Seeds and pollen grains are key adaptations for life on land characteristic common to all seed plans seeds reduced and dependent gametophytes heterospory produce 2 types of spores megaspores female microspores male ovules structures that house the female gametophytes pollen structures that house male gametophytes and gets dispersed gametophytes of seed plants develop within the walls of spores retained within tissues of the parent Gametophyte Sporophyte relationships sporophyte no longer dependent on water for fertilization non vascular and seedless non vascular rely on water at some point so the sperm can swim out and fertilize the eggs Ovules and Production of Eggs an ovule consists of megasporangium megaspore and protective integuments megaspore develops into the female gametophyte Pollen and Production of sperm microspores develop into pollen grains contains male gametophyte pollen can be dispersed by air or animals eliminates the need for water in fertilization don t need water because pollen grains can be dispersed by the winds or by animals if a pollen grain germinates it gives rise to a pollen tube that discharges sperm into the female gametophyte within the ovule figure 30 3b in above picture A seed once fertilization takes place the whole ovule becomes the seed includes sporophyte embryo food supply protective coat food source gets transferred to the embryo which is the new sporophyte stage figure 30 3c in above picture Which does not apply to gymnosperms or angiosperms single spore type seed bearing vascular plants will have vascular tissue the dominant stage is the sporophyte stage with is diploid and they do have cuticles with stomata they also have heterospory so they produce 2 types of spores In seed bearing plants microspores develop into pollen grains seed happens after fertilization occurs and it produces a new embryo the whole structure of the ovule becomes the seed Concept 30 2 Gymnosperms bear naked seeds typically on cones Gymnosperms include phylum cycadophyta sago palms phylum gingkophyta only 1 species within this phylum tolerant to adverse conditions tolerate high levels of pollution phylum gnetophyta found in desert regions reproductive stages they produce cones phylum coniferophyta includes pines firs junipers sequoias A closer look at the lifecycle of the gymnosperm key features of the gymnosperm life cycle include dominance of the sporophyte generation the role of pollen in transferring sperm to ovules no water required because they are dispersed by the wind the development of seeds from fertilized ovules Pine Life Cycle alternation of generations sporophyte stage pine tree is the mature sporophyte microsporangia have microsporocytes megasporangium has megasporocytes which undergoes meiosis to form 4 megaspores 1 develops into the female gametophyte the other 3 disintegrate in pines the female gametophyte contains archegonia each of which contain an egg in pines a megaspore repeatedly grows and divides giving rise to a female gametophye the female gametopye is the site in which egg bearing gametophytes develop in pines an embryo is an immature sporophyte the diploid embryo will develop into a seedling and then into a mature pine tree in pines pollen grains get to the ovule via the micropyle the gametophye tissue that surrounds the pine embryo functions as a haploid food reserve the tissue is a source of nourishment for the embryo in the pine diploid microsporangia form haploid microspores by meiosis How many generations are represented in the seed of a gymnosperm three seed coat is derived from parent sporophtye the food reserve is the gametophyte and the embryo is the new sporophyte Gymnosperms include cycads conifers ginkgo biloba and gnetopytes gymnosperms do not include flowering plants or plants with fruits Concept 30 3 The reproductive adaptations of angiosperms include flowers and fruits angiosperms means covered seeds and the things that cover those seeds are fruits derived traits are the reproductive structures called flowers and fruits they are the most widespread and diverse of all plants 250 000 species 90 of all extent plant species so they are the dominant plant today Angiosperm Diversity southern magnolias some groups found within this clade include amborella trichopoda water lilies star anise and most recent common ancestor of all living angiosperms is at 150 million years ago all come from 2 groups monocots include pine trees etc embryo has 1 seed leaf vascular tissue in parallel vascular bundles throughout the stem eudicots include 2 seed leaves vascular tissue Flowers specialized for sexual reproduction a structure used for reproduction specialized shot with modified leaves sepals enclose the flower protect the flower when it is not in bloom petals brightly colored attract pollinators stamens produce pollen consists of the filament and the anther carpels produce ovules consists of the sigma the style and the ovary which holds the ovules Fruits a mature ovary ovary matures after fertilization of ovules can be carried by wind water or animals enhancing dispersal The angiosperm life cycle double fertilization a derived feature it is unique to angiosperms flowering plants occurs when a pollen tube discharges 2 sperm into the male gametophyte within an ovule the first sperm fertilizes the egg the second sperm combines with 2 nuclei in the center cell of the female gametophyte and initiates development of the endosperm endosperm nourishes the developing embryo within the ovary of a flower are the ovules the ovules have megasporangium which contain the megasporocytes which undergo meiosis to create 4 megaspores 3 of the megaspores separate and get reabsorbed and one survives and develops into the female gametophyte The female gametophyte is made up of 3 antipodal cells 2 polar nuclei an egg cell and 2 synergids the pollen tube then deposits 2 sperm nuclei into the ovule where we have the female gametophyte 1 sperm nucleus fuses with the egg cell and the other fuses with the two polar nuclei We end up with a zygote that will develop into an embryo and the endosperm which develops into the nutrients for the cell The outer integuments of the ovule becomes the seed coating and the ovary of the cell becomes the fruit The seed gets disperse and lands in a suitable place in the soil it germinates and grows into a new sporophyte stage This is alternation of generations The


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LSU BIOL 1202 - Chapter 30: Plant Diversity 2

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