Angiosperms - Tulane University - New Orleans_ LA

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					Angiosperms - Flowering Plants
• • • • • • • • Charles Darwin called the evolution of the angiosperms “an abominable mystery” The evolution of angiosperms remains a mystery to this day Flowering plants evolved sometime during the Cretaceous, while the dinosaurs were at their peak Water lilies one of the first clades to evolve They quickly became the dominant plants, although gymnosperms continue to rule in cold, dry, or sandy habitats Oldest known angiosperm (recently discovered) is Archaefructus, ~ 122-145 my (upper Jurassic), discovered in mainland China in 2002 Most primitive living angiosperm (also recently discovered) is Amborella Grows as a shrub or small tree on South Pacific island of New Caledonia Angiosperms - Division Anthophyta • • More than 255,000 species of flowering plants grouped in over 300 families Flowering plants are superior competitors > Able to survive in a greater variety of habitats > Mature more quickly > Produce greater number of seeds Flowering plants are superior competitors > Fruit for seed dispersal > Wider-bore vessels to conduct water > Animals aid in pollination - can survive as small scattered populations, whereas wind-pollinated species need dense populations More diverse and specialized group > Gymnosperms are woody perennials, angiosperms can be perennials or annuals > Leaves are thin blades, diversity of shapes > Woody tissues are more complex and highly specialized Angiosperms - Economic Importance • • • • • Food - fruits, vegetables, grains, nuts, spices Wood - homes, tools, ships… Oils and waxes - olive oil, perfumes, soap… Drugs - coffee, chocolate, wine, beer… Medicines - quinine, digitalis, codeine…

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All angiosperms have flowers Flowers are reproductive structures that are formed from four sets of modified leaves Amazing diversity of floral structure Linnaeus used these differences in his first attempt at classifying plants Angeios = Greek for vessel (container) Ovules are encased in an ovary, not lying naked on the scales of a strobilus, as they are in gymnosperms Ovules in the ovary develop into seeds The ovary walls form a fruit to help disperse the seeds Fruit is a marvelous example of coevolution between plants and animals Coevolution occurs when an evolutionary change in one organism leads to an evolutionary change in another organism that interacts with it Flowering plants show two great examples of coevolution > Evolution of fruit dispersal > Evolution of animal pollination Fruits function to disperse seeds > Animals eat fruit, but don’t digest seeds > Tiny hooks and spines to attach to animal > Also dispersed by wind, water (coconuts) Flowers that rely on wind pollination are tiny and inconspicuous (like oak trees, maple trees, corn, grasses) Flowers that are pollinated by animals have showy petals to attract the pollinators Flowers advertise their reward of nectar, sugar water to attract pollinators Nectar contains sugar (sucrose, fructose or glucose), amino acids, vitamins, oils etc. Made by glands called nectaries, usually near the base of the ovary Nectar sipped by bees is the sugar source for honey! Flowering plants go to great lengths to avoid pollinating themselves > Chemical – Pollen and ovule are chemically incompatible > Architectural – Stamens and pistils are arranged to avoid contact > Temporal – Pollen and pistil mature at different times Flowers consist of four sets of modified leaves on a shortened stem > Sepals - protect floral parts in the bud > Petals - attract pollinators > Stamens - anthers and filaments > Carpels - form the pistil (stigma, style, ovary)

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What the heck is a carpel?? Cut into the pistil and you will see one or more tiny chambers, each chamber holding one or more sporangia on tiny stalks These sporangia are the ovules - each carpel can hold one or several ovules Goethe, German writer, philosopher, and (in his spare time) noted botanist, proposed that carpels evolved from leaves Chambers in the pistil were probably formed from a sporophyll Edges of the leaf folded over and fused together to form a protective chamber Goethe’s “foliar theory of the carpel” is still the best hypothesis for explaining the evolution of the carpel Sporophylls are leaves modified to hold spores Carpels are leaves modified to hold seeds What we are used to calling the pistil consists of the fusion of several carpels along the midrib of the modified leaves The root “carp” means fruit – appropriate, this portion of the flower becomes the fruit Sporophytes form sporangia Stamens are highly modified sporophylls Sporangia are located on the stamens, inside the anthers, atop a slender filament Each anther holds four microsporangia - microspores develop in microsporangia Microspore mother cell divides by meiosis to form four haploid microspores Each microspore develops into a multicellular pollen grain Pollen grains are the male gametophytes Microspore divides into a tube cell (will form pollen tube) and a sperm cell (nucleus will act as sperm) Mature male gametophyte is teeny-tiny, only a few cells Cross walls between two pairs of microsporangia break down Results in two long pollen sacs full of developing gametophytes Pollen grain are wrapped in sporopollenin, the same biopolymer that protects spores Surface of pollen grains can be very complex, species specific patterns - floral thumbprint Pollen shapes are critical for flowers - lets them recognize pollen of their own species Pollen in fossil record also allows us to reconstruct ancient environments Meanwhile, inside the ovary…. Sporophytes form sporangia Megaspores develop in megasporangia

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Megasporangia are located in the ovary, at the base of the pistil Each pistil consists of one or more carpels (modified leaves - sporophylls)

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Each carpel holds one or more ovules Ovules = megasporangium + integument Ovules are attached to the walls of the ovary by a short stalk (just like babies) Each ovule (megasporangia) will ultimately contain a single egg Megaspore mother cell divides by meiosis to form four haploid megaspores Three megaspores degenerate, the fourth divides by mitosis three times to form 8 haploid nuclei This large cell with 8 nuclei is the female gametophyte, called the embryo sac Weird cellular square dance ensues… One nucleus from each group of four now migrates to the center of the embryo sac Remaining three nuclei in each group migrate to each pole Cell walls form around the the three nuclei Female gametophyte now consists of 7 cells, 3 at the top, 3 at the bottom, 1 large cell in the middle with two nuclei One of the bottom 3 cells will act as the egg Pollen grain reaches the stigma of the pistil, grows a long tube (pollen tube) to penetrate the neck of the pistil and reach the ovary As the pollen tube approaches the embryo sac, the sperm nucleus divides in two, mature male gametophyte now contains three nuclei - two sperm nuclei, one tube Pollen tube enters the micropyle, discharges the sperm nuclei into the embryo sac Two central female nuclei fuse together to make a 2N nucleus One sperm nucleus fuses with the 2N nucleus to make a 3N nucleus - the other sperm nucleus fertilizes the egg Embryo sac now contains a diploid zygote, which will grow up into an adult sporophyte Also contains a 3N nucleus, which begins to divide repeatedly, forms the nutrition for the embryo - called endosperm (seed within) This process is called double fertilization, and it is unique to angiosperms Ephedra has double fertilization, suggesting that gnetophytes are related to angiosperms, but Ephedra doesn’t form endosperm Ovule = megasporangium + integuments Megasporangium holds developing embryo Integument develops into a hard protective jacket, the seed coat, which protects the embryo from dessication and mechanical damage Walls of the ovary develop into the fruit Each section of orange, each chamber in the tomato is a carpel, now filled with little embryos inside seeds


				
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posted:11/28/2009
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