Chapter 19 Notes
Chapter 19 Notes BIOL 3040
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This 5 page Class Notes was uploaded by Min-Young Kim on Wednesday February 10, 2016. The Class Notes belongs to BIOL 3040 at Clemson University taught by Christina Wells in Spring 2016. Since its upload, it has received 28 views. For similar materials see Biology of Plants in Biology at Clemson University.
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Date Created: 02/10/16
Chapter 19 Notes: Angiosperms -‐ Angiosperms make up monophyletic phylum Anthophyta – largest phylum of photosynthetic organisms o Monocotyledonae (monocots) ~90,000 species o Eudicotyledonae (eudicots) ~200,000 species o Almost all angiosperms are free-‐living, but a few parasitic and myco-‐ heterotrophic forms exist (mistletoe, Rafflesia) -‐ Parasitic plants form specialized absorptive organs called haustoria that penetrate tissues of hosts -‐ Myco-‐heterotrophic plants: lack chlorophyll and are non-‐photosynthetic, have obligate relationships with mycorrhizal fungi that are also associated with angiosperm -‐ Flower is a determinate shoot that bears sporophylls (sporangium-‐bearing leaves). o Definitive structure = carpel – contains ovules which develop into seeds after fertilization. Carpel develops into fruit wall o Flowers may be clustered into aggregations called inflorescences o Stalk of inflorescence or solitary flower known as peduncle o Stalk of individual flower in inflorescence is pedicel o Receptacle – part of flower stalk to which flower parts are attached o Sepals and petals attached to receptacle below stamens and carpels § Sepals form calyx, petals form corolla § Calyx and corolla form perianth § Stamens (microsporophylls) collectively called androecium § Stamen consists of slender stalk (filament) and two-‐lobed anther containing four microsporangia, or pollen sacs, in two pairs. § Carpels (ovule bearing parts) collectively known as gynoecium (megasporophylls enclosing one or more ovules) § One or more carpels, may be separate or fused, called pistil o Four whorls from outside: § Sepals (calyx) § Petals (corolla) § Stamens (androecium) § Carpels (gynoecium) o Individual carpels or fused carpels differentiated into three parts: lower part (ovary) which encloses ovules, middle part (style) which pollen tubes grow, upper part (stigma) which receives pollen. o Common ovary of fused carpels generally partitioned into two or more locules – chambers of ovary that contain ovules o Portion of ovary where ovules originate and remain attached until maturity = placenta. Placentation (arrangement) varies among groups. § Parietal – ovules borne on ovary wall or on extensions of it § Axile – ovules borne on central column of tissue in partitioned ovary § Free central – ovules borne on central column of tissue not connected by partitions with ovary wall § Basal – single ovule at base § Apical – single ovule at apex o Perfect – flowers with both stamens and carpels (bisexual) o Imperfect – either stamens or carpels missing (unisexual) § Either staminate or carpellate o Monoecious – staminate and carpellate flowers occur on same plant o Dioecious – staminate and carpellate flowers occur on separate plants o Complete flowers – all four floral whorls o Incomplete flowers – lack any whorl o Connotation – floral parts united with other members of same whorl o Adnation – floral parts united with members of other whorls (stamens with corolla) o Superior – if whorls attached to receptacle below the ovary o Inferior – whorls attached near the top of the ovary o Hypogynous – perianth and stamens situated on receptacle beneath ovary and free from it and from calyx o Epigynous – perianth and stamens arise from top of ovary o Perigynous – stamens and petals adnate to calyx, form short tube arising from base of ovary o Radially symmetrical – different whorls made of up of members of similar shape that radiate from center of flower and equidistant from each other § Regular o Bilaterally symmetrical – one or more members of at least one whorl are different from other members of same whorl § Irregular -‐ Angiosperm gametophytes much reduced. o Mature microgametophyte consists of only three cells o Mature megagametophyte (embryo sac) retained for entire existence within tissues of sporophyte, or specifically ovule (typically seven cells). o Both antheridia and archegonia lacking; pollination indirect § Pollen deposited on stigma, pollen tube grows through or on surface of tissues of carpel to convey two nonmotile sperm to female gametophyte; fertilization, ovule develops into seed, which is enclosed in ovary; ovary develops into a fruit o Microsporogenesis: formation of microspores within microsporangia, or pollen sacs, of anther o Microgametogenesis: later development of microgametophyte to three celled stage § Anther is uniform mass of cells until it differentiates into four columns of sporogenous cells within anther. Each column surrounded by sterile cells, develop into wall of pollen sac. Outermost layers trigger anther, whereas innermost layer of pollen sac wall forms nutritive tapetum (forms lipid-‐rich coat to surface layer of pollen grain). § Sporogenous cells become microsporocytes (pollen mother cells) that divide mitotically. Each diploid microsporocyte gives rise to tetrad of haploid microspores. § Microsporogenesis completed with formation of single-‐celled microspores (pollen grains) § During meiosis, nuclear division followed by cell wall formation, four microspore protoplasts walled off after second meiotic division; major features of pollen grains established § Pollen grains develop resistant outer wall, exine and inner wall, intine. • Exine interrupted by pores (preferable for pollen tube initiation) • Exine composed of resistant substance sporopollenin (synthesized primarily by tapetum) • Intine composed of cellulose and pectin, laid down by microspore protoplasts • Angiosperm pollen coat secreted onto textured exine by tapetum and is unique to all but few angiosperms • Microgametogenesis is uniform, begins when microspore divides mitotically, forming two symmetrical cells. Division forms large vegetative cell, or tube cell, and small generative cell, which moves interior or pollen grain. • Two celled pollen grain is immature microgametophyte o 2/3 angiosperm species, microgametophyte remains in two celled stage when pollen grains released o Rest of species, generative nucleus divides, giving rise to two sperm, resulting in three celled microgametophyte • Pollen grains vary considerably in size and shape o Undergo mitosis before dispersal (contrast to meiosis in seedless plants); have two or three nuclei when shed; germinate through apertures o Megasporogenesis: involves meiosis and results in formation of megaspores within ovule in nucellus § Early in development of ovule, single megasporocyte arises in nucellus; diploid megasporocyte divides meiotically to form four haploid megaspores, generally arranged in linear tetrad; in most seed plants, three of four megaspores disintegrate. o Megagametogenesis: development of megaspore into the embryo sac § One nucleus from each group migrates into center of eight-‐ nucleate cell and become polar nuclei. Remaining three eggs at micropylar end organized into egg apparatus (egg cell and two synergids). Three nuclei at chalazal end form antipodals. Central cell contains two polar nuclei. Eight-‐nucleate, seven celled structure is mature megagametophyte, or embryo sac o Funiculus: complex structure in ovule bearing a nucellus enclosed by one or two integuments o Micropyle: small opening at one end of the ovule o Functional megaspore enlarges, nucleus of megaspore divides mitotically three times. Eight nuclei arranged in two groups of four, one at micropylar end and other at chalazal end. -‐ Most ancient angiosperm lineages do not have Polygonum-‐type embryo sacs -‐ Dehiscence of anther (opening of pollen sacs): pollen grains transferred to stigmas (pollination). o Once in contact with stigma, pollen grains take up water from cells of stigma surface. Pollen grain germinates, forms pollen tube generative cell divides within pollen tube, forming two sperm. This is mature gametophyte. -‐ Wet stigmas: consist of glandular tissue that secretes proteins, amino acids, and lipids. -‐ Dry stigmas: hydrated layer consisting of proteins, carbohydrates, and small amount of lipid -‐ Pollen tubes grow downward between cells of stigma and enter style; grow between transmitting tissue -‐ Many monocots and certain eudicots have open stigmas (lined by glandular epidermis on which pollen tube grows) -‐ Pollen tube enters ovary, reaches ovule, grows out of transmitting tissue, follows funiculus, enters micropyle, conveying male germ unit (two sperm cells and vegetative nucleus) in process -‐ Pollen tubes of angiosperms have considerably longer distances to grow from site of reception to that of fertilization – evolution favored greater growth rates in angiosperms o Plastic and rapidly extending tip and reinforced lateral wall composed of callose o Many pollen tubes also deposit callose plugs that seal off older portions of tube from apical portion -‐ Guidance of pollen tube through style governed by cells of transmitting tissue -‐ When pollen tube reaches embryo sac, enters one of synergids near filiform apparatus and discharges contents into degenerated synergid. -‐ Actin form sperm cells and extend to egg cell and central cell -‐ Double fertilization: union of one sperm cell with egg and other sperm cell with polar nuclei o Primary endosperm nucleus divides, forming endosperm o Zygote develops into embryo o Integuments develop into seed coat o Ovary wall and related structures develop into fruit -‐ Triple fusion: most common type of embryo sac formation where one of sperm nuclei fuses with two polar nuclei and results in triploid primary endosperm nucleus -‐ Embryogeny in angiosperms resembles that of seedless vascular plants: first nuclear division of zygote accompanied by cell wall formation -‐ In seeds of some groups of angiosperms, nucellus proliferates into food-‐ storage tissue known as perisperm -‐ Angiosperm seeds differ from gymnosperms in origin of stored food – by endosperm (rather than female gametophyte) -‐ Ovule develops into seed, and ovary develops into fruit. Ovary wall, pericarp, thickens and becomes differentiated into different layers.
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