BIOL208_CH19.pdf

BIOL 208 CH 19 Joanne Nelson, Ts’msyen, MPH, PhD  Ray F. Evert Susan E. Eichhorn Raven Biology of Plants Eighth Edition CHAPTER 19 Introduction to the Angiosperms © 2013 W. H. Freeman and Company Chapter Outline Diversity in the Phylum Anthophyta The...

BIOL 208 CH 19 Joanne Nelson, Ts’msyen, MPH, PhD  Ray F. Evert Susan E. Eichhorn Raven Biology of Plants Eighth Edition CHAPTER 19 Introduction to the Angiosperms © 2013 W. H. Freeman and Company Chapter Outline Diversity in the Phylum Anthophyta The Flower The Angiosperm Life Cycle Learning Outcomes After reading this chapter, you should be able to answer the following: 1. What is a flower, and what are its principal parts? 2. Describe some of the variations in flower structure. 3. By what processes do angiosperms form microgametophytes (male gametophytes)? How are these processes both similar to and different from those that give rise to megagametophytes (female gametophytes)? 4. What is the structure, or composition, of the mature microgametophyte in angiosperms? Of the mature megagametophyte? 5. Describe double fertilization in angiosperms. What are the products of this process? Diversity in the Phylum Anthophyta Angiosperms: The Largest Photosynthetic Organisms Comprises phylum Anthophyta, comprising 300,000 to 450,000 species. Exhibits diverse vegetative and floral features. Size ranges from Eucalyptus trees to duckweeds. Some vines climb into tropical rainforest canopy, others grow in it. Adapted for growth in arid regions, like cacti. Flowering plants have dominated land for over 100 million years. Angiosperms are seed plants with unique characteristics: flowers, fruits, and unique life-cycle. Source Diversity in the Phylum Anthophyta Angiosperms: Monophyletic and Diversity Angiosperms are monophyletic, derived from a single common ancestor. They consist of several evolutionary lines, including monocotyledonae (monocots) with at least 90,000 species and eudicotyledonae (eudicots) with at least 200,000 species. Monocots include familiar plants like grasses, lilies, irises, orchids, cattails, palms, rice, and bananas. Eudicots are more diverse, including trees, shrubs, and many herbs. The classification system of "dicots" overemphasizes monocots' distinctiveness. Diversity in the Phylum Anthophyta Angiosperm Nutrition Modes Most angiosperms are free-living, with a few parasitic and myco-heterotrophic forms. About 200 species of parasitic monocots and 2800 species of parasitic eudicots exist. Parasitic plants form haustoria, specialized absorptive organs. Myco-heterotrophic plants lack chlorophyll and Indian pipe (Monotropa uniflora), a myco- have obligate relationships with mycorrhizal heterotroph, lacks chloroplasts and fungi. obtains its food from the roots Fungus forms a bridge transferring of other, photosynthetic plants via the carbohydrates from photosynthetic to myco- fungal hyphae associated with its roots. heterotrophic plants. ©Joanne Nelson The Flower Four Whorls of the Flower 1. Carpels – female parts, gynoecium 2. Stamens – male parts, androecium 3. Corolla – petals 4. Calyx - sepals Source The Flower Flower Structure and Inflorescences Flower is a determinate shoot with limited growth duration. It bears sporophylls, sporangium- bearing leaves. The definitive structure of the flower is the carpel, which contains ovules and the fruit wall. The part of the flower stalk attached to the flower parts is the receptacle.  Flowers consist of two sets of sterile appendages: sepals (calyx) and petals (corolla). The calyx and corolla form the perianth. Stamens: pollen-bearing parts, microsporophylls. slender stalks with a two-lobed anther containing four microsporangia. Carpels: ovule-bearing parts, megasporophylls folded lengthwise. Flower can have one or more carpels separate, partially fused, or fused three parts: ovary, style, and stigma The Flower Flower Structure and Inflorescences Flowers can be clustered into inflorescences. The stalk of an inflorescence or solitary flower is a peduncle, and the stalk of an individual flower is a pedicel.  Inflorescence Racemose Cymose young flower old flower young flower old flower Indefinite Definite Peduncle grows continuously Peduncle terminates in a flower It does NOT terminate in a flower Further flowers are produced laterally Acropetal order – youngest at top Basipetal order – lateral flowers are younger  Racemose corymb Umbel Catkin Spadix Head Raceme Spike Raceme Spike Head Catkin Spadix Racemose corymb Umbel Viburnum lantana Alnus rubra Rubus spectabilis Chamaenerion angustifolium or wayfaringtree red alder salmonberry Epilobium angustifolium Lysichiton americanus fireweed Plantago major western skunk cabbage Heracleum maximum whiteman’s footprint cow-parsnip Solitary Non-solitary Rosa nutkana Nootka rose Calypso bulbosa fairy slipper The Flower Ovary encloses ovules. Style grows pollen tubes. Stigma receives pollen. The common ovary is partitioned into two or more locules. The number of locules is usually related to the number of carpels in the gynoecium. Source The Flower Placenta: The ovary portion where ovules originate and remain attached until maturity. Variations in placentation - arrangement of ovaries in ovules. Parietal: Ovules are borne on the ovary wall or extensions. Axile: Ovules are borne on a central tissue column in a partitioned ovary. Free central: Ovules are borne on a central tissue column not connected to the ovary wall. Basal or apical placentation: A single ovule occurs at the base or apex of a unilocular ovary. Placentation Marginal Parietal Axile Free Central Basal/Apical Examples Pea Curcubita, cantelope Citrus Dianthus cayophyllus Talinum Photo The Flower Most flowers have both stamens and carpels, making them perfect (bisexual). If either stamens or carpels are missing, the flower is imperfect (unisexual). The flower type depends on the part present, either staminate or carpellate (or pistillate). The Flower If both occur on the same plant, the species is monoecious (from Greek words monos, "single," and oikos, "house"). If both occur on separate plants, the species is dioecious (two houses). The Flower Four floral whorls: sepals, petals, stamens, or carpels. Complete flowers have all four. Incomplete flowers lack any whorl. Imperfect flowers lack either stamens or carpels. Not all incomplete flowers are imperfect. The Flower Incomplete Imperfect lacks any whorl (sepals, petals, lack either stamens or carpels e.g. stamens, or carpels) are either only ♂ or ♀ Incomplete lacking Incomplete lacking Incomplete lacking Incomplete lacking sepals AND petals AND ♀ AND ♂ AND Perfect with ♂ and Perfect with ♂ and Imperfect with only Imperfect with only ♀ ♀ ♂ ♀ The Flower Apocarpous - Flower with Free Carpels. Shown in 92: apocarpous gynoecium composed of three carpels. Syncarpous - The carpels of the gynoecium are united in a compound ovary as in 93, 94 and 95. Source The Flower Floral parts can be spiral on a receptacle or attached in a whorl. Parts can be united with other members of the same whorl (connation) or with members of other whorls (adnation). Adnation is the union of stamens with the corolla, common in various plants. When parts are not joined, prefixes apo- or poly may be used. When parts are joined or connate, prefixes syn- or sym- are used. Synsepalous calyx has sepals joined, while aposepalous or polysepalous calyx doesn't. Source The Flower  Adnate Connate The Flower United with members of united with other other whorls members of the same whorl Photo Cohesion of Monadelphous Diadelphous Polydelphous Syngenesious Synandrous Stamens Arrangement filaments united, anthers united by their stamens united by anthers into anthers fused; filaments anthers and filaments are free to form two groups three or more groups free united through whole length Examples Hibiscus, Abutilon Crotalaria Castor, Citrus Helianthus annus Cucurbita Illustration Cohesion of Monadelphous Diadelphous Polydelphous Syngenesious Synandrous Stamens Arrangement filaments united, united by their stamens united by anthers into anthers fused; filaments anthers and filaments anthers free to form two groups three or more groups free are united through whole length Examples Hibiscus, Abutilon Astragalus crassicarpus Citrus Helianthus annus Cucurbita Photo The Flower Flower parts arrangement varies, including spiral or whorled arrangement.  Sepals, petals, and stamens' insertion on floral axis varies with ovary or ovaries. Lilies have superior ovary, sepals, petals, and stamens attached below receptacle. Other flowers have inferior ovary, with sepals, petals, and stamens attached near top. Intermediate conditions exist where ovary part is inferior. Hypogynous flowers: Perianth and stamens located beneath the ovary, free from the calyx. Epigynous flowers: Perianth and stamens arise from the top of the ovary, like apple blossoms. Perigynous flowers: Stamens and petals adnate to the calyx, forming a short tube (hypanthium) from the base of the ovary, like cherry flowers. The Flower Radially Symmetrical Flowers: Consist of similar-shaped whorls radiating from the center and equidistant from each other. Examples include roses and tulips. Bilaterally Symmetrical Flowers: Members of at least one whorl differ from other members of the same whorl. Examples include snapdragons and garden peas. Some regular flowers have irregular color patterns, giving pollinators an image similar to a structurally irregular flower. The Angiosperm Life Cycle Angiosperm Gametophytes Reduced in size compared to other heterosporous plants. Mature microgametophyte consists of three cells. Mature megagametophyte (embryo sac) typically has seven cells. Antheridia and archegonia are absent. Pollination is indirect, with pollen deposited on stigma and pollen tube growing on carpel tissue. Fertilization leads to ovule development into a seed and ovary development into a fruit. The Angiosperm Life Cycle Microgametophyte Formation Processes Microsporogenesis: Formation of single-celled precursors of pollen grains within anther microsporangia. Microgametogenesis: Later development to three-celled stage. The Angiosperm Life Cycle Anther Formation and Microsporogenesis Anther initially consists of a uniform mass of cells, with a partially differentiated epidermis. Four columns of fertile cells become visible within the anther, surrounded by sterile cells. The outermost layers trigger the anther's opening, while the innermost layer forms the nutritive tapetum. The sporogenous cells transform into microsporocytes, which divide meiotically. Each diploid microsporocyte produces a tetrad of haploid microspores. Microsporogenesis concludes with the formation of single-celled microspores or pollen grains. The Angiosperm Life Cycle Pollen grains: outer wall (exine) and inner wall (intine) exine surface: smooth or sculptured with pores or linear apertures Apertures are sites for pollen tube initiation and can contract or expand in response to osmotic pressure. Exine: composed of sporopollenin, a polymer primarily synthesized by the tapetum, providing a protective barrier against UV irradiation, dehydration, and pathogen attack. Intine: composed of cellulose and pectin, is laid down by microspore protoplasts. pollen coat: often scented, pigmented, and enzyme-rich, is secreted onto the textured exine by the tapetum. The Angiosperm Life Cycle Microspore division forms two asymmetrical cells within the original microspore wall. Large vegetative cell (tube cell) and small generative cell move to the pollen grain interior. In two-thirds of angiosperm species, the microgametophyte is in this two-celled stage when pollen grains are liberated. In remaining species, the generative nucleus divides, resulting in two male gametes and a three-celled microgametophyte. Mature pollen grains may contain starch or oils, providing a nutritious food source for animals. The Angiosperm Life Cycle Pollen grain development. Development of the pollen grain from diploid (2n) microspore mother cell to 3-celled microgametophyte. Each micropore mother cell in a pollen sac undergoes meiosis to produce four haploid (n) microspores. Each microspore divides once to produce a 2-celled pollen grain. The generative cell divides again to yield two sperm. The tube cell will form the pollen tube that delivers the sperm following pollination. Credit: Diagram by E.J. Hermsen (DEAL), modified after Foster & Gifford (1974). Germinated eudicot pollen grain. Two stages in the development of a germinated eudicot pollen grain. The generative cell of the two- celled stage divides to give rise to the sperm cells of the three-celled stage. Note that the pollen tube is growing through one of three apertures in the pollen wall. Depending on the plant, sperm may be formed before or after the pollen tube begins to develop. Credit: Drawing of germinated eudicot pollen grain, fig. 118 from Bergen & Caldwell (1914) Introduction to Botany (no known copyright Source restrictions). Image modified from original. The Angiosperm Life Cycle Pollen grains vary in size and shape, from 10 micrometers to 350 micrometers in diameter. They range from spherical to rod-shaped and differ in number and arrangement of apertures. Identified by size, number, arrangement, type of apertures, and exine sculpturing. Widely represented in the fossil record due to their tough, highly resistant exine. Studies of fossil pollen provide insights into past plant types, communities, and climates. Source The Angiosperm Life Cycle Pollen Grain Dispersal vs Spores Pollen grains undergo mitosis before dispersal and have two or three nuclei when shed, unlike spores' one. Spores germinate through a Y- shaped suture Pollen grains germinate through their apertures. The Angiosperm Life Cycle Megasporogenesis and Megagametogenesis in Embryo Formation Megasporogenesis involves meiosis, forming megaspores in ovule in nucellus. Megagametogenesis develops megaspore into embryo sac. The Angiosperm Life Cycle Ovule Structure Consists of a funiculus or stalk with a nucellus enclosed by one or two integuments. One to many ovules may arise from the placentae, ovule-bearing regions of the ovary wall. Initially, the ovule is entirely nucellus, but soon develops integuments enveloping the nucellus. Leaves a small opening, the micropyle, at one end of the ovule.  The Angiosperm Life Cycle Polygonum Type Megasporogenesis and Megagametogenesis About 70% of extant angiosperms undergo this pattern. A single megasporocyte emerges in the nucellus early in ovule development. Diploid megasporocyte divides to form four haploid megaspores. Three of the four disintegrate in most seed plants. The farthest megaspore survives and develops into the megagametophyte. The Angiosperm Life Cycle Megaspore enlarges with nucellus expansion. Nucellus of megaspore divides mitotically. Eight nuclei are divided into two groups: one near micropylar end and another at chalazal end. One nucleus from each group migrates to the center of the eight-nucleate cell, forming polar nuclei. Three nuclei at micropylar end form the egg apparatus, consisting of an egg cell and two short- lived cellular synergids. Cell wall formation occurs around three nuclei at chalazal end, forming antipodals. The central cell contains the two polar nuclei, forming the mature megagametophyte or embryo sac.  The Angiosperm Life Cycle The Fritillaria type, found in Lilium, is an unusual pattern where all four megaspore nuclei participate in the formation of the embryo sac. Three of the nuclei move to the chalazal end of the embryo sac, while the remaining nucleus is at the micropylar end. At the micropylar end, the single haploid nucleus undergoes mitosis, yielding two haploid nuclei. At the chalazal end, mitotic spindles of the three sets of chromosomes unite, resulting in two triploid nuclei. A second four-nucleate stage is produced, with two haploid nuclei at the micropylar end and two triploid nuclei at the chalazal end.  The Angiosperm Life Cycle Historically, the belief was that the first flowering plants had Polygonum-type embryo sacs. Molecular studies in 1999 identified three angiosperm lineages: Amborellaceae, Nymphaeales, and Austrobaileyales. The Oenothera-type embryo sac of Nymphaeales and Austrobaileyales contains four cells and four nuclei at maturity. Amborella's mature embryo sac resembles a Polygonum-type sac but has eight cells and nine nuclei, an egg apparatus, three antipodals, and a binucleate central cell. No ancient flowering plant lineage produces a seven-celled, eight-nucleate embryo sac.  The Angiosperm Life Cycle Pollen grains transfer to stigmas through various methods after dehiscence of the anther. Viable, compatible pollen grains absorb water from stigma surface cells. After hydration, pollen grain germinates, forming a pollen tube. If not already divided, generative cell divides within the tube, forming two sperm. Germinated pollen grain, with vegetative nucleus and two sperm cells, forms the mature microgametophyte. The Angiosperm Life Cycle Stigma and style are modified for pollen grain germination and tube growth. Wet stigma: Nicotiana Wet stigmas secrete proteins, tabacum var. Virginia Source amino acids, and lipids. Dry stigmas have a hydrated layer of proteins, carbohydrates, and a small amount of lipid. Dry stigma: Arabidopsis thaliana (thale cress) Source The Angiosperm Life Cycle Pollen tubes grow between stigma cells and enter the style, specialized tissue called transmitting tissue. Open or hollow stigmas in monocots and certain eudicots have glandular epidermis for pollen tube growth. Pollen tube enters the ovary and ovule, transferring sperm cells and vegetative nucleus to the micropyle. Two sperm cells are physically associated with the vegetative nucleus, forming the male germ unit. Source The Angiosperm Life Cycle Angiosperm pollen tubes have greater growth Angiosperm pollen tubes have a unique distances than gymnosperms. structure with a plastic tip and a reinforced Evolution has favored greater growth rates in lateral wall made of callose. angiosperms, about 1000 times greater than Callose plugs help maintain positive turgor in gymnosperms. the apical, growing portion of the pollen tubes, enabling them to reach greater distances. The bright spots at the → are callose plugs Source The Angiosperm Life Cycle Pollen tube guidance through the style is governed by cells of transmitting tissue. After entering the ovary, it is guided by diffusible chemoattractants produced at the micropylar end of the ovule. Source The Angiosperm Life Cycle Pollen Tube and Embryo Sac Process Pollen tube enters synergid near filiform apparatus and discharges into degenerated synergid. Actin "coronas" form near sperm cells and extend to fertilization targets. Coronas mark sperm cell and nucleus In WT (wild-type) pollen tubes, long actin filaments migration pathways. are distributed throughout the entire tube in a net axial array that is largely parallel to the direction of Migration involves actin and myosin elongation, except at the very tip (a,a’), and a interactions, similar to cytoplasmic distinct cortical actin fringe is formed in the streaming. subapical zone of the tube (arrows in (a,a’)). One sperm nucleus unites with egg Source and polar cell nuclei. The Angiosperm Life Cycle Ovule with Polygonum-type embryo sac. Idealized diagram of an ovule with a Polygonum-type embryo sac, showing the seven cells and eight nuclei surrounded by a thin nucellus (megasporangium) and double integuments. Note that the antipodals may break down and the polar nuclei may fuse to form a diploid nucleus prior to fertilization. The nucellus may also break down, so may not be observed in mature ovules. Credit: E.J. Hermsen (DEAL). The Angiosperm Life Cycle Mature seven-celled, eight- nucleate female gametophyte (embryo sac) 3 antipodal cells 1 embryo sac (2 polar nuclei) 1 egg cell 2 synergids The Angiosperm Life Cycle Gymnosperms and Double Fertilization Gymnosperms have only one functional sperm cell, one unites with the egg and the other dies. Double fertilization involves both sperm cells, forming an embryo and endosperm. One sperm fuses with the egg – diploid (zygote) One sperm fuses with two polar nuclei – triploid (endosperm) This process is a defining characteristic of angiosperms. Source Double fertilization also occurs in Ephedra and Gnetum, but the second fertilization event doesn't yield endosperm but an extra embryo that eventually aborts. The Angiosperm Life Cycle Angiosperm Formation and Nucleus Formation Polygonum type angiosperms have triploid (3n) primary endosperm nucleus due to triple fusion. Polar nuclei 2 x n + Sperm cell 1 x n = Primary Endosperm Nucleus 3n The Angiosperm Life Cycle one polar nucleus is 3n polar nuclei fuse to 4n the other is n secondary nucleus Angiosperm Formation and Nucleus Formation Lilium (Fritillaria type) angiosperms have pentaploid (5n) primary endosperm nucleus due to triploid (3n) and haploid (n) nuclei fusing with (4n) sperm (n) Nymphaeales and Austrobaileyales have diploid primary endosperm nucleus due to haploid central cell nucleus fusion with sperm nucleus. Vegetative nucleus degenerates during double fertilization, and (4n) synergid and antipodals degenerate (5n) near fertilization or embryo after triple fusion differentiation. primary endosperm nucleus is 5n The Angiosperm Lifecycle Lilium (Fritillaria type) lifecycle egg pollen tube zygote 2n primary endosperm nucleus 5n antipodal cells The Angiosperm Life Cycle Double Fertilization Process 1. Primary endosperm nucleus divides to form endosperm. 2. Zygote develops into embryo. 3. Integuments form seed coat. 4. Ovary wall and related structures form fruit. The Angiosperm Life Cycle The Angiosperm Life Cycle Embryogenesis in Angiosperms Unlike gymnosperms, angiosperm embryogeny starts with a freenuclear stage. Similar to seedless vascular plants, first nuclear division of zygote precedes cell wall formation. Monocot embryos undergo similar cell division sequences to other angiosperms. Monocot embryos form one As the ovule matures into a seed, the zygote develops into the embryo, consisting of cotyledon(s) (C) and hypocotyl (D), the cotyledon, unlike other angiosperm endosperm (B) develops into a nutrient tissue food supply, and embryos. the integuments become the seed coat (A). The Angiosperm Life Cycle Endosperm Formation Starts with mitotic division of primary endosperm nucleus, usually before zygote division. Nuclear-type endosperm formation involves multiple free-nuclear divisions before cell wall formation. Cellular-type endosperm formation follows initial and subsequent mitoses, followed by cytokinesis. Endosperm tissue provides essential food materials for the developing embryo and young seedling. A-E represents cellular endosperm and F to J represents nuclear division. Source The Angiosperm Life Cycle Endosperm Formation Nucellus proliferates into food- storage tissue known as perisperm in some angiosperm groups. Some seeds contain both endosperm and perisperm, like beet (Beta). Most or all storage tissues are absorbed by the developing embryo before seed becomes dormant. Principal food materials stored in seeds are carbohydrates, proteins, Beta vulgaris (sugar beet) Source and lipids. The Angiosperm Life Cycle Angiosperm Seeds vs Gymnosperms Gymnosperms store food by female gametophyte, angiosperms by endosperm. Nutritive tissue builds up after fertilization in Gnetum and angiosperms. Nutritive tissue in other seed plants forms before fertilization. Gymnosperm: Pinus seed. Source The Angiosperm Life Cycle Ovule Development into Fruit Ovule develops into seed, forming fruit. Ovary wall thickens and differentiates into exocarp, mesocarp, and endocarp. Exocarp and endocarp layers are more prominent Tomato flower and fruit. Source in fleshy fruits. The Angiosperm Life Cycle Angiosperm Lifecycle vs Gymnosperm Lifecycle The Angiosperm Life Cycle Carmichael, J.S. and Friedman, W.E. (1996), Double fertilization in Gnetum gnemon (Gnetaceae): its bearing on the evolution of sexual reproduction within the Gnetales and the anthophyte clade. American Journal of Botany, 83: 767-780. https://doi.org/10.1002/j.1537- 2197.1996.tb12766.x Source  Double fertilization in Gnetum Second fertilization event doesn't yield endosperm Extra embryo eventually aborts Carmichael, J.S. and Friedman, W.E. (1996), Double fertilization in Gnetum gnemon (Gnetaceae): its bearing on the evolution of sexual reproduction within the Gnetales and the anthophyte clade. American Journal of Botany, 83: 767-780. https://doi.org/10.1002/j.1537-2197.1996.tb12766.x Schematic of the process of double fertilization in Ephedra trifurca. Both sperm nuclei and the egg nucleus and ventral canal nucleus begin with the 1C complement of DNA (A) Following the entry of two sperm nuclei into the egg cell, the first sperm nucleus migrates to the egg nucleus (B) and initiates contact (C). The second sperm nucleus lags behind the first sperm nucleus and converges with the ventral canal nucleus, which dislodges from its initially apical position within the egg cell. Both pairs of male and female nuclei appear to maintain their individual zones of nucleoplasm, after contact has been made, and a chalazal pattern of migration takes place. All four gamete nuclei pass through the synthesis phase of the cell cycle (B-F). This results in the production of two pairs of nuclei, each nucleus with the 2C quantity of DNA. Each pair fuses (G) to produce two nuclei with the 4C content of DNA. Shortly after the first and second fertilization events have been completed, each of the nuclei enters into mitosis (H). Recall that the extra embryo eventually aborts Hay Fever 10-18% of people in Northern Hemisphere suffer from hay fever. Proteins found in pollen grain walls act as allergens and antigens, triggering immune reactions. These proteins often contribute to genetic self-incompatibility. Smaller particles of the tapetum, smaller than pollen grains, can become airborne during anther splits. Source Hay Fever Wind-borne pollen from grasses, birch trees, and ragweed is a significant agent of hay fever due to its direct air exposure. Pollen quantity is the primary factor determining allergic response. Large amounts of windborne pollen, like maize and pines, rarely cause difficulty. Certain flowers' scent can cause reactions resembling hay fever, possibly increasing nasal membrane sensitivity. Source Hay Fever Spring: Associated with tree pollen from oaks, elms, maples, poplars, pecans, and birches. Summer: Predominantly grass pollen, with Bermuda grass, timothy grass, and orchard grass important in different regions. Fall: Ragweed and grasses become major irritants. Individual susceptibility to different plants varies greatly. Source Hay Fever Wide cultivation of newly introduced plants like rapeseed important new sources. Arid Southwest US increased lawn and golf course irrigation and introduction of various weeds makes hay fever common. Source Hay Fever Hay fever incidence in the U.S. has rapidly increased over 60 years. Pollen count is falling in many areas. Increase partly due to improved detection. Understanding the cause requires better understanding of human immune system. Source SUMMARY Anthophyta Classification Flower Structure Overview Monocotyledonae: Largest class Up to four whorls of appendages: sepals (calyx), petals with 90,000 species. (corolla), stamens (androecium), and carpels (gynoecium). Eudicotyledonae: Largest class Sepals and petals are sterile, while petals attract pollinators. with 200,000 species. Stamens are divided into a filament and an anther, containing four pollen sacs. Distinctive Features of Flowering Carpels are differentiated into the ovary and the style, Plants ending in the receptive stigma. Ovule enclosure within carpels. Variations in flower structure: incomplete flowers lack one Nutritive endosperm or more of the four whorls, complete flowers possess all production in seeds. four. Unique reproductive structure, Perfect flowers have both stamens and carpels, while flower. imperfect flowers are unisexual and bear either stamens or carpels. Flowers can be regular or irregular. SUMMARY Pollination in Angiosperms Pollination occurs through the transfer During angiosperm fertilization, one of pollen from anther to stigma. sperm unites with the egg, producing a A pollen grain is an immature male diploid zygote. gametophyte (microgametophyte), The other unites with the two polar initially containing a vegetative cell and nuclei, giving rise to the primary a generative cell. endosperm nucleus, often triploid (3n). The mature microgametophyte is the The primary endosperm nucleus germinated pollen grain with its divides to produce a unique nutritive vegetative nucleus and two sperm. tissue, the endosperm. The mature female gametophyte of an Not all ancient basal-grade lineages of angiosperm is called an embryo sac, angiosperms have seven-celled, eight- which typically has seven cells and nucleate embryo sacs. eight nuclei. SUMMARY Ovarian Development and Fruits Ovaries develop into fruits enclosing seeds. Fruit is a defining characteristic of angiosperms. QUESTIONS 1. Distinguish among or between the following: calyx, corolla, and perianth; stigma, style, and ovary; complete and incomplete; perfect and imperfect; androecium and gynoecium. 2. Diagram and label as completely as possible a complete hypogynous flower, in which none of the floral parts are joined. 3. An imperfect flower is automatically incomplete, but not all incomplete flowers are imperfect. Explain. 4. Diagram and label completely a mature male gametophyte (germinated pollen grain) and a mature seven-celled, eight-nucleate female gametophyte (embryo sac) of an angiosperm. Compare these gametophytes with their counterparts in pine. 5. Double fertilization followed by the formation of endosperm is unique to angiosperms. How does double fertilization in the gnetophytes Ephedra and Gnetum differ from that in angiosperms? QUIZLET