BIO 311D 1st Edition Lecture 30Outline of Last Lecture I. Adaptations for acquiring resources were key steps in the evolution of vascular plantsII. Shoot Architecture and Light CaptureIII. Root Architecture and Acquisition of Water and MineralsIV. Different mechanisms transport substances over short or long distancesV. Short-Distance Transport of Solutes Across Plasma MembranesVI. How Solutes and Pressure Affect Water PotentialVII. Water Movement Across Plant Cell MembranesVIII. Aquaporins: Facilitating Diffusion of WaterIX. Long-Distance Transport: The Role of Bulk FlowOutline of Current Lecture I. Flowers, double fertilization, and fruits are unique features of the angiosperm life cycleII. Flower Structure and FunctionIII. Development of Male Gametophytes in Pollen GrainsIV. Development of Female Gametophytes (Embryo Sacs)V. PollinationVI. Coevolution of Flower and PollinatorVII. Double FertilizationVIII. Seed Development, Form, and FunctionIX. Endosperm DevelopmentX. Structure of the Mature SeedCurrent LectureFlowers, double fertilization, and fruits are unique features of the angiosperm life cycle• Plant lifecycles are characterized by the alternation between a multicellular haploid (n) generation and a multicellular diploid (2n) generation• Diploid sporophytes (2n) produce spores (n) by meiosis; these grow into haploid gametophytes (n) • Gametophytes produce haploid gametes (n) by mitosis; fertilization of gametes produces a sporophyte• In angiosperms, the sporophyte is the dominant generation, the large plant that we see• The gametophytes are reduced in size and depend on the sporophyte for nutrients• The angiosperm life cycle is characterized by “three Fs”: flowers, double fertilization, andfruitsFlower Structure and Function• Flowers are the reproductive shoots of the angiosperm sporophyte; they attach to a partof the stem called the receptacle• Flowers consist of four floral organs: sepals, petals, stamens, and carpels• Stamens and carpels are reproductive organs; sepals and petals are sterile• A stamen consists of a filament topped by an anther with pollen sacs that produce pollen • A carpel has a long style with a stigma on which pollen may land• At the base of the style is an ovary containing one or more ovules• A single carpel or group of fused carpels is called a pistil• Complete flowers contain all four floral organs• Incomplete flowers lack one or more floral organs, for example stamens or carpels• Clusters of flowers are called inflorescencesDevelopment of Male Gametophytes in Pollen Grains• Pollen develops from microspores within the microsporangia, or pollen sacs, of anthers• Each microspore undergoes mitosis to produce two cells: the generative cell and the tube cell• A pollen grain consists of the two-celled male gametophyte and the spore wall• If pollination succeeds, a pollen grain produces a pollen tube that grows down into the ovary and discharges two sperm cells near the embryo sacDevelopment of Female Gametophytes (Embryo Sacs)• The embryo sac, or female gametophyte, develops within the ovule• Within an ovule, two integuments surround a megasporangium• One cell in the megasporangium undergoes meiosis, producing four megaspores, only one of which survives• The megaspore divides, producing a large cell with eight nucleiPollination• In angiosperms, pollination is the transfer of pollen from an anther to a stigma• Pollination can be by wind, water, or animals• Wind-pollinated species (e.g., grasses and many trees) release large amounts of pollenCoevolution of Flower and Pollinator• Coevolution is the evolution of interacting species in response to changes in each other• Many flowering plants have coevolved with specific pollinators• The shapes and sizes of flowers often correspond to the pollen transporting parts of their animal pollinators– For example, Darwin correctly predicted a moth with a 28 cm long tongue based on the morphology of a particular flowerDouble Fertilization• After landing on a receptive stigma, a pollen grain produces a pollen tube that extends between the cells of the style toward the ovary• Double fertilization results from the discharge of two sperm from the pollen tube into the embryo sac• One sperm fertilizes the egg, and the other combines with the polar nuclei, giving rise tothe triploid food-storing endosperm (3n) Seed Development, Form, and Function• After double fertilization, each ovule develops into a seed• The ovary develops into a fruit enclosing the seed(s)Endosperm Development• Endosperm development usually precedes embryo development• In most monocots and some eudicots, endosperm stores nutrients that can be used by the seedling• In other eudicots, the food reserves of the endosperm are exported to the cotyledonsEmbryo Development• The first mitotic division of the zygote splits the fertilized egg into a basal cell and a terminal cell• The basal cell produces a multicellular suspensor, which anchors the embryo to the parent plant• The terminal cell gives rise to most of the embryo• The cotyledons form and the embryo elongatesStructure of the Mature Seed• The embryo and its food supply are enclosed by a hard, protective seed coat• The seed enters a state of dormancy• A mature seed is only about 5–15% water• In some eudicots, such as the common garden bean, the embryo consists of the embryonic axis attached to two thick cotyledons (seed leaves)• Below the cotyledons the embryonic axis is called the hypocotyl and terminates in the radicle (embryonic root); above the cotyledons it is called the epicotyl• The plumule comprises the epicotyl, young leaves, and shoot apical meristem• A monocot embryo has one cotyledon• Grasses, such as maize and wheat, have a special cotyledon called a scutellum• Two sheathes enclose the embryo of a grass seed: a coleoptile covering the young shootand a coleorhiza covering the young rootSeed Dormancy: An Adaptation for Tough Times• Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling• The breaking of seed dormancy often requires environmental cues, such as temperature or lighting changesSeed Germination and Seedling Development• Germination depends on imbibition, the uptake of water due to low water potential of the dry seed• The radicle (embryonic root) emerges first • Next, the shoot tip breaks through the