BIO 311D 1st Edition Lecture 30 Outline of Last Lecture I Adaptations for acquiring resources were key steps in the evolution of vascular plants II Shoot Architecture and Light Capture III Root Architecture and Acquisition of Water and Minerals IV Different mechanisms transport substances over short or long distances V Short Distance Transport of Solutes Across Plasma Membranes VI How Solutes and Pressure Affect Water Potential VII Water Movement Across Plant Cell Membranes VIII Aquaporins Facilitating Diffusion of Water IX Long Distance Transport The Role of Bulk Flow Outline of Current Lecture I Flowers double fertilization and fruits are unique features of the angiosperm life cycle II Flower Structure and Function III Development of Male Gametophytes in Pollen Grains IV Development of Female Gametophytes Embryo Sacs V Pollination VI Coevolution of Flower and Pollinator VII Double Fertilization VIII Seed Development Form and Function IX Endosperm Development X Structure of the Mature Seed Current Lecture Flowers 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 and fruits Flower Structure and Function Flowers are the reproductive shoots of the angiosperm sporophyte they attach to a part of 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 inflorescences Development 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 sac Development 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 nuclei Pollination 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 pollen Coevolution 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 flower Double 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 to the 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 cotyledons Embryo 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 elongates Structure 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 shoot and a coleorhiza covering the young root Seed 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 changes Seed 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 soil surface In many eudicots a hook forms in the hypocotyl and growth pushes the hook above ground Light causes the hook to straighten and pull the cotyledons and shoot tip up In maize and other grasses which are monocots the coleoptile pushes up through