Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Reproductive Life Cycles of Vascular Plants Plants The most primitive nonvascular and vascular plants sexually reproduce by spores. The more advanced vascular plants produce seeds. Non-vascular Mosses 400 million years ago Vascular Seedless plants Lycopods Horsetails Spores Ferns Seed plants Seeds Cycads Ginkgo Conifers Angiosperms 300 million years ago 200 million years ago 100 million years ago 55 million years ago Seed plants Seed plants are separated into gymnosperms and angiosperms. The seed habit vs. spore production: (1) Rather than producing a single spore type (homospory), seed plants produce a separate female megaspore and male microspore (heterospory). (2) The female gametophyte is retained on the mother plant (sporophyte) and is enclosed within a protective maternal seed coat. (3) The ovule has an opening designed to receive pollen that does not depend on water for male gamete transfer. Seed evolution Seed Plants - Extinct seed producing plants The evolution of the seed habit began during the Devonian period about 350 to 385 million years ago in progymnosperms. Fossil leaf Progymnosperms were sporeproducing, but showed heterospory. Heterospory is a first step in the evolution of a true seed. Progymnosperms are considered the common ancestor of all seed plants. Archaeopteris Seed evolution Seed Plants - Extinct seed producing plants The “seed ferns” in the late Devonian were the first plants to produce seedlike structures enclosed in female tissue called cupules. The cupules contained a single seed (megaspore) within protective coverings. Cupules Cupules Alethopteris Seed Plants - Gymnosperms Gymnosperms are the oldest living seed producing plants. The term gymnosperm means “naked seeds”. Gymnosperms include the cycads, ginkgo, gnetophytes (Ephedra, Gnetum) and the conifers (like pine, fir, and hemlock). Fir (Abies) cone Cycads Cycads appear in the fossil record 320 million years ago. Encephalartos Cycads The living cycads are palm-like in character and mostly tropical and subtropical. Cycas Cycads Cycads produce male and female cone-like sporangia on separate plants. Intact sporophyll cone Shattered cone Seed Zamia floridana Male shedding pollen Female Female with seeds Cycads Unlike other gymnosperms, cycad sperm is motile and swims. Male cone (strobili) Cycads Cycas nicely shows that the female is a modified leaf called a megasporophyll with attached sporangia. Cycas Cycads Females in other cycads are more cone-like. Encephalartos Lepidozamia Cycads Cycad seeds are covered with a fleshy sarcotesta that resembles and functions like a true fruit. Encephalartos Ginkgo This is a single genus family. Only extant member from a family developed over 120 million years ago. Ginkgo is a unique gymnosperm because it has a broad leaf and is deciduous. Ginkgo Female ovule in Ginkgo are produced in pairs at the tips of reproductive short shoots. There is a small opening at the tip of the ovule to permit sperm to enter. Reproductive short shoot Pair of ovules Ovule opening Ginkgo Males are produced on separate trees and are clusters of sperm containing microsporangia that release pollen. Microsporangia Ginkgo Ginkgo seeds are produced in autumn and have an outer fleshy and inner hard seed coat. Endosperm Embryo Intact seed with fleshy seed coat covering. Seeds with the outer coat removed. Cut seed showing embryo and endosperm. Gnetophytes Gnetophytes include Gnetum, Ephedra, and Welwitschia. Gnetophytes appear to be the closest living relatives to the Angiosperms. Ephedra can have double fertilization. Ephedra female (left) and male (right) plants. Gnetophytes Welwitschia is an unusual desert plant that produces only two opposite strap-like leaves on either side of a central disc (stem). Gnetophytes Reproductive structures are produced on the margin of the central disc. Seeds have a papery outer seed coat. Welwitschia Conifers Conifers (cone-bearing) represent the largest group of genera in the gynmosperms dating back to 290 million years ago. Abies Picea Pseudotsuga Conifers Conifers include: Araucariaceae – Agathis and Araucaria. Pinaceae – Abies, Cedrus, Larix, Picea, Pinus, Pseudolarix, Pseudotsuga, Tsuga, and Sciadopitys. Cupressaceae – Chamaecyparis, Cupressus, Juniperus, Microbiota,Thuja, Platycladus, and Thujopsis. Taxodiaceae – Taxodium, Metasequoia, Cryptomeria, Cunninghamia, Taiwania, Sequoia, and Sequoiadendron. Taxaceae – Cephalotaxus, Taxus, and Torreya. Taxaceae Members of the Taxaceae produce males and females on separate plants. The female is cone-like and produces a drop of fluid at the tip of the ovules to capture air borne pollen. Taxus Cephalotaxus Taxaceae Males shed wind-borne pollen. Cephalotaxus Taxus Wind-borne pollen Taxaceae Unlike many of the other conifers, members of the Taxaceae produce seeds with a fleshy outer coat. In Taxus, the red coat is an aril. Taxus Cephalotaxus Conifer life cycle Conifer Reproductive Cycle - Pine Pine reproduction takes two growing seasons to complete. Conifer life cycle Pollen formation The male gametophyte is produced in a staminate cone. Staminate cone Conifer life cycle Pollen formation Four haploid pollen grains are produced through meiosis. Conifer life cycle Pollen formation The male gametophyte is a winged pollen grain spread by the wind. Pollen grain Pollen in the staminate cone. Conifer life cycle Ovule formation In most gymnosperms, the female gametophyte is produced in the axils of the ovulate cone between protective scales. The ovulate cone consists of many spirally arranged ovuliferous scales subtended by a cone bract. Ovulate cone Each ovuliferous scale has a pair of ovules on its surface. The ovuliferous scale will form the seed wing that covers the mature seed. Spruce (Picea) Ovuliferous scale Conifer life cycle Ovule formation Megaspore mother cell The haploid female gametophyte is developed within the nucellus (megasporangium) from the megaspore mother cell by meiosis. Nucellus Cone bract Ovuliferous scale Nucellus Megaspore mother cell Ovuliferous scale Cone bract Conifer life cycle Ovule formation Within the female gametophyte, several archegonia are formed each with one haploid egg cell. Micropyle Integuments Archegonia Ovule Conifer life cycle Pollen germination The pollen grain contains two nuclei – one is the tube cell and one is the generative cell. Following pollen germination, the generative nucleus divides into additional sperm nuclei. Pollen grain Germinating pollen Sperm nuclei Pollen tube Generative cell Tube cell Tube nucleus Conifer life cycle Fertilization The pollen germinates and the pollen tube enters the ovule and deposits the sperm nuclei. The sperm nucleus and egg nucleus fuse to complete fertilization and form the 2n zygote. However, fertilization in most conifers does not occur until months after the pollen tube enters the ovule. In pines, it can take over a year between pollination and egg cell fertilization. Micropyle Pollen Ovule Egg nucleus Conifer life cycle Fertilization Fusion of the egg and sperm cells completes fertilization and results in a new zygote. Pollen tube Sperm nucleus Egg nucleus Archegonium Conifer life cycle Fertilization Following gamete fusion, the suspensor cells elongate and several multiple embryos (polyembryos) are formed inside a single ovule. Initially, there can be as many as 12 developing embryos. However, it is usual that one embryo becomes dominate and continues to develop as the seed matures. Suspensors Embryos Conifer life cycle Fertilization In most conifers, the mature seed is attached to a wing derived from the ovuliferous scale and the embryo that will be the next sporophytic generation. Seed coat Embryo A pair of mature seeds in pine Cone bract Wing Endosperm (1n) Female gametophyte Spruce (Picea) seed Seed Wing Seed Conifer life cycle Fertilization In gymnosperms, there is only a single fertilization event between the sperm and egg nuclei. Therefore, the endosperm is the female gametophyte (megasporangia) that is haploid. Endosperm Female gametophyte (1n) Seed coat Embryo Mature pine seed Angiosperm life cycle Angiosperms are true flowering plants. The term angiosperm means “enclosed seeds” and refers to the female ovary tissue that forms the fruit surrounding angiosperm seeds. Angiosperms are the dominant plant type on Earth with approximately 250,000 species, compared with only about 8,000 living species of gymnosperms. Akebia quinata Angiosperm life cycle Angiosperms Toward the end of the Mesozoic era (140 million years ago), angiosperms began to appear. They became dominant by 90 million years ago as they replaced gymnosperms. The sudden abundance of angiosperms in the fossil record was famously called “an abominable mystery” by Charles Darwin. Charles Darwin Angiosperm life cycle Angiosperms One reason for angiosperm success and diversity is the mutualistic co-evolution of animals (especially insects) as pollinators and seed dispersers. Charles Darwin Angiosperm life cycle Angiosperms present an incredible diversity of flower forms and colors. Angiosperm life cycle Flowers are the sexual organs in angiosperms. The male organ is the stamen and the female is the pistil. Stamen Anther Pollen Filament Petals Stigma Stigma Style Anthers Pistil Petal Ovule Ovary Ovary Sepal Style Receptacle Pedicel Receptacle Pedicel Angiosperm life cycle Angiosperm life cycle Angiosperm life cycle Pollen development (Microsporogenesis) Male gametes are formed in the pollen grains (microspores) that are produced within the stamen of the flower. Stamen Anther Anther Pollen grains Pollen grain Filament Angiosperm life cycle Pollen development (Microsporogenesis) There are four pollen sacs in each lily anther. Microsporangia (Pollen sacs) Microspore mother cells Cross-section lily stamen Angiosperm life cycle Pollen development (Microsporogenesis) Microspore mother cells (Microsporocytes) will divide via meiosis to become the pollen grains. The tapetum is a layer of nutritive cells surrounding the developing pollen. Microsporangia (Pollen sacs) Microspore mother cells initiating division Tapetum Angiosperm life cycle Pollen development (Microsporogenesis) Many pollen grains matures within each pollen sac. Eventually the anther will open to release the pollen. Pollen sacs Pollen Angiosperm life cycle Pollen development (Microsporogenesis) The outer layer of the pollen grain is called the exine. The exine provides protection for the pollen grain. The exine tends to be smooth in wind-pollinated plants and rough or spiked in insect-pollinated plants. Hibiscus pollen with a rough exine indicating that the pollen is carried by insects. Angiosperm life cycle Pollen development (Microsporogenesis) A mature pollen grain typically contains two nuclei; one generative nucleus and one tube nucleus. The outer surface of the pollen grain has an outer exine and inner intine interrupted by several pores. Pore Pore Tube nucleus Tube nucleus Intine Generative nucleus Exine Pore Generative nucleus Angiosperm life cycle Pollen germination The pollen tube will exit (germinate) through one of the pores. Generative nuclei Tube nucleus Pore Pollen tube Tube nucleus Generative nuclei Angiosperm life cycle Pollen germination The pollen tube moves down the style towards the ovule. The journey may be short, less than ½ an inch in beet; or it might be a long distance (over 5 to 15 inches) as in lily or corn. Pollen Stigma Style Pollen tube Generative nuclei Tube nucleus Angiosperm life cycle Pollen germination The tube nucleus acts to guide the pollen tube, while the generative nuclei will eventually fuse with female egg cells. The pollen tube enters the micropyle (a natural opening between the integuments) releasing the generative nuclei into the embryo sac. Pollen Pollen tube Micropyle Integuments Embryo sac Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) In the angiosperm flower, the female gametophyte consists of nucellar tissue that is surrounded by either a single or a double outer tissue layer called the integuments. The integuments will become the seed coat. Integuments Integuments Nucellus Nucellus Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) In the nucellus, a megaspore mother cell forms that will undergo meiosis and become the female egg cells within the egg sac. Outer Integument Inner Integuments Megaspore mother cell Nucellus Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Ovule development over time. The integuments grow to cover the nucellus and continues to enclose the embryo sac creating the ovule. Funiculus Nucellus Integuments Micropyle Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Embryo sac Integuments Integuments Micropyle Nucellus Megaspore mother cell forms in the nucellus. Funiculus Megaspore mother cell undergoes meiosis. Haploid cells form in the embryo sac. Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Ovules vary in their orientation and shape in the ovary. Three common types include orthotropous, anatropous, and hemianatropous. Ovary Integuments Ovule Egg sac orthotropous anatropous hemianatropous Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Soon after the completion of meiosis, the egg sac is formed. Funiculus Ovary Ovule A gap is retained between the enveloping integuments called the micropyle. This is the opening where the pollen tube will enter the embryo sac. Micropyle Integuments Egg sac Nucellus Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Nuclei in the embryo sac from by meiosis. In meiosis I, there is an initial cell division to give two cells with diploid nuclei. At the end of meiosis II, there are four linear haploid nuclei formed. Only one nucleus survives to duplicates to form the archegonia in gymnosperms or the contents of the embryo sac in angiosperms. Angiosperm life cycle Ovule formation Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) In angiosperms, the most common arrangement of cells in the embryo sac is called the Polygonum type and occurs in about two-thirds of flowering plants Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) The Polygonum type of embryo sac has seven cells (eight nuclei) that occupy specific locations that dictate their function. Egg sac Angiosperm life cycle Ovule formation Embryo sac development (Megagametogenesis) Egg sac organization in lily Andipodals Central cell polar nuclei Egg and synergids Angiosperm life cycle Fertilization In angiosperms, sexual reproduction involves double fertilization. Once the pollen tube enters the micropyle it releases the male nuclei into the embryo sac. For double fertilization, one male and one female egg nuclei fuse to form the zygote and a second male and two female egg nuclei fuse to form the triploid endosperm. Pollen Pollen tube Micropyle Integuments Embryo sac Angiosperm life cycle Fertilization The female synergid cells are closely associated with the egg cell and function to attract and guide male nuclei to the egg cell for fertilization. Synergids produce a chemical that attracts the pollen tube to the micropyle, arrests its growth, and ensures the proper release of the sperm cells into the ovule. Angiosperm life cycle Fertilization The exact function of antipodal cells is not completely understood, but they disintegrate soon after fertilization of the egg cell. Angiosperm life cycle Fertilization The pollen tube enters the ovule through the micropyle and deposits the two male nuclei into the embryo sac. Female egg nuclei Embryo sac Pollen tube entering the ovule Angiosperm life cycle Fertilization Double fertilization One generative nucleus fuses with the egg cell to form the zygote (2n embryo) Embryo sac Central cell Egg cell The second generative nucleus fuses with the central cell and its two polar nuclei to form the 3n endosperm. Double fertilization in lily. Angiosperm life cycle Fertilization Most seeds of angiosperms mature to include three basic parts. The diploid embryo from the fusion of male and female gametes. The triploid endosperm from the fusion of one male and two female gametes. The diploid seed coat developed from the integuments derived from diploid maternal tissue. Seed coat Embryo Endosperm