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Gymnosperms: gymnos “naked” sperma “seed”, seed plants with exposed seeds (no fruit). The seeds are single or borne on the scales of cones. (Only flowering plants produce fruit) average = 70.0 In the gymnosperms, the gametophytes are not free living. The female gametophyte is maintained in the megasporangia. The male gametophyte is the pollen and is released and travels to the female gametophyte for fertilization. These are adaptations to reduce the vulnerability of the gametophyte. After fertilization the new sporophyte generation (the embryo in the seed) is released. This transition required the evolution of pollen and seeds found in gymnosperms. Monilophytes True ferns Horsetails heterospory (in some) Ophioglossalean ferns leptosporangia heterospory (in some) Whisk ferns Marattialean ferns megaphylls Isoetes Selaginella (Zosterophyllophyta) (Lepidodendrids) heterospory heterospory Seed plants chloroplast DNA inversion Lycopodium group reduced gametophyte Lycophytes (Other Rhyniophytes) (Aglaophyton) (Rhyniophytes) lignin (Rhynia) strongly lignified vascular tissue (xylem and phloem) Evolution of seeds from retained megaspores. branched, independent sporophyte Bryophytes Conifers Reduction of the gametophyte Sporophyte (2n) Bryophyte Sporophyte (2n) Gametophyte (n) (a) Sporophyte dependent on gametophyte (mosses and other bryophytes). Gametophyte (n) (Progymnosperms) (Seed ferns) Conifers Cycads Gingkos Gnetophytes Cephalotaxaceae Podocarpaceae Gnetophytes Welwitschia small, independent gametophyte (ferns and other seedless vascular plants). Taxaceae Microscopic female gametophytes (n) inside these parts of flowers (dependent) Gnetum Pinaceae Ephedra Flowering plants Cycads Angiosperm Sporophyte (2n) (independent) (c) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). Cupressaceae Araucariaceae Sciadopityaceae Ginkgo Microscopic male gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Monilophyte (b) Large sporophyte and Microscopic female gametophytes (n) in ovulate cones (dependent) Gymnosperm Gymnosperms: Sporophyte (2n), the flowering plant (independent) (“Seed ferns”) “Progymnosperms” (Archaeopteris) seeds Monilophytes wood Earliest Tracheophytes 1 Progymnosperms such as this drawing of an “Archaeopteris” had secondary stem growth similar to today’s conifers. They had vascular bundles surrounding a pith. But they produced spores on leaves like ferns. The evolution of pollen is important because previously a film of water was required for sperm to swim to the archegonia. With pollen the sperm only need to swim (or move) down the pollen tube. Pollen - male gametophyte, protected by a spore wall These existed in the Devonian (around 400 M years ago. The evolution of wood is important because it provided strength for very tall and long-lived trees that could out-compete other plants. 25m tall Evolution of leaves in progymnosperms What is a seed? “Baby plant in a box with its lunch” Dr. David Biesboer, 1978. Advantages of seeds: The young sporophyte no longer needs to grow and photosynthesize immediately (it can go dormant). The sporophyte can be dispersed (air, animal vector). The sporophyte is protected. Disadvantage: very costly to make. The first seed plants appeared in the Devonian (400 M years ago). These were small trees with fern-like leaves. The name “seed fern” was invented to describe them. Development of the seed Ovule - this is the megasporangium surrounded by the integument before the ovule becomes a seed. Integument - protective covering of the ovule, sporophyte tissue that becomes the seed coat. The integument has an opening at one end called the micropyle that allows the pollen tube entry. What were the precursors for the development of seeds? 1. Heterospory - this arose independently several times. It allowed for one mobile spore - microspore and one larger nutrient laden megaspore. 2. Retention of the megaspore within the sporophyte and reduction of megaspore number from 4 to 1. 3. Development of the protective layer called the integument. The megasporangium is called a nucellus in seed plants, it is 2n sporophyte tissue and produces the megasporocyte that undergoes meiosis. The megaspore is the first product of meiosis (so it is haploid) and it gives rise to the megagametophyte (egg sac). integument Evolution of the seed in progynmosperms This leads to another problem - delivery of the male gamete. 3 aborted megaspores functional megaspore 2 Pollen is an immature microgametophyte. In heterosporous species like Selaginella, the microspore is released and then the microgametophyte grows within the spore wall and produces sperm which are released. rays resin duct All gymnosperms have secondary xylem and phloem, rays and bark. The wood is regular because there are no vessels (except in Gnetophytes). The phloem contains sieve cells (not sieve elements and companion cells). summerwood Pollen is derived from this. Mature pollen leaves the microsporangium consisting of 2-5 haploid nuclei. When it reaches the ovule, it produces 2 sperm cells and a pollen tube. springwood one annual growth ring Many gymnosperms produce resin that accumulates in resin ducts and inhibits insect feeding. tracheids Ginkgo - one species left, only found in the wild in China, planted at monasteries in Japan and China for centuries. Now it is planted as an ornamental tree throughout the world, and all over the St. Paul campus. They have stinky seeds (not fruit). Characteristics of Gingko: Dioecious (male and female plants). Large trees with fan shaped leaves. The microgametophytes produce large flagellated sperm. Ovules are borne on stalks rather than cones. Gingko male cones Leaves with dichotomous venation In Ginkgo, the pollen enters the micropyle, grows a bit, and produces large sperm. Gingko ovules Gingko embryo 3 Cycads - Mesozoic dinosaur chow. There are 125 extant species, mostly tropical Cycad characteristics: They are all dioecious, there are male and female sporophytes. (Monoecious plants produce both male and female reproductive structures.) They have compound leaves (like ferns). They grow slowly and live a long time (1000 years). The seeds are eaten by birds, and dispersed in that way. The seeds are poisonous to humans causing a slow degenerative brain disease. Pollination can take up to 10 years. Some are pollinated by insects. Ovulate cone Here is a human removing a male cone from a cycad. Structure of the cycad ovule. Male cone w/pollen sacs Cycad ovule Conifers (“cone bearer”) - pines, firs, spruces, hemlocks, 650 species Woody seed cone (some lack woody cones - yews, junipers the fleshy covering of their seeds is not a fruit, it is integument or modified scales). Conifer families: Pinaceae: pines, firs, spruces Cupressaceae: junipers, cypresses, redwoods Taxaceae: Yews Araucariaceae: monkey puzzle tree, Norfolk Island Pine Podocarpaceae: southern hemisphere conifers Wind pollination - lots and lots of pollen produced Produce cones w/ ovules - two year cycle 4 Pinaceae Characteristics -Needle-like leaves singly or in fascicles (clusters). -Most are monecious (bisexual plants with male and female cones) -Two seeds per seed scale Pines, genus Pinus is the largest genus with 93 species. -Needles clustered in 2-5 per fascicle (except for pinyon pine which is a single leaved pine). -Needles oval to triangular in cross section. -Cones hang down and vary in size, usually they shed their seeds, some “closed-cone pines” such as the jack pine require heat to open the cone. Fire removes the competition. -Bristlecone pines (Pinus longaeva) are the longest living organism (5000 years). mesophyll vascular bundle phloem xylem resin duct Cross section of a pine needle. Showing thick cuticle, sunken stomata, no air space, fiberous epidermis. All modifications to reduce desiccation. Fascicles of pine needles. Pollen germination leads to pollen tube growth and the production of two sperm cells within the ovule. endodermis Female gametophyte (n) guard cells mesophyll stomatal pore resin duct Male gametophyte (within germinating pollen grain) (n) Discharged sperm nucleus (n) Micropyle Pollen grain (n) (b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle, the only opening through the integument, allows entry of a pollen grain. The pollen grain contains a male gametophyte, which develops a pollen tube that discharges sperm. schlerenchyma epidermis The seed develops from the ovule. The seed is the sporophyte embryo, along with its food supply, packaged in a protective coat. Egg nucleus (n) Spore wall Pine life cycle as an example for gymnosperms Important features: -Dominant sporophyte generation Seed coat (derived from Integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte) (c) Gymnosperm seed. Fertilization initiates the transformation of the ovule into a seed, which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument. -Development of seeds from fertilized ovules -Pollen delivers the male gametophyte to the ovule The megasporangium (nucellus) produces a megasporocyte that undergoes meiosis. One megaspore survives to become the female gametophyte. In the seed, the embryo is surrounded by female gamotophyte (1n) tissue. 5 Structure of the female cone of pine. Structure of the male cone of pine. pollen tube integument archegonium with egg micropylar chamber female gametophyte nucellus Conifers Pinaceae Characteristics Cephalotaxaceae Podocarpaceae Gnetophytes Taxaceae -Needle-like leaves singly or in fascicles (clusters). Cupressaceae Araucariaceae Sciadopityaceae -Most are monecious (bisexual plants with male and female cones) -Two seeds per seed scale Welwitschia Gnetum Pinaceae Ephedra Pines, genus Pinus is the largest genus with 93 species. Flowering plants Ginkgo -Needles clustered in 2-5 per fascicle (except for pinyon pine which is a single leaved pine). Cycads -Needles oval to triangular in cross section. (“Seed ferns”) “Progymnosperms” (Archaeopteris) seeds Monilophytes wood -Cones hang down and vary in size, usually they shed their seeds, some “closed-cone pines” such as the jack pine require heat to open the cone. Fire removes the competition. -Bristlecone pines (Pinus longaeva) are the longest living organism (5000 years). Earliest Tracheophytes 6 Pinaceae, the pine family Firs (genus Abies) Examples: Each year’s growth is a whorl of branches. Pine (genus Pinus) Firs (genus Abies) Douglas firs (genus Pseudotsuga) Spruce (genus Picea) Hemlocks (genus Tsuga) Larch and tamaracks (genus Larix) Cedars (genus Cedrus) Single flat, blunt needles Erect cones Cone shatter at maturity Spruce (genus Picea) Hemlocks (genus Tsuga) Needles are single, angular in cross section and sharply pointed Needles are flat with a short petiole cones hang down Cones are small and hang down Black spruce has “closed cones” Tsuga canadensis Douglas firs (genus Pseudotsuga) Young hemlock trees Pseudotsuga menzeiesii is one of the main trees cut for lumber in the Pacific NW Single needles resemble spruce but are softer Cones hang down Easily identified by its cone with three-lobed bracts that stick out between the scales 7 Larch and tamaracks (genus Larix) Unusual because they are deciduous (they lose all of their leaves in the Fall) Older branches have clusters of multiple needles, oneyear old branches have single or spiral needles. scale bract Cones of Pseudotsuga menzeiesii note three lobed bract. Cedars (genus Cedrus) needles are single Female cone at pollination Male cones Cones of Deodar cedar Seed cones Families: Pinaceae: pines, firs, spruces, hemlock, larches, cedars Cupressaceae: junipers, cypresses, redwoods Taxaceae: Yews Araucariaceae: monkey puzzle tree, Norfolk Island Pine Podocarpaceae: southern hemisphere conifers Cupressaceae: Junipers, Cypresses and Redwoods shrubs or trees small scale-like leaves monoecious or dioecious cone scales are woody (as in cypresses) or fleshy (as in juniper) 130 species, distributed world wide some species are “closed-cone” Examples: dawn redwood (Metasequoia glyptostroboides) California coast redwood (Sequoia sempervirens) Sierra redwood (Sequoiadendron giganteum) bald cypress (Taxodium distichum) 8 dawn redwood (Metasequoia glyptostroboides) deciduous, grows well here (can grow up to 200 ft. tall), was thought to be extinct but rediscovered in 1944 in China. California coast redwood (Sequoia sempervirens) Sierra redwood (Sequoiadendron giganteum) bald cypress (Taxodium distichum) Taxaceae: Yews shrubs or trees dark, linear, pointed leaves single seeds with a freshy covering (aril) these are the only conifers to lack cones Examples English Yew (Taxus baccata) English Yew (Taxus baccata). The wood is famous for its use in bows. 9 Podocarpaceae - Southern Hemisphere Conifers Araucariaceae - Southern Hemisphere Conifers shrubs or trees leaves vary from short needles to long oblong blades one ovule per cone scale seeds spread by birds mostly dioecious -generally large trees (monkey puzzle tree, southern pine) -the tallest trees in the tropics -leaves vary from needle-like to flat and broad -cones are large and globe shaped (disintegrate when ripe) cones genus Araucaria leaves ovules genus Podocarpus branches Gnetophytes -three genera: Ephedra, Gnetum, and Welwitschia -vessel elements in xylem -ovules surrounded by fleshy layer -similarities to flowering plants represent convergent evolution -convergent evolution: independent evolution of a trait such as wings in reptiles and mammals. Ephedra viridis (Mormon tea) It has some stimulant properties. Ephedrine and pseudoephedrine are the main active ingredients. Gnetum leyboldii - hard to distinguish from a flowering plant Welwitchia mirabilis Male and female reproductive structures of Welwitchia mirabillis pollen strobili microsporophylls Native to Namib desert. It has two leaves. It grows slowly, this plant is about 1000 years old. microsporangiate bearing structure Female cones megasporophylls Discovered by Friedrich Welwitsch (1806-1874) Austrian explorer and botanist. seed wings bract 10 Monocots Angiosperms “seed within a vessel” developed around 135 M years ago Please watch this movie about angiosperm lifecycle http://www.youtube.com/watch?v=H_UyDtaa8Ow Important developments in Angiosperms. - ovules enclosed in ovaries (the ovary forms the fruit) - pollen enclosed in stamen - double fertilization - further reduced male and female gametophytes increases the speed of egg development and pollination showy flowered non-showy flowered commelinids Lily group Ginger group Eudicots Grass group Alismatales Palms Magnoliids Caryophyllids The angiosperms are the magnoliids, the monocots and the eudicots. Laurels Magnolias Peperomias Pipe vines Star anise group single cotyledon tricolpate pollen Rosids Asterids Proteales Ranunculales Water lilies Amborella Living gymnosperms Gnetophytes Conifers vessels double fertilization fruit flowers Ginkgo Cycads Magnolia virginiana Unknown gymnosperms Next time: angiosperms (flowering plants) 11