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gymnosperms - The Plant Kingdom Seed Plants Chapter 28 Two...

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Unformatted text preview: The Plant Kingdom Seed Plants Chapter 28 Two Groups of Seed Plants Gymnosperms Seeds are totally exposed or borne on the scales of cone Ovary wall does not surround the ovules Angiosperms Flowering plants Produce seeds within a fruit (a mature ovary) Gymnosperm and Angiosperm Evolution Major Innovations of Seed Plants Seeds Recall that the ferns have their gameteproducing organs (antheridia, archegonia) located directly on an unprotected gametophyte. Some extinct relatives of modern ferns developed structures that enclosed the gametophyte. "Seed ferns." The "progymnosperms' were similar to the seed ferns No ferns that are living today produce seeds Modern Seed Plants The gametophytes are very reduced compared to ferns and bryophytes. As with ferns, the sporophyte is the dominant portion of the life cycle. Pollen Instead of having sperm that swim free in external water (as in ferns), seed plants protect their sperm within the male gametophyte. Another term for this male gametophyte is pollen. The sperm cells (or sperm nuclei) are protected from drying out by the pollen grain that move through the air and deposits the sperm very near their final destination the egg cell. Pine pollen Gymnosperms What are they? Gymnosperms Gymno naked, sperm seed Refers to the presence of ovules that are exposed at the time of fertilization Compare female pine cone to an apple. Gymnosperms are seed plants (seeds develop from a ripened ovule) but do not form fruits a characteristic only of angiosperms. Angiosperms have ovules enclosed by carpels. Phylogeny Although controversial, recent evidence (molecular) indicates that living gymnosperms are monophyletic. They may have arisen from progymnosperms or seed ferns, all of which are extinct today. Taxonomy The four major groups of gymnosperms are: Cycadophyta Ginkgophyta Gnetophyta Coniferophyta Cycads 910 genera. Plants are shrubs or palmlike trees. Leaves Clustered in a tight clusters at the terminal parts of the stem (rosette). They are pinnately or bipinnately compound. The cones can be quite large with many spirally arranged scales Cycads (Phylum Cycadophyta) Palmlike or fernlike in appearance but reproduce with pollen and seeds in conelike structures Male and female cones are produced on separate plants (dioecious). Pollen sacs occur on the undersurface of the scales of microsporangiate cones. Ovulate (megasporangiate) cones can either have large scales or leaflike sporophylls with marginal ovules. Male cone Female cone Cycas male cone Zamia female cone Cycas megasporophylls Cycads Cycads are the last survivors of a previously more widespread and diverse group of Mesozoic (Triassic, Jurassic) plants. Some cycads are grown as ornamentals or for food (sago starch from Cycas). The one cycad genus that occurs in the U.S. is Zamia. Ginkgo Maidenhair Tree One species (Ginkgo biloba). Tree was discovered in China (cultivated for thousands of years) is considered a living fossil. Leaves, wood, etc. of the living plants are identical to fossil Ginkgo. Leaves alternate or fascicled on short (spur) shoots. They are fanlike with many dichotomously branched veins. Plants are dioecious. Pollen is borne in axillary spikelike clusters of sporangiophores that resemble stamens. There are usually two naked ovules on the tip of a forked peduncle. Ginkgo The seed is large, drupelike (looks like a fruit but isn't!) and has a fleshy foul smelling outer layer Butyric acid rotten butter smell I call it dog vomit! Once you smell it you never forget it Inner part of seed (nucellus and megagametophyte) is edible used in Asian cooking. Gnetophyta Families each with only one genus Gnetum Ephedra Welwitschia). These three genera are diverse and do not appear related. Gnetum The most angiospermlike of the three, is pantropical. It is a vine or small tree with opposite, simple, pinnatelyveined leaves. Pollen and ovules are borne in flower like structures. cones Ovules on female Ephedra Mormon tea of the W. U.S. Shrub or woody vine with jointed green stems Simple often scalelike, deciduous leaves. Pollen and ovules borne in flowerlike structures. male Female Welwitschia A large herb found in the Namib deserts of Africa. It produces only two leathery, straplike leaves its entire life these get torn and battered by the wind into gnarled masses. Plants are dioecious, the pollen and ovules borne on stalked cones. Male pollen cones Welwitschia microsporangiate cones Female plant Welwitschia cones with seeds Conifers Largest of the gymnosperm divisions. Includes the following families: Taxaceae (Taxus or yew) Podocarpaceae (Podocarpus) Cupressaceae (includes Taxodium or bald cypress, Sequoiadendro the giant sequoia, and Juniperus the juniper or cedar) Araucariaceae (Norfolk Island pine) Pinaceae (Pinus pine, Picea spruce, Abies fir, Larix larch). Taxus yew Podocarpus Taxodium bald cypress Sequoiadendron the giant sequoia Juniperus juniper or cedar Araucaria Norfolk Island Pine Pinus pine Pinus pine Picea spruce Abies fir Larix larch Conifers The name conifer comes from the Latin for conebearer and refers to the reproductive structures that produce the pollen and the seed. Many conifers have evergreen leaves that have morphological and anatomical adaptations to dry conditions (e.g. thick cuticle, sunken stomata). Economically, conifers such as pine, spruce, fir, etc. produce most of the wood used for lumber and paper pulp. Pine Life Cycle 1 Pine tree A mature sporophyte Pine gametophytes Extremely small Nutritionally dependent on sporophyte generation Pine Life Cycle 2 Pine is heterosporous Produces microspores and megaspores in separate cones Male cones produce microspores that develop into pollen grains (immature male gametophytes) Carried by air currents to female cones Pine Life Cycle 3 Female cones produce megaspores One of each four megaspores (meiosis) develops into a female gametophyte within an ovule (megasporangium) Pine Life Cycle 4 Pollination Transfer of pollen to female cones Pollen tube Grows through megasporangium to egg within archegonium After fertilization Zygote develops into embryo encased inside seed adapted for wind dispersal Pine Life Cycle Male cone Each scale bears two microsporangia Microsporangium 2 Microspores, each of which develops into a pollen grain Scale from a Pollen grains are 3 transferred to the male cone 4 female cone by wind 3 Scale from a Female female cone gametophyte Megasporangium Each scale bears two ovules Megaspore (megasporangia) Growing pollen Ovule tube Fertilization Meiosis HAPLOID (n) GAMETOPHYTE GENERATION Immature female cone DIPLOID (2n) SPOROPHYTE GENERATION Zygote Second sperm Papery nucleus wings Pollen tube Seed coat 5 Sperm nucleus united with egg nucleus Mature female cone (seed cone) Male cones (pollen cones) 1 6 Embryo Pine (mature sporophyte) Newly germinated seedling Two seeds on the Female upper surface of the gametophyte scale (nutritive tissue) Fig. 28-4, p. 604 Pine Life Cycle Begin with the mature pine tree. Pine needles are actually the leaves that occur in clusters (fascicles). Pine Life Cycle The plant produces both male and female cones (strobili). Female Cone The ovule is actually a specialized female sporangium (megasporangium) enclosed by integuments. The ovule develops on an ovuliferous scale that is subtended by a bract. Female Cone When the female cone is very small and young, meiosis takes place within the megasporangium. The female gametophyte (megagametophyte) develops an archegonium which is composed of just a few cells forming a wall and an egg cell. In addition, the megagametophyte also consists of a nutritive tissue called the nucellus. A porelike opening between the integuments is called the micropyle. micropyle archegonium egg Female Cone By the time the female cone is mature, pollination and fertilization have already occurred and the seed is developing from an ovule. Serotinous cones require fire for seed release from Male Cone The male cone produces pollen sacs (microsporangia) Meiosis produces a tetrad of microspores that mature into pollen grains. Pollen One pollen grain forms from each microspore. The mature pollen grain is the male gametophyte (microgametophyte) composed of only three cells: prothallial, generative and tube cells. The pollen grain has wings on each side that aid in dispersal through the air. Pollination Pollination occurs when wind carries pollen to the micropylar opening of the ovule. Some gymnosperms have pollination droplet that the pollen grains adhere to. Upon evaporation, the pollen is drawn into the micropyle. Once there, they form a pollen tube. Fertilization Fertilization occurs when the sperm nucleus migrates down the pollen tube, into the archegonium, and finally fuses with the egg nucleus. sperm are complete, motile cells that actively swim. In some gymnosperms (e.g. cycads) the In others, free nuclei are released into the archegonium from the pollen tubes. Fertilization Several eggs may be fertilized at once and may begin developing as independent embryos at the apex of long suspensor cells. the mature ovule. Usually, only one embryo survives within Seed The mature ovule is the seed. consists of an outer testa (seed coat developed from the integument), a thin nucellar layer (used up), a ripened megagametophyte, and an embryo. for the seedling (sometimes referred to incorrectly as endosperm which is found only in angiosperms). The female gametophyte act as the nutritive tissue Embryos may have two to many cotyledons. ...
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