BIOL208_CH18.pdf

BIOL 208 CH 18 Joanne Nelson, Ts’msyen, MPH, PhD  Ray F. Evert Susan E. Eichhorn Raven Biology of Plants Eighth Edition CHAPTER 18 Gymnosperms © 2013 W. H. Freeman and Company Another tenet of indigenous plant knowledge is that we can learn a plant’s use by wh...

BIOL 208 CH 18 Joanne Nelson, Ts’msyen, MPH, PhD  Ray F. Evert Susan E. Eichhorn Raven Biology of Plants Eighth Edition CHAPTER 18 Gymnosperms © 2013 W. H. Freeman and Company Another tenet of indigenous plant knowledge is that we can learn a plant’s use by where it occurs. For example, it’s well known that a medicinal plant frequently occurs in the vicinity of the source of the illness. Source Source Chapter Outline Evolution of the Seed Progymnosperms Extinct Gymnosperms Living Gymnosperms Phylum Coniferophyta Other Living Gymnosperm Phyla: Cycadophyta, Ginkgophyta, and Gnetophyta Learning Outcomes After reading this chapter, you should be able to answer the following: 1. What is a seed, and why was the evolution of the seed such an important innovation for plants? 2. According to the current hypothesis, from which group of plants did seed plants evolve? What is the evidence for this hypothesis? 3. How do the mechanisms by which sperm reach the eggs in gymnosperms and in seedless vascular plants differ? 4. Give the distinguishing features of the four phyla of living gymnosperms. 5. In what ways do gnetophytes resemble angiosperms? Evolution of the Seed Seed plants are heterosporous, producing megaspores and microspores. Megagametophytes (female gametophytes) and microgametophytes (male gametophytes) are produced. Seed production is a modified form of heterospory, forming an ovule. A seed is a matured ovule containing an embryo. The immature ovule consists of a megasporangium and one or two additional tissue layers, the integuments. Evolution of the Seed Evolution of Ovule 1. Retention of megaspores within the megasporangium, a fleshy nucellus in seed plants. 2. Reduction of megaspore mother cells to one in each megasporangium. 3. Survival of only one of four megaspores produced by the spore mother cell, leaving a single functional megaspore. Evolution of the Seed Evolution of Ovule 4. Formation of a female gametophyte inside the single functional megasporangium. 5. Development of the embryo within the female gametophyte retained within the megasporangium. 6. Formation of an integument enveloping the megasporangium, except for the micropyle opening. 7. Modification of the apex of the megasporangium to receive microspores. Evolution of the Seed Early Evolution of Vascular Plants The exact order of events in vascular plant history is unknown due to incomplete fossil record. Oldest ovules or seeds from Late Devonian (365 million years ago) are known. Elkinsia polymorpha is an early seed plant with a nucellus and integumentary lobes. The ovules were surrounded by sterile structures called cupules. The integuments evolved through gradual fusion until the only opening left was the micropyle. Evolution of the Seed Seed Composition and Formation Ovule in modern seed plants: Nucellus enveloped by integuments with micropyle. Nucellus contains megagametophyte of nutritive tissue and archegonia. After fertilization, integuments form a seed coat. Embryo develos within the seed before Source dispersal, except in Ginkgo and many cycads. All seeds contain stored food. Evolution of the Seed Seed Plant Evolution Originated in Late Devonian period 365 million years ago. Over 50 million years, diverse seed- bearing plants evolved. Groups include seed ferns, cordaites, and conifers. Source  Evolution of the Seed Seed Plants and Megaphylls Seed plants have megaphylls, large leaves with multiple veins. Five phyla with living representatives: Coniferophyta, Cycadophyta, Gingophyta, Gnetophyta, and Anthophyta. Anthophyta includes angiosperms, while gymnosperms are evolutionary lineages. Gymnosperms are often dominant over wide areas, despite having fewer species than angiosperms. Source Progymnosperms A group of plants that existed around 290 million years ago. Reproduced through freely dispersed spores and produced secondary xylem similar to living conifers. Also produced secondary phloem. Both progymnosperms and Paleozoic ferns likely evolved from ancient trimerophytes. Progymnosperms had more elaborate branch systems and complex vascular systems. Significant evolutionary advance over trimerophytes and ferns. Presence of bifacial vascular cambium, producing secondary xylem and secondary phloem. Characteristic of seed plants, believed to have evolved first in progymnosperms. Source Progymnosperms Archaeopteris Type Progymnosperm Evolution Emergence in Devonian period (370 million years ago) and Mississippian period (340 million years ago). Main component of earliest forests until extinction. Lateral branch systems flattened in one plane, bore structures considered leaves. Evolution of eustele, a system of vascular tissues in discrete strands arranged around a pith. Larger branches of Archaeopteris-type progymnosperms had a pith. Homosporous majority of Archaeopteris species, some heterosporous. Wood production and heterospory predate seed evolution. Progymnosperms Archaeopteris Fossil Logs: Callixylon Up to a meter in diameter and 10 meters long. Species may have been large trees. Formation of extensive forests in some regions. Individuals may have resembled Section of Upper Devonian petrified wood (Callixylon conifers in branching patterns. trifilievii) Source Progymnosperms Seed Plant Evolution Seed plants evolved from plants similar to progymnosperms. Seed appeared in common ancestor of all seed plants. Detailed understanding of early seed plant evolution remains challenging. Extinct Gymnosperms Seed ferns (Pteridospermales) are diverse, artificial groups ranging from Devonian to Jurassic. They range from slender, branched plants to tree fern-like plants. Several extinct Mesozoic plants are sometimes included with seed ferns. A series of Devonian-Carboniferous seed ferns, including medullosans, are at the base of seed plant phylogeny. The relationship between different seed fern groups to living gymnosperms remains uncertain. Source Extinct Gymnosperms Bennettitales: Extinct Gymnosperms Bennettitales, or cycadeoids, were extinct gymnosperms with palm-like leaves. They disappeared during the Cretaceous. Some paleobotanists suggest they may have been part of the same evolutionary line as angiosperms. Reproductively, Bennettitales were distinct from cycads, with bisexual flower-like structures in some species. Living Gymnosperms Molecular Analysis of Gymnosperms An earlier hypothesis suggested a clade of "anthophytes" combining gnetophytes, Bennettitales, and angiosperms, Some molecular analyses suggest a monophyletic relationship emphasizing flowerlike reproductive structures. between gnetophytes and conifers. However, subsequent studies do not support this hypothesis. The gnetifer hypothesis suggests monophyletic conifers are sister to monophyletic gnetophytes. The phylogenetic relationships among seed plant lineages remain uncertain. The gnepine hypothesis links gnetophytes with the Pinaceae family, placing them as sister to other conifer families. Living Gymnosperms Ferns and other seedless vascular plants require water for motile flagellated sperm. Gymnosperms transport sperm to eggs without water. Pollen grain is transferred by wind to a megagametophyte within an ovule. Endosporic microgametophyte produces pollen tube after pollination. Gymnosperms and other seed plants Source do not form antheridia-filled sperm for egg fertilization. Living Gymnosperms Cycads and Ginkgo Fertilization Process Fertilization transitions between ferns and seedless plants, involving free-swimming sperm and nonmotile sperm. Microgametophytes produce a pollen tube, which doesn't penetrate the archegonium. Pollen grain bursts near the archegonium, releasing sperm cells. Sperm then swim to an archegonium, one of which fertilizes the egg. Living Gymnosperms In conifers, gnetophytes, and angiosperms, nonmotile sperm is transported directly to the egg cell by Ginkgo angiosperms pollen tubes. This innovation eliminates the need for free water for fertilization in seedless plants. The presence of haustorial pollen tubes in Ginkgo and cycads suggests the pollen tube evolved to absorb nutrients for sperm production. This conveyance of nonmotile sperm Diversity of pollen tube pathways in seed plants is a later evolutionary modification. Source Phylum Coniferophyta Coniferophyta: The Most Widespread Gymnosperm Phyla Comprises 70 genera with 630 species. The tallest vascular plant is the redwood (Sequoia sempervirens) of coastal California and southwestern Oregon. Conifers, including pines, firs, and spruces, are of great commercial value. Their stately forests are a significant natural resource in the north temperate zone. During the Early Tertiary period, some genera were more widespread, resulting in a diverse conifer flora. Source Coniferophyta Conifer Evolution Conifers' history dates back to the Late Carboniferous period, 300 million years ago. Modern conifers' needlelike leaves exhibit drought-resistant properties, potentially beneficial in certain habitats. Diversification of the phylum during the Permian period (290 to 245 million years ago) may have favored structural adaptations like conifer leaves. Source Coniferophyta Pines: A Unique Coniferous Plant Pines (genus Pinus) dominate North America and Eurasia. There are approximately 100 species, each with unique leaf arrangement. Pine seedlings have needlelike leaves spirally arranged and singly borne on stems. After a year or two of growth, pines produce leaves in bundles or fascicles, each containing one to eight leaves. These fascicles, wrapped by small leaves, are short shoots with restricted apical meristem activity. Under unusual circumstances, the apical meristem within the fascicle of needles may reactivate and grow into a new shoot or even an entire pine tree. Source Coniferophyta Pine Leaf Structure Are suitable for water-scarce conditions. Epidermis is covered by a thick cuticle, reducing evaporation. Hypodermis is composed of compact, thick- walled cells. Stomata are sunken below the leaf surface. Mesophyll is composed of parenchyma cells with wall ridges and penetrated by resin ducts. Vascular bundles in the center are surrounded by transfusion tissue which conducts materials between the mesophyll and vascular bundles. Endodermis separates the transfusion tissue from the mesophyll. Coniferophyta Pine Needle Retention and Photosynthetic Balance Most pine species retain needles for two to four years. Photosynthetic balance depends on several years' needle crops. Bristlecone pine (Pinus longaeva) retains needles for up to 45 years. Evergreen leaves are exposed to damage longer than deciduous leaves, which are replaced annually. Great Basin Bristlecone Pine, Pinus longaeva Source Coniferophyta Secondary Growth in Conifer Stems Secondary growth begins early in pine and conifer stems, forming substantial secondary xylem or wood. Secondary xylem is produced inside the vascular cambium, while secondary phloem is produced outside. Conifer xylem primarily consists of tracheids, while phloem consists of sieve cells. Both types of tissue are traversed radially by narrow rays. Epidermis is replaced with periderm, a protective tissue from cortical cells. As secondary growth continues, subsequent periderms are produced by active cell division deeper in the bark. Source Coniferophyta Pine and Conifer Microsporangia Microsporangia and megasporangia are borne in separate cones on the same tree. Microsporangiate cones are on lower branches, megasporangiate cones are on upper branches. In some pines, ovulate cones are on the same branch, closer to the tip. Ovulate cones are pollinated by pollen from another tree, enhancing outcrossing. Source Coniferophyta Pine Microsporangiate Cones and Microspore Mother Cells Pine microsporangiate cones are small, typically 1 to 2 centimeters long. Microsporophylls are spirally arranged and membranous, bearing two microsporangia on their lower surface. Young microsporangiums contain many microsporocytes or microspore mother cells. In early spring, microspore mother cells undergo meiosis, producing four haploid microspores. Each microspore develops into a winged pollen grain, the immature microgametophyte. Pollen grains are shed in large quantities, some carried by wind to ovulate cones. Coniferophyta Pinecone Structure and Ovules Pinecones are larger and more complex than pollen-bearing cones. Ovuliferous scales, or cone scales, are seed-scale complexes, not simply megasporophylls. Each seed-scale complex consists of the ovuliferous scale and a sterile bract. The scales are arranged spirally around the cone axis. Each ovule has a multicellular nucellus (the megasporangium) surrounded by a massive integument with an opening (the micropyle). Each megasporangium contains a single megasporocyte, which undergoes meiosis, resulting in a series of four megaspores. Coniferophyta Pine Pollination Process Pollination occurs in spring, with ovulate cone scales separated. Pollen grains adhere to pollination drops, which contain water-soluble compounds and proteins for pathogen defense and pollen development. Pollination drops carry pollen grains to the nucellus, which has a slight depression. After pollination, scales grow together to protect developing ovules. Pollen grain germinates to form a pollen tube, but meiosis doesn't occur in the nucellus. Failure of pollination results in ovule abortion, about 95% of the time in gymnosperms. Coniferophyta Pine Megagametophyte Development Process Four megaspores produced a month post- pollination. Only one develops into a megagametophyte. Development slows down, often taking six months to complete. Mitosis proceeds without immediate cell wall formation. Source Cell wall formation begins 13 months after 2000 free nuclei are present. Archegonia differentiate at the micropylar end 15 months post-pollination, setting the stage for fertilization. Coniferophyta Pine Pollen Grain Germination and Development Pollen grain germinates 12 months prior, producing a pollen tube. Pollen tube digests through nucellus tissues to developing megagametophyte. A year after pollination, generative cell of Source microgametophyte divides into sterile and spermatogenous cells. Spermatogenous cell divides to produce two sperm before reaching megagametophyte. Microgametophyte matures, not forming antheridia. Coniferophyta Pine Pollination Process in Archegonium Pollen tube reaches egg cell 15 months post-pollination. Pollen discharges cytoplasm and sperm into egg cytoplasm. One sperm nucleus unites with egg nucleus, other degenerates. Eggs of all archegonia fertilize and develop into embryos. Polyembryony occurs, with 3-4 percent of pine seeds having multiple embryos and producing multiple seedlings. Coniferophyta Pine Life Cycle and Embryo Development Four tiers of cells produce near the lower end of the archegonium. Each cell of the uppermost tier forms an embryo. Suspensor cells force the developing embryos through the archegonium wall and into the megagametophyte. Up to 16 embryos can be initiated in a seed, but only one develops fully. During embryogeny, the embryo develops into a seed coat. Source Coniferophyta Pine Conifer Seed Structure Comprises two diploid sporophyte generations: seed coat and embryo, and one haploid gametophyte. Gametophyte serves as a food reserve. Embryo consists of a hypocotyl-root axis with root cap and apical meristem. Integument consists of three layers, with the middle layer becoming hard Source and serving as the seed coat. Coniferophyta Pine Seed Distribution Pine seeds are shed from cones in the second year after initial cone appearance and pollination. At maturity, cone scales separate, and winged seeds flutter through the air. Lodgepole pines (Pinus contorta) do not separate until exposed to extreme heat. Fire-resistant cones release accumulated seed crop, reestablish the species. Other species like limber pine, whitebark pine, and pinyon pines store seeds for later consumption by nutcrackers. Birds miss many of the seeds they store, aiding in pine dispersal. Source Coniferophyta Other Living Conifers Most conifers, unlike pines, have a homogeneous group. Reproductive cycle takes only a year, with seeds produced in the same season as pollination. Pollination-fertilization time Softwood trees in BC (top left) Pinus Contorta (top centre) ranges from three days to three Spruce (top right) Fir (bottom left) Cedar (bottom right) or four weeks, not 15 months. Western Hemlock Source Coniferophyta Conifer Genera and a New Species Important conifer genera include firs, larches, spruces, hemlocks, Douglas firs, cypresses, and junipers. Abies, Larix, Picea, Tsuga, and Pseudotsuga are all Pinaceae. Cupressus and Juniperus belong to the Cupressaceae. A new species of Cupressaceae, Callitropsis vietnamensis, was reported from Ha Giang. In yews, a solitary ovule is surrounded by a fleshy, cuplike structure, the aril. Coniferophyta There are a few Monkey Puzzle trees in Prince Rupert! Araucariaceae Found naturally only in the Southern Hemisphere. Developed greatest diversity in Jurassic and Cretaceous periods. Extinct in the Northern Hemisphere in the Late Cretaceous. Only three surviving genera exist: Agathis, Araucaria, and Wollemia. Wollemia nobilis, discovered in 1994, is the world's rarest plant species. Panama pine, a valuable timber tree in South America, and other species like monkey-puzzle tree and Norfolk Monkey Puzzle Tree, Island pine are cultivated in mild climates. Araucaria Araucana Source Coniferophyta Redwoods and Cupressaceae: A Historical Overview Redwoods and their relatives, previously part of Taxodiaceae, are now part of Cupressaceae. Found in the Triassic, with fossils dating back to the Middle Jurassic. Represented by diverse species, remnants of populations from the Tertiary period. Notable species include the coast redwood, Sequoia sempervirens, Sequoiadendron giganteum, and bald cypresses in the southeastern United States and Mexico. Coast redwood, Sequoiadendron giganteum, Agassiz Tree in Calaveras State Park Source Coniferophyta Metasequoia: A Living Fossil Metasequoia, the dawn redwood, was widespread in the Tertiary period and was the most abundant conifer in western and Arctic North America from the Late Cretaceous to the Miocene epoch. It survived in Japan and eastern Siberia until a few million years ago. The genus was first described from fossil material by Japanese paleobotanist Shigeru Miki in 1941. Chinese forester Tsang Wang discovered a unique tree in China in 1948, revealing the fossil Metasequoia had "come to life." In 1980, about 8000 to 10,000 trees still existed in the Metasequoia valley, but they were not reproducing due to harvesting and lack of suitable habitat. Thousands of seeds have been distributed worldwide, allowing the "living fossil" to grow in parks and gardens worldwide. Dawn Redwood, Metasequoia glyptostroboides Source Other Living Gymnosperm Phyla: Cycadophyta, Ginkgophyta, and Gnetophyta Gymnosperms: Cycads and Bennettitales Cycads, palm-like plants, appeared 250 million years ago in the Permian period. Known as the "Age of Cycads and Dinosaurs" due to their abundance in the Mesozoic era. Consist of 11 genera, with about 300 species. Florida Arrowroot, Zamia integrifolia Zamia integrifolia, native to Florida, is Source the only cycad native to the U.S. Cycadophyta, Ginkgophyta, and Gnetophyta Cycad Plant Characteristics Large plants, some reaching 18 meters or more. Have a distinct trunk covered with shed leaves. Functional leaves cluster at the top of the stem, resembling palms. Exhibit true, sluggish, secondary growth from a vascular cambium. Often highly toxic, containing neurotoxins and carcinogenic compounds. Form upward, branching dichotomously near the soil surface, called coralloid roots. Cortical cells of coralloid roots host the The coralloid roots of Cycas under ex situ protection in cyanobacterium Anabaena cycadeae, which fixes Kunming Botanical Garden, Yunnan, China: (a) atmospheric nitrogen and contributes nitrogenous morphology of the whole root of Cycas; (b) growing substances. status of cycad coralloid root; (c) cross sectional illustration of coralloid root; (d) longitudinal illustration of coralloid root. Source Cycadophyta, Ginkgophyta, and Gnetophyta Cycad Reproductive Units and Fertilization Process Cycads have reduced leaves with attached sporangia clustered in cone-like structures near the plant's apex. Pollen and ovulate cones are borne on different plants. Pollen tubes formed by microgametophytes are typically Encephalartos ferox, ovulate cones unbranched or slightly branched. Growth of pollen tube results in significant destruction of nucellar tissue. Sperm is released from the basal end of the microgametophyte before fertilization, resulting in two sperm. Cycas siamensis, female with seeds Cycadophyta, Ginkgophyta, and Gnetophyta Insect Pollination of Cycads Beetles associated with male cones of cycads. Weevils of the Rhopalotria genus live on male cones of Zamia. Pollen-eating beetles have been present throughout cycad history. Cycads are now overwhelmingly insect-pollinated. Source Cycadophyta, Ginkgophyta, and Gnetophyta Ginkgo Biloba: Maidenhair Tree Known for fan-shaped leaves with open, branched, dichotomous veins. Attractive, slow-growing tree with potential height of 30 meters or more. Leaves on short shoots are mostly entire, while long shoots and seedlings are deeply lobed. Deciduous tree with golden leaves before autumn. Cycadophyta, Ginkgophyta, and Gnetophyta Ginkgo Biloba: Living Survivor of Ginkgophyta Only living member of the phylum Ginkgophyta, with minimal changes over 150 million years. Shares features with other gymnosperm genera dating back to the Early Permian period. Preserved in temple grounds in China and Japan. Introduced to temperate regions for 200 years, cultivated in parks and gardens. Highly resistant to air pollution, commonly Gingko trees line the streets of cultivated in urban parks and city streets. Vancouver's Chinatown Source Cycadophyta, Ginkgophyta, and Gnetophyta Ginkgo Seeds and Their Characteristics Ginkgo ovules and microsporangia are borne on different individuals. Ovules are borne in pairs on short stalks and ripen to produce fleshy-coated seeds. The rotting flesh of the seed coat is vile due to butanoic and hexanoic acids. Male Ginkgo is preferred for parks and street plantings. Source The kernel of the seed has a fishy taste and is a prized delicacy in China and Japan. Cycadophyta, Ginkgophyta, and Gnetophyta Ginkgo Fertilization Process Fertilization within ovules may not occur until shedding from parent tree. Microgametophyte forms a branched, haustorium-like system from unbranched pollen tube. Pollen tube growth is strictly intercellular, no damage to adjacent nucellar cells. Basal end develops into saclike structure, containing two large, multi-flagellated sperm. Saclike portion of pollen tube ruptures, Source releasing sperm to eggs within ovule megagametophyte. Cycadophyta, Ginkgophyta, and Gnetophyta Ginkgo and Coccomyxa-like Green Alga Association Ginkgo hosts an immature "precursor" state of this alga with no discernible nucleus or mitochondria. Chloroplast appears nonfunctional, with Electron micrographs of two endosymbiotic algae freshly escaped from in vitro-cultured Ginkgo biloba cells. a Immature precursor diffuse electron-dense regions indicating alga showing within its cytoplasm large lipid droplets (black arrows) thylakoid-like membranous structures. and electron-dense material (white arrows), which subsequently evolved into thylakoids. b Mature alga with a nucleus (black arrowhead), Mature algae with eukaryotic traits and mitochondria (white arrows), a cup-shaped chloroplast (white normal functional chloroplast found only arrowhead), and lipid droplets (black arrows). Scale bar = 1.0 µm Source in dying host cells. Association found in tissues of Ginkgo trees from Asia, Europe, and North America. Cycadophyta, Ginkgophyta, and Gnetophyta Gnetophytes Overview Gnetum: Contains 35 species of trees and climbing vines with large, leathery leaves resembling eudicots. Found in moist tropics. Ephedra: Approximately 40 species of profusely branched shrubs with small, scale-like leaves. Mostly found in arid or desert regions. Welwitschia: Known for its bizarre vascular plant, buried in sandy soil, with a massive, woody, concave disk. Produces only two strap-shaped leaves that split lengthwise with age. Grows in the coastal desert of southwestern Africa, in Angola, Namibia, and South Africa. Source Cycadophyta, Ginkgophyta, and Gnetophyta Gnetophyta Genera and Characteristics Gnetophyta genera are related but differ greatly in characteristics. They share angiosperm-like features like similar strobili to angiosperm inflorescences. Ephedra's megagametophyte typically (a) Ephedra minuta Florin (b) Gnetum parvifolium contains two or three archegonia. (Warb.) (c) Welwitschia mirabilis Hook. f (d) Current analysis suggests these Simplified phylogenetic trees of the Gnetales within features were independently derived seed plants Source in gnetophytes and angiosperms. Cycadophyta, Ginkgophyta, and Gnetophyta Gymnosperm Fertilization Pine gymnosperms have only one functional sperm nuclei from the germinating pollen grain. Double fertilization, defined as two fertilization events in a single megagametophyte by two sperm from a single pollen tube, occurs in Ephedra and Gnetum. In Ephedra, each microgametophyte produces a single binucleate sperm cell, one fertilizes the egg nucleus, and the other may fuse with the ventral canal nucleus. In Gnetum, each pollen tube contains a single binucleate sperm cell, and each sperm nuclei fuses with a separate, undifferentiated female nucleus within the free-nuclear megagametophyte. Unlike flowering plants, in Ephedra and Gnetum, the second fertilization event produces an extra embryo, which ultimately aborts. A large megagametophyte in Ephedra and Gnetum nourishes the surviving embryo as it develops within the seed. Source Cycadophyta, Ginkgophyta, and Gnetophyta None of the living gnetophytes could possibly be an ancestor of any angiosperm Each gnetophyte has unique specializations. Some species produce nectar and are visited by insects. Wind and insects play crucial roles in pollination. Insect interactions with ephedra: Ephedra is a prime example of insect- Lepisiota frauenfeldi (A) Pollen mediated pollination. gathering (B) Pollination drop sucking (C) Musca domestica (D) Myzus persicae, Aphis fabae (E) On internode (F) Needle-like proboscis. SUMMARY Seed Development from Ovule Seeds offer plants a selective advantage due to their survival characteristics. Prerequisites include heterospory, single megaspore retention, embryo development, and integuments. All seeds have an embryo, stored food, and a seed coat. In gymnosperms, stored food is provided by the haploid megagametophyte. All seed plants bear megaphylls. Seed Plant Evolution from Progymnosperms Progymnosperms, extinct Paleozoic vascular plants, are potential progenitors of gymnosperms and angiosperms. Major extinct gymnosperm groups include seed ferns (Pteridospermales) and cycadeoids (Bennettitales). Seed ferns are diverse and unnatural, while cycadeoids have leaves resembling cycads but different reproductive structures. SUMMARY Gymnosperm Life Cycle Overview Gymnosperms consist of four phyla: Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta. Life cycles are similar: alternation of heteromorphic generations with large, independent sporophytes and reduced gametophytes. Ovules are exposed on megasporophyll surfaces. Megagametophyte of most gymnosperms is multicellular with several archegonia. Microgametophytes develop inside pollen grains. Antheridia are absent in all seed plants. Male gametes arise directly from the spermatogenous cell. Sperm of seed plants are nonmotile except for cycads and Ginkgo. SUMMARY Gymnosperm Pollination and Fertilization Pollination transfers pollen from microsporangium to megasporangium. Fertilization occurs when one sperm unites with the egg. The second sperm, disintegrated, has no apparent function. Each ovule develops into a seed, containing an embryo. Pollination and pollen tube formation eliminate the need for water for sperm to reach the egg. Sperm are conveyed to eggs through a combination of pollination and pollen. Gymnosperms: Four Phyla with Living Representatives Conifers: Largest and most widespread phylum of living gymnosperms, with 70 genera and 630 species. Cycads: Consists of 11 genera and 300 species, mainly tropical but extending away from the equator in warmer regions. Ginkgophyta: Only one living species, the maidenhair tree (Ginkgo biloba). Gnetophyta: Shows features of conifers and angiosperms, including similar strobili, vessels in xylem, lack of archegonia in Gnetum and Welwitschia, and double fertilization in Ephedra and Gnetum. QUESTIONS 1. One of the most important evolutionary advances in the progymnosperms was the presence of a bifacial vascular cambium. What is a bifacial vascular cambium, and where is it found besides in the progymnosperms? 2. In what way do the Bennettitales, or cycadeoids, resemble cycads? How do they differ from the cycads? 3. The potential for polyembryony occurs twice in the pine life cycle. Explain. 4. Diagram and label the components of each of the following: a pine ovule with a mature megagametophyte; a mature pine microgametophyte (germinated pollen grain with sperm); and a mature pine seed. 5. Evidence exists in the cycads and Ginkgo that the first pollen tubes were haustorial structures, not true sperm conveyors. Explain. 6. Explain how the fertilization events in Ephedra differ from those in other gymnosperms. Quizlet