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Date Created: 10/05/15
Chapter 18 EVOLUTION OF THE SEED Seed plants dominate the modern ora Seeds have a great survival value SEED SPORE l Multicellular embryo Single cell 2 Food supplied by tissue Food only in the cell 3 Multicellular seed coat Covering not cellular 4 Diploid sporophyte Haploid cell 5 Product of fertilization Product of meiosis The immature ovule consists of the megasporangium surrounded by one or two layers of tissue called integuments The seed is a mature ovule containing an embryo Events in the evolution of the ovule l Retention of the megaspores Within the megasporangium o The megasporangium is eshy is seed plants and is called the nucellus 2 Reduction in the number of megaspore mother cells I one in each 39 to 3 Survival of only one of the four megaspores produced by the spore mother cell leaving a single functional megaspore in the megasporangium 4 Formation of a highly reduced megagametophyte inside the single functional megaspore o Endosporic megagametophyte that is no longer free living and is retained Within the megasporangium 5 Development of the young sporophyte Within the megagametophyte retained Within the megasporangium 6 Formation of an integument that completely envelops the megasporangium except for an opening at the apeX called the micropyle 7 Modi cation of the apeX of the megagametophyte to receive the microspores or pollen grains Fossil record The oldest ovules or seeds are from the Late Devonian 365 million years old The oldest seedbearing plant is Elkinsia polymorpha The earliest seed plants produced their seeds along their branches without specialized structures such as cones or owers unlike most living seed plants The seeds were produced singly or in pairs and were surrounded by a loose cupule This small cuplike structure was lobed and branched dichotomously in the earliest seeds producing a somewhat sheltered chamber at one end of the seed Within this cupule the seed was enclosed by a more tightly adpressed tissue called the integument The integument is a layer of tissue found in all seeds it is produced by the parent plant and develops into the seed coat As the integument evolved to enclose the seed more tightly an opening was left at one end called the micropyle which permitted pollen to enter and provide sperm to fertilize the egg cell Both the integuments and cupule are believed to be the result of reduced and fused branches or leaves There are ve phyla of seed plants with living relatives Cycadophyta Ginkgophyta Coniferophyta Gnetophyta and Anthophyta PROGYMNOSPERM The phylum Progymnospermophyta were important components of the vegetation from the Middle Devonian Devonian 417354 mya through the Lower Mississipian Mississippian 354323 mya As their name implies they had characteristics intermediate between those of the seedless vascular trimerophytes and those of the seed plants Like the true gymnosperms progynmosperrns commonly had secondary growth xylem and phloem of their vascular tissues ie they produced wood and some grew to be tall trees Bifacial vascular cambium is an important advance over trimerophytes and ferns Bifacial vascular cambium produces xylem on one side and phloem on the other This type of vascular cambium is characteristic of seed plants and evolved rst in the progymnosperms Unlike the gymnosperms however they did not produce seeds but rather released their spores as do ferns Some progymnosperms were homosporous producing many identical spores while others were heterosporous producing two different kinds of spores This heterosporous group is thought to be ancestors or at least close relatives of the seed plants Two major groups are recognized Aneurophyton type Archaeopteris type Devonian Devonian and Early Carboniferous 380362 mya 370340 mya protostele eustele telomes unwebbed terminal branches webbed heliX arrangement of telomes twodimensional branching ultimate branches dichotomous fertile branches occur on the same aXis with with multiple homosporous sporangia photosynthetic lea ets in others whole branch systems are dedicated to reproduction homosporous some were heterosporous Extinct gymnosperms Pteridospermophyta or seed ferns is a diverse group that lived from the Devonian 417354 mya to the Jurassic 206144 mya Pteridosperms evolved in the latest Devonian and became more common in the Carboniferous Pteridosperms bore seeds and either simple or compleX pollen organs on pinnatelycompound dichotomizing femlike leaves They had protostelic or eustelic stems good organ differentiation into frondlike leaves stems and roots secondary vascular tissues from a bifacial vascular cambium Eight groups of pteridosperms are presently recognized siX of them are known from the Paleozoic and three from the Mesozoic The cycadeoids Bennettitales are a group of eXtinct seed plants that bore palmlike pinnately compound leaves that look super cially much like cycad leaves The cycadeoids are an eXtinct group of gynmosperms that are mostly known from Mesozoic 24065 mya Jurassic to Cretaceous strata in England Europe Greenland India the Black Hills of South Dakota and MeXico They disappeared at the end of the Cretaceous Their large columnar trunks up to 2 meters bear numerous tightlyadpressed leaves and intermixed helically arranged cones on the tru The Bennettitales reproductively differ from cycads in having owerlike reproductive structures that were bisexual in some species Currently it is still uncertain exactly where the Bennettitales t in phylogenetically LIVING GYMNOSPERMS Gymnosperm means quotnaked seedquot 0 Their ovules are exposed on the surface of sporophylls and analogous structures In 1825 the Scottish botanist Robert Brown distinguished gymnosperms from the other major group of seed plants the angiosperms whose seeds are surrounded by an ovary wall There are four phyla of extant gynmosperrns Cycadophyta Ginkgophyta Coniferophyta and Gnetophyta Their phylogenetic relationship to each other remains uncertain Some recent molecular studies suggest that these groups are monophyletic and form a clade sister to the angiosperms Several hypotheses exist 0 Gnepine hypothesis Close relationship between gnetophytes and conifers 7 more speci cally the Pinaceae rendering the conifers polyphyletic o Anthophyte hypothesis gnetophytes Bennettitales and angiosperms form a clade known as Anthophyta This is based on morphological studies that consider their reproductive structures to be owerlike The microgametophyte of gymnosperms These phyla have in common 0 Ovules and seeds exposed on the surface of the sporophylls o The female gametophyte produces several archegonia there are some exceptions Polyem bryony may result from multiple fertilizations o The seeds of many gymnosperms are borne in cones and are not visible The cones however are not the same as fruits 0 Furthermore at maturity the cone expands to reveal the naked seeds Pollination is the transfer of pollen the microgametophyte to the vicinity of the megagametophyte During pollination the immature male gametes or pollen grains sift among the cone scales and land directly on the ovules which contain the immature female gametes rather than on elements of a ower the stigma and carpel as in angiosperms The endosporic microgametophyte produces a pollen tube Cycads and Ginkgo have a motile sperm and fertilization is transitional between seedless plants and seed plants 0 The pollen of cycads and Ginkgo produces a haustorial pollen tube that penetrates the nucellus and may grow for several months 0 Eventually the multi agellated sperm is released near the archegonium Conifers and gnetophytes have nonmotile sperms 0 In conifers gnetophytes and angiosperrns the pollen tube bring the sperm to egg cell 0 This is considered a later development to haustorial pollen tube Some of the divisions of gynmosperrns are not closely related to others having been distinct groups for hundreds of millions of years Currently about 6070 genera are recognized with a total of 700800 species Gymnosperms are distributed throughout the world with eXtensive latitudinal and longitudinal ranges Phylum Coniferophyta This is the most numerous most widespread and most ecologically important of gynmosperm taxa It comprises about 70 genera and 650 species Conifers probably dominated the upland forests since the late Carboniferous some 300 mya and continued to be important components of the vegetation to this day In the wake of dramatic cooling and drying near the close of the Paleozoic conifers eXpanded their range to become dominant elements of tropical and temperate plant communities Conifers remained ecosystem dominants through most of the Mesozoic only yielding their position to the diversifying angiosperrns in the Cretaceous CHARACTERISTICS 1 Almost all known conifers extinct and living are woody 2 Most are trees The most common growth form for conifers is a single erect trunk with branches emerging at nearly right angles to the main aXis 3 Most conifers are evergreen but a few LariX and Metasequoia are deciduous 4 The leaves of eXtant conifers may be needlelike awlshaped or attened 5 Conifer leaves have parallel veins 6 Abundant secondary growth occurs from a bifacial vascular cambium 7 Xylem is composed entirely of tracheids and wood is generally parenchyma poor and thus pycnoxylic 8 Tracheids of many conifers have a characteristic circular bordered pitting on element walls 9 They have cones Cone is a colloquial term for a woody strobilus Not all conifers produce woody strobili In some taxa ovulebearing structures are reduced to eshy berrylike forms eg Juniper Taxus Podocarpus 10 Ovules are attached to a scale in various orientations A bract subtends the scale in turn In pollen cones microsporangia are borne on a single bract without a trace of a second subtending structure Family Pinaceae Molecular data indicate a basal split between the Pinaceae and a clade that includes all of the other conifers The family Pinaceae are known in the fossil record since the Cretaceous A clearly monophyletic group united by characters seen in the mature seed cones o bractscale complexes consisting of welldeveloped scales that are free from the subtending bracts for most of their length two inverted ovules on the adaXial face of each scale a usually obvious seed wing developed from the cone scale The Pinaceae are resinous trees or rarely shrubs comprising about 9 genera and 225 species found mostly in temperate regions of the Northern Hemisphere The leaves are spirally disposed and are linear and needlelike The male or microsporangiate strobili are small terminal or more often clustered along the stem aXis and consist of many papery microsporophylls each with two microsporangia on the lower surface The pollen grains typically have two bladderlike wings The female cones or megasporangiate strobili are woody and often large consisting of many ovuliferous scales each with a pair of adaXial ovules and a more or less distinct subtending bract Pines Trees or shrubs aromatic evergreen crown usually conic when young often rounded or attopped with age During pollination the immature male gametes or pollen grains sift among the cone scales and land directly on the ovules which contain the immature female gametes Branches usually in pseudowhorls Shoots dimorphic with long shoots and dwarf shoots Dwarf shoots borne in close spirals from axils of scaly bracts and bearing fascicles of leaves needles shoot with determinate growth Long shoots have indeterminate growth Leaves dimorphic spirally arranged o Foliage leaves needles l256 per fascicle persisting 212 or more years sessile sheathed at base by 1215 overlapping scale leaves 0 Scale leaves usually persisting for life of fascicle or shed after rst season Leaves are well adapted to scarcity of water 0 Leaves with thick cuticle covers the epidermis below which is the hypodermis which consists of one or more layers of thickwalled cells 0 The stomata are sunken o The mesophyll is made of parenchyma cells with conspicuous wall ridges that project into the cells increasing their surface area 0 There may be two or more resin ducts in the mesophyll 0 One or two vascular bundles made of xylem and phloem are surrounded by transfusion tissue which consists of parenchyma cells and some dead tracheids o The transfusion tissue is thought to conduct material between the mesophyll and the vascular bun o The endodermis surrounds the transfusion tissue Pines produce much secondary xylem or wood The xylem consists of tracheids and ray parenchyma the phloem is made of sieve tubes which are typical of conifers The rays are made of living parenchyma that connect the dead xylem with the living phloem Sieve cells are tapered at both ends like tracheids The tapered ends overlap Sieve cells lose their nucleus and are assisted physiologically by albuminous cells The epidermis is eventually replaced by the periderm Cork cambium phellogen produces cells toward the outside phellem and toward the inside phelloderm Plants monoecious Staminate cones numerous and small in a dense spikelike cluster around base of current year s growth Ovulate cones solitary to few maturing in 152 or 3 years The cones have spirally arranged woody scales each being subtended by a short or long bract and bearing 2 adaXial ovules Two seeds at the base of the cone scale winged Native to all continents and some oceanic islands of the northern hemisphere chie y in boreal temperate or mountainous tropical regions reaching its southernmost distribution shortly below the Equator in southeast Asia Sumatra Introduced as ornamental and timber trees in much of the Southern Hemisphere Pine Life Cycle Microstrobilus A Microsporangiate cones Small one to two centimeters long Microsporophylls are spirally arranged and relatively membranous Each microsporophyll supports two sporangia on its lower surface There are many microsporocytes that undergo meiosis and each produce four haploid microspores Each microspore develops into a winged pollen grain Each pollen grain contains two prothallial cells a generative cell and a tube cell This pollen grain is the microgametophyte Megastrobilus A Ovulate cone Ovulate cones are larger and more compleX than pollenbearing cones The ovuliferous scales bear the ovules Ovuliferous scales are determinate branch systems known as seed scale complex Each seedscale compleX consists of an ovuliferous scale two ovules born on its upper surface and a sterile bract The scales are arranged spirally around the aXis of the cone B Structure of the ovule Each ovule consists ofa megasporangium also known as the multicellular nucellus and a massive integument surrounding the multicellular nucellus The opening or microper faces the cone aXis Each megasporangium contains a single megasporocyte or spore mother cell which immediately undergoes meiosis Meiosis produces four linear megaspores but only one survives and the other degenerate The functional megaspore slowly develops into a multinucleated gametophyte It may take 13 months After a year of development the female gametophyte cell wall formation begins around the free nuclei Archegonia 2 or 3 develop at the micropylar end of the ovule C Pollination Pollination occurs in the spring The scales of the ovulate cone are widely separated Pollen may adhere to pollination drops secreted by the micropylar canal at the open end of the ovule As the pollination drops evaporate they pull the pollen in contact with the nucellus Shortly after coming in contact with the nucellus the pollen grain germinates and produces a pollen tube that slowly digested its way through the nucellus growing towards the developing megagametophyte About a year after pollination the generative cell divides and produces a stalk cell and spermatogenous cell The spermatogenous cell divides to produce two sperm nuclei 15 months after pollination pollen tube reaches the egg in the archegonium and discharges its cytoplasm and two sperms into it One sperm nucleus unites with the egg nucleus and the other degenerates Usually all archegonia are fertilized and produce embryos polyembryony but only one develops About 34 of the pine seeds have several live embryos and produce several seedlings The life cycle eXtends over a period of two years D Em bryogenesis Four tiers of cells are formed from the zygote the farthest away cells from the micropylar en develops into the embryo The cells below the embryo elongate and form the suspensor that pushes the developing embryo deep into the megagametophyte The megagametophyte serves as the nutritive tissue of the embryo The seed consists of diploid sporophytic generations 7 the integuments and the embryo 7 and one haploid generation the megagametophyte The embryo consists of a hypocotylroot aXis with a root cap at one end and an apical meristem with several cotyledons N 8 at the other end MEGASTROBILUS MICROSTROBILUS FEMALE GAMETOPHYTE YEAR MALE GAMETOPHYTE ZYGOTE I initiation of microstrobilus initiation of megastrobilus sporogenous tissue 11 meiosis shedding of pollen ovule development megasporocyte pollen germination stalk and formed pollination meiosis free body cell formed nuclear gametophyte III stalk and body cells move to additional free nuclei cellular gameto lower end of pollen tube two phyte archegonia formed fertilization sperms produced rapid growth zygote and embryo development of pollen tube through megasporangium maturation of seed shedding of seed Other conifers The family Araucariaceae occurs in the Southern Hemisphere and lands that were part of Gondwana It has three living genera Araucaria Agathis and Wollemia Araucaria is an important ornamental Wollemia was discovered in 1994 is southern Australia The family Taxodiaceae includes Sequoia Sequoiadendron the bald cypress Taxodium and the dawn redwood Metasequoia discovered in 1944 in China Other important conifers are Abies rs Picea spruce LariX larch hemlocks Tsuga Juniperus junipers and Cupressus cypresses PHYLUM CYCADOPHYTA Cycads are an ancient group of seed plants with a crown of large compound leaves and a stout trunk They appeared in the Permian about 250 mya Cycads are a unique assemblage of plants although grouped with the gynmosperrns they are unrelated to any other group of living plants They are a minor component of the ora in tropical and subtropical regions today but during the Jurassic Period they were a common sight in many parts of the world For this reason the Jurassic is often referred to as the quotAge of Cycadsquot Today cycads may be regarded as relicts consisting of small populations distributed disjunctly on many continents The Cycadaceae are woody unbranched or sparsely branched palmlike seedbearing trees or shrubs with thick pithy stems The main roots of cycads are thickened and eshy and may have storage capacities Vascular tissue has tracheids only and lacks vessels Cycads have a sluggish secondary growth from vascular cambium The leaves are frondlike pinnately compound usually stiff and often with sharply pointed lea ets that eXhibit circinnate vernation Leaves are alternate and spirally arranged in a cluster at the summit of the stem Cycads are dioecious The ovules and seeds are born naked on the petioles of reduced leaves that are loosely clustered or more highly modi ed and compacted into female or ovulate cones Male plants produce male or microsporangiate cones that bear many scales each with an abundance of microsporangia scattered over the lower surface The pollen grains or microgametophytes that develop from the microspores eventually produces two multi agellated motile sperm cells Cycads are considered to be insect pollinated Pollen tube is usually unbranched and grows into the female gametophyte The developing embryo derives its nutrients from the female gametophyte Seeds are typically large sometimes up to the size of goose eggs Many species are highly toxic with neurotoxins and carcinogens PHYLUM GINKGOPHYTA Ginkgo biloba is the last surviving member of a lineage that originated in the late Paleozoic and became quite diverse during the Mesozoic The living plant has distinctive fanshaped leaves produced in clusters at the ends of short lateral shoots and it is very easy to recognize Although probably eXtinct in the wild the plant was domesticated thousands of years ago in China Muchbranched deciduous trees with long shoots and spur or short shoots Secondary growth well developed Leaves alternate but often appearing whorled on slowgrowing short shoots simple fanshaped with dichotomous venation often with 2 lobes Plants dioecious Microsporangia in pairs on slender microsporophylls modi ed leaves which are loosely arranged in small pendent strobili borne on spur shoots Male gametophyte pollen forms an eXtensive haustorium in between the cells of the nucellus Male gametophyte produces two large swimming sperm multi agellated within the pollen tube Ovules borne in pairs on slender stalks on spur shoots Fertilization occurs after they have been shed from the parent tree The microgametophyte forms a highly branched haustorium system that develops from an initially unbranched pollen tube The growth of the haustorium is intercellular Seeds with seed coat differentiated into 3 layers an outer eshy layerfetid with odor of rancid butter when seed is mature a middle bony layer and a thin inner eshy layer PHYLUM GNETOPHYTA The phylum Gnetophyta consists of three families three genera and about 70 species Plants are trees shrubs lianas or stumpy turniplike nuu Leaves are simple opposite or whorled straplike in w 1 sp 39 1 lil in Gnetaceae 30 spp or scalelike in Ephedraceae 35 spp Secondary xylem contains tracheids and vessels as in angiosperms although the homology of the vessels in the two phyla is in question Plants are usually dioecious with quot owersquot unisexual in compound strobili or quotin orescencesquot Ovulate and microsporangiate cones on separate plants except in some species of Ephedra Double fertilization is recorded in Ephedra as in angiosperms although the two are not exactly homologous The second fertilization produces eXtra embryos rather than nutritive tissue EXtra embryos degenerate Gnetum and Ephedra produce a large female gametophyte that nourishes the embryo Wind pollination is the common method of pollen transfer but insects also pollinate Ephedra Vessels lack of archegonia and double fertilization are angiospermlike features that place the Gnetophyta closer to the Anthophyta owering plants
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