Ch 5 Woody and Seed Plants REVIEWER

Botany Chapter 5 Evolution and Diversity of Woody and Seed plants LIGNOPHYTES—WOODY PLANTS 

Lignophyta – a MONOphyletic lineage MONOphyletic lineage of euphyllous vascular plants secondary growth  – growth of the vascular and cork o cambia  initiates after the vertical extension of stems and roots due to cell expansion (primary growth)  vascular cambium  – shared the derived feature wood and a cork cambium  gives rise to wood and  cork cambium  – produces cork

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vascular cambium  – a sheath, or hollow cylinder, of cells that develops within the stems and roots as a continuous layer, between the xylem and phloem in extant, eustelic spermatophytes cells - divide mostly tangentially  (parallel  (parallel to a o tangential plane), resulting initially in two concentric layers of cells  1st layer – remain as the vascular cambium  2nd layer – divide indefinitely – bifacial growth  secondary xylem = wood  – produced to the inside of the cambium  secondary phloem  – produced to the outside bifacial growth  – layers of cells are produced produced both to o the inside and outside of a continuously generated cambium  more secondary xylem is produced than secondary phloem Secondary cambium  – independently evolved in fossil lineages within the lycophytes (e.g., Lepidodendron) and equisetophytes (e.g., Calamites) Unifacial growth – produces secondary xylem (wood) o to the inside but no outer secondary phloem

Secondary growth  – results in an increase of the width or girth of stems and root o expansion of the new cells generated by t he cambium radial divisions  – increasing the number of cells o within a given growth ring  regular growth periods – annual rings of wood  Dendrochronology – tree-ring dating cork cambium  – similar to a vascular cambium, only it differentiates near the periphery of the stem or root axis periderm – outer bark = cork cambium + cork o cork – outermost layer of periderm o suberin – waxy polymer similar to cutin that is quite o resistant to water loss Secondary xylem (wood) – functions in structural support, enabling the plant to grow tall and acquire massive systems of lateral branches vascular cambium  – precursor to the formation of o intricately branched shrubs or trees with tall overstory canopies Cork – functions as a thick layer of cells that protects o the delicate vascular cambium and secondary phloem from mechanical damage, predation, and desiccation monopodial growth  – a single main shoot develops branches from lateral (usually axillary) buds o arisen prior to the monilophyte –lignophyte split enabled woody plants the capability of forming o extensive (sometimes massive) woody branching systems, permitting them to survive and reproduce more effectively

SPERMATOPHYTES—SEED PLANTS 

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Spermatophyta – MONOphyletic Seed – major evolutionary novelty Embryo – immature diploid sporophyte developing o tissue and from the zygote, surrounded by nutritive tissue and enveloped by a seed coat  Radicle – immature root  epicotyl – shoot apical meristem  cotyledons – young seed leaves  hypocotyl – transition region b/n root and stem ovule – immature seed, prior to fertilization o

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Seed Evolution o (1) Heterospory - formation of two types of haploid spores within two types of sporangia  (a) megaspores – large, fewer-numbered  develop via meiosis in the megasporangium the megasporangium  female gametophyte – bears only archegonia  (b) microspores – small, more numerous  products of meiosis in the microsporangium  male gametophyte – bears only antheridia homospory – ancestral condition, in which a  single spore type forms  heterosporous fossil plants w/o seeds  species of Archeopteris of Archeopteris  heterospory – evolved independently in other, nonseed plants  extant lycophytes Selaginella and Selaginella and Isoetes and in the water ferns

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(2) Endospory – complete development of the female gametophyte within the original spore wall  exospory - ancestral condition, in which the spore germinates and grows as an external gametophyte  correlated with evolution of endosporic male gametophytes

(3) Reduction of megaspore number to one  megasporocyte (megaspore megasporocyte (megaspore mother cell) (2n) – cells within the megasporangium that undergo meiosis  reduced from several to one  gives rise to four haploid megaspores  four haploid megaspores – three consistently abort, leaving only one functional megaspore

(4) Retention of the megaspore  Megaspore  released from the sporangium – ancestral condition  retained within the megasporangium  reduction in thickness of the megaspore wall

(5) Evolution of the integument & micropyle  Integument – layer of tissue that envelopes th e megasporangium  grows from the base of the megasporangium and envelopes it, except at the distal end  a continuous sheath that completely surrounds the nucellus  nucellus – megasporangium surrounded by an integument  telomes – ancestral branches that surrounded the megasporangium  “preovules” – ovules prior to the evolution of integuments  Lagenostome – a rim or ring of tissue at the apex of the megasporangium which functioned to funnel pollen grains to a pollination chamber Micropyle – a small pore at the distal end of the  integument  replaced the ancestral lagenostome as the site of entry of pollen grains (or in angiosperms, of pollen tubes)  functions in the mechanics of pollination droplet formation and resorption  single integument represents the ancestral condition of spermatophytes  in angiosperms. 2nd integument layer evolved later

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Pollen grains – extremely reduced male gametophytes, consisting of only a few cells o “immature” male gametophytes – not fully differentiated at the time of their release Wind dispersal – ancestral means of pollen transport Pollen tube – exosporic which functions as a haustorial organ, obtaining nutrition by absorption from the surrounding sporophytic tissue

POLLINATION DROPLET 

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pollination droplet  – droplet of liquid that is secreted by the young ovule through the micropyle mostly water + some sugars / amino acids  – formed o by the breakdown of cells at the distal end of the megasporangium (nucellus) o transports pollen grains through the micropyle may be apomorphic for at least the extant seed plant o lineages pollination chamber  – cavity formed by the breakdown of cells ovules of angiosperms lack pollination droplets or pollination chambers, as flowering plants have evolved a different mechanism of pollen grain transfer

Pollen – microsporangium → micropyle of the ovule (angiosperms: stigmatic tissue of the carpel) → male gametophyte –  mitotic divisions and differentiation → pollen tube → sperm → fertilization

POLLEN TUBE 

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Haustorial – ancestral type of pollen type in extant seed plants found in cycads and ginkgophytes o male gametophyte feeds (like a parasite) off the tissues of the nucellus zooidogamy ( zooidogamy (zooin, animal + gamos, marriage) – motile sperm is delivered from this male gametophyte into a fertilization chamber, where the sperm swims to the archegonium containing the egg siphonogamy (siphono, tube + gamos, marriage) o in the conifers, including Gnetales pollen tubes deliver nonmotile sperm cells to the o archegonium or egg o A type of siphonogamy evolved independently in the angiosperms  pollen tubes grow through stylar tissue prior to delivering the sperm to the egg of a female gametophyte

OVULE AND SEED DEVELOPMENT 

POLLEN GRAINS 

pollen grains  – an immature, endosporic male gametophyte Endospory in Endospory in pollen grain evolution – similar to the o same process in seed evolution, involving the development of the male gametophyte within t he original spore wall

megasporocyte – develops within the megasporangium of the ovule after pollination a single cell that undergoes meiosis, producing a o tetrad of  four  four haploid megaspores  three megaspores that are distal (away from the ovule base) abort  proximal megaspore (near the ovule base) continues to develop In the pollination chamber, resorbed pollen grains → o mature male gametophytes → pollen tubes  In gymnosperms, these male gametophytes may live in the megasporangial tissue for some time, generally several months to a year

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functional megaspore  – greatly expands, accompanied by numerous mitotic divisions, to form the endosporic female gametophyte gymnosperms, archegonia o In the seeds of gymnosperms, differentiate at the apex of the female gametophyte plants , each archegonium has a o in the nonseed land plants, large egg cell and a short line of neck cells (plus typically a ventral canal cell or nucleus)

SEED ADAPTATIONS 

archegonial chamber  – cavity between the megasporangium and female gametophyte o (in cycads and Ginkgo) male gametophytes release motile sperm cells o (in conifers) pollen tube of the male gametophyte delivers sperm cells directly into the archegonial neck **ovules of some Gnetales and all angiosperms lack o archegonia Pollination – delivery of the pollen grains to the ovule

adaptive significance of the seed provide protection provide protection,, mostly by means of the seed coat, from mechanical damage, desiccation, and often predation unit  of sexual reproduction function as the dispersal unit of o  a fleshy outer seed coat layer may function to aid in animal dispersal  differentiation of the seed coat into one or more wings functions in seed dispersal by wind function in dormancy mechanisms that mechanisms  that ensure o germination of the seed only under ideal conditions of temperature, sunlight, or moisture upon germination, the nutritive tissue surrounding tissue surrounding o energy for the young seedling, the embryo provides energy for aiding in successful establishment o

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Fertilization – actual union of sperm and egg (in flowering plants) fertilization generally occurs very o soon after pollination

(in seed plants) female gametophyte (which develops within the megaspore) remains attached to and nutritionally dependent upon the sporophyte o reverse condition as is found in the l iverworts, hornworts, and mosses

EUSTELE 

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eustele – a primary stem vasculature that consists of a single ring of discrete vascular bundles Each vascular bundle contains an internal strand of o xylem and an external strand of phloem that are radially oriented Protoxylem endarch  – toward the center of the stem Lycophytes: exarch protoxylem o o most monilophytes: mesarch protoxylem

Pollen – Pollen – microsporangium → micropyle of the ovule (angiosperms: stigmatic tissue of the carpel) → male gametophyte gametophyte – mitotic divisions and differentiation → pollen tube → sperm sperm → archegonia → egg cell → fertilization fertilization → Z →  Zygote ygote (2n) → e mbryo (immature sporophyte) → tissue of the female gametophyte continues to surround the embryo and serves as nutritive tissue for the embryo upon seed germination (except i n angiosperms) → The megasporangium (nucellus) eventually degenerates → integument matures into a peripheral seed coat , which may differentiate into various hard and/or fleshy layers

Note single ring of vascular bundles, with xylem inside, phloem outside. B. C. Close-up of Helianthus stem cross-section, an example of a eustele. C. Close-up vascular bundle, showing xylem, phloem, and associated fibers.

DIVERSITY OF WOODY AND SEED PLANTS

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Diversity of Gymnospermae cycads (Cycadophyta) – most basal lineage Ginkgo (Ginkgophyta) Ginkgo (Ginkgophyta) o conifers (Coniferae) conifers (Coniferae) o  Gnetales – either sister to the conifers or within the conifers, as the sister group to the Pinaceae o

Cycadophyta—Cycads 

ARCHEOPTERIS 

 Archeopteris – fossil plant and a well-known lignophyte that lacked seeds a large tree, with wood like a c onifer but leaves like a o fern Sporangia – producing spores, were born on fertile o branch systems Some species of Archeopteris were heterosporous o

“PTERIDOSPERMS”— “SEED FERNS” 

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Pteridosperms (Seed Ferns) - a PARAphyletic group PARAphyletic group of fossil plants that had fernlike foliage, yet bore seeds Medullosa – a well-known example of a seed fern fernlike leaves – in the form genera Alethopteris o genera Alethopteris and  and Neuropteris pollen-bearing organs  – in Dolerotheca, which had o huge pollen grains seeds of Medullosa – placed in the form genus o Pachytesta

GYMNOSPERMAE—GYMNOSPERMS 

2 sister groups of Spermatophyta (seed plants) Gymnospermae Angiospermae o o

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Gymnospermae (after gymnos, naked + sperm, seed) – ovules are not enclosed by a surr ounding carpel layer (thus, being “naked”) at the time of pollination o developing seeds – often enclosed by megasporophylls or ovuliferous scales, after pollination o nonflowering seed plants

Cycadophyta (Cycadales) – MONOphyletic lineage consisting of plants with a mostly short, erect stem or trunk, rarely tall and palmlike (as in Microcycas) approximately 320 –340 species in 11 genera o found in southeastern North America, Mexico, o Central America, some Caribbean islands, South America, eastern and southeastern Asia, Australia, and parts of Africa leaves – spirally arranged, mostly pinnately o compound Bowenia  – only genus with bipinnately  compound leaves loss of axillary branching on branching  on the aerial trunk o  no lateral (axillary) branching circinate vernation o  cycad pinnae (Cycas) or leaves (some Zamiaceae; e.g., Bowenia) dioecious – either male or female o pollen cones/strobili (male cones/strobili) – consist o microsporophylls, each of which of an axis bearing microsporophylls, bears numerous microsporangia  microsporangia  – produce many microspores (n), each of which develops into a pollen grain  motile sperm cells 2 families families - differing primarily in the absence and o presence of seed cones  Cycadaceae – no seed cones  seeds are produced on the margins of numerous megasporophylls Zamiaceae – have seed [ovulate] cones/strobili  (female cones/strobili)  Seed cones – consist of an axis bearing megasporophylls, which bears two seeds economic importance o  source of food starch (sometimes termed “sago”)  Cycas revoluta (“sago palm”) – planted horticulturally

Cycadaceae Cycads Family 1 genus ( Cycas, incl. Epicycas)/100 –110 species dioecious trees or perennial herbs trunks or subterranean stems large, coriaceous, evergreen, pinnate leaves (vernation involute circinate) large, determinate pollen cones No seed cones numerous toothed to divided megasporophylls arising from apex of trunk, each bearing one or more marginal ovules/seeds

Zamiaceae Coontie Family 10 genera/ca. 220 –230 species (incl. Boweniaceae, Stangeriaceae) dioecious trees or perennial herbs trunks or subterranean stems

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leaves pinnate [rarely bipinnate] determinate pollen and seed cones With seed cone peltate megasporophylls each bearing 2 [3] adaxially marginal ovules/seed o o

Ginkgophyta 

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Ginkgophyta (ginkgophytes) – extensive fossil record but contain only one extant species, Ginkgo biloba Ginkgo biloba – native only to certain remote regions o of China  a highly branched, woody tree  short shoots in addition to long shoots  distinctive obtriangular (fan-shaped), often twolobed leaves with dichotomous venation dioecious o  Male Ginkgo trees bear “cones” without sporophyll  pollen cones – consist of a central axis with lateral stalks each of which bears two microsporangia  ancestrally motile sperm  Female Ginkgo trees do not bear cones  female reproductive structures each consist of an axis having two terminal ovules Ginkgoaceae (Maidenhair Ginkgoaceae (Maidenhair Tree family) resinous, dioecious trees o roots: roots: mycorrhizal o stem: trunk is tall, with numerous lateral branches o leaves: simple, leaves: simple, spiral, petiolate, deciduous, and o flabelliform (fan-shaped), with open dichotomous venation  born on both long shoots and stout short s hoots male reproductive structure: structure : catkin-like, each o consisting of an axis bearing microsporophylls with paired, pendant microsporangia ovulate reproductive structure: structure : consisting of an axis o bearing generally two erect ovules each with a basal collar seed – with an outer fleshy and inner hard o integument layer

fascicle (in fascicle (in Pinus) – a specialized short shoot consisting of stem tissue, one or more needleshaped leaves, and persistent basal bud scales loss of sperm cell motility – apomorphy with Gnetales  siphonogamy  male gametophyte of conifers delivers the sperm cells more directly to the egg by the growth of the pollen tube to the archegonial neck  more than one archegonium per seed, multiple fertilization events may occur, resulting in multiple young embryos, but usually only o ne survives in the mature seed dioecious Seed cones – compound structure, consisting of an axis that bears modified leaves called bract  ovuliferous scale – separate seed-bearing structure  modified lateral shoot system Cordaitales (fossil group) – had seed cones that  consisted of an axis bearing bracts  in the axils of the bracts was a shoot (branch) system, which consisted of an axis bearing sterile leaves and one to many ovules  compound cone hypothesis  – veins of the ovuliferous scales of extant conifers are inverted (upside down), with respect to the bract, indicating that these veins may have been derived from ancestral leaves oriented 180˚ relative to the bract Pseudotsuga (Douglas-fir) Pseudotsuga (Douglas-fir) – bracts are elongated  and can be seen on the outside of the ovuli ferous scales  Podocarpaceae and Podocarpaceae and Taxaceae – seed cones are greatly reduced  bracts and bracts and ovuliferous scales – only present as developmental remnants or modified to form a fleshy structure, with the entire cone bearing only one ovule 

Coniferae—Conifers 

Coniferae (Pinophyta or Coniferophyta) – ancient group of land plants that were once dominant in most plant communities worldwide MONOphyletic group MONOphyletic group of highly branched trees or o shrubs with simple leaves Leaves: Leaves: linear, acicular (needle-like), or subulate (awlo shaped) short shoots  – adjacent internodes are very short in o length

Evolution of the compound conifer cone. A,B. A,B. CORDAITALES, Lebachia. A. Compound cone, bearing numerous, spirally arranged bracts, each subtending a fertile short shoot. B. Close-up B. Close-up of bract and axillary fertile shoot system (left = abaxial view, right = side view), bearing spiral leaves and single ovule . C,D. Fossil conifers. C conifers.  C.. Voltzia, shoot system of compound cone, with fi ve sterile leaves and three ovules. D. Ulmannia, shoot system of compound cone (left = abaxial view, right = adaxial view), showing outer bract and inner fused shoot system (ovuliferous scale) and ovule.

Chapter 6 Evolution of Flowering Plants 

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Angiosperms (Flowering plants, Magnoliophyta, or Anthophyta) – MONOphyletic group currently thought to be the sister group to the gymnosperms 95% of all land plant species  – most numerous, o diverse, and “successful” extant plant group

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primordia – bumplike mounds of tissue formed by actively dividing regions of cells develop in whorls from outside to inside o  sepal (outer tepal) primordia → petal (inner tepal) primordia → stamen primordia (often in two or more whorls) → carpel primordia floral development – homologous to leaves o  initiating like leaf primordia of a vegetative shoot  being innervated by veins  often having a dorsiventral shape “ABC” model of floral development  Arabidopsis thaliana ( thaliana (Drosophila of the plant world) o o Sepals – by A Petals – by A and B o Stamens – by B and C o o Carpels – by C SEPALLATA class SEPALLATA class – needed in combination with those o of the A, B, and C classes to effect proper floral organ identity

ANGIOSPERM APOMORPHIES (1) FLOWER 

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Flower – a modified, determinate shoot system bearing stamens, collectively called the androecium, androecium, one or more stamens, carpels (making up one or more and/or one or more carpels (making pistils), collectively called the gynoecium bisexual (perfect) – both stamens and carpels o unisexual (imperfect) – only stamens or carpels o

perianth (tepal) – consisting of modified leaves at the base of the shoot system protects the other floral parts during floral o development and an attractant for pollination perianth of two discrete whorls or series of parts: o  an outer calyx (outer tepal) and an inner corolla (inner tepal)  calyx – green and photosynthetic, composed of sepals or calyx lobes (if fused) leaf-like sepals or  corolla – typically colorful, showy, and odoriferous and is composed of individual petals or corolla lobes (if fused) tepals/ tepals/perianth parts/ parts /perianth segments  – spirally o arranged units with no clear point of differentiation

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transcription factors  – induce the expression of other genes that bring about the development of the 4 floral organs Animal pollination – appears to be the primitive condition in the angiosperms, separating them from the predominantly wind-pollinated gymnosperms may include bees, butterflies and moths, flies, bats, o and birds  Ranunculus sp.(buttercup) – bee-pollinated  Calonyction sp. (moon flower) – moth-pollinated  Stapelia sp. (star flower) – fly-pollinated Selenicereus  (night-blooming cereus) &  Couroupita guianensis (cannonball tree) – batpollinated  Strelitzia reginae (bird of paradise) – birdpollinated water pollinated or pollinated or wind pollinated  – flowers of many groups that are quite reduced in size or structural complexity, often lacking a perianth altogether Phyllospadix torreyi  (surf-grass)  (surf-grass) – water-pollinated o o Grass – wind-pollinated

(2) STAMENS 

stamen – male reproductive organ of a flower o modified microsporophylls, modified leaves that bear microsporangia o Microsporangia → microspores → pollen grains

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laminar (leaf-like) structure (in some stamen) – to which the anther is attached or embedded sessile – Some stamens that lack a filament (or o lamina) and is directly attached to the rest of the flower stamens of most flowering plants have two parts filament – stalk o anther – pollen bearing part o Anther synangium – a fusion product of sporangia o o unique in (ancestrally) containing two pairs of microsporangia, usually arranged in a bilateral symmetry theca – discrete half of the anther which contains o each pair of microsporangia  bithecal – two thecae  tetrasporangiate – 4 microsporangia anther locule  – single, contiguous chamber from the o 2 microsporangia that coalesced at maturity ** anthers of some angiosperms are monothecal or o bisporangiate (secondarily bisporangiate (secondarily reduced to a single theca) adaptive value of the stamens of angiosperms over the microsporophylls of gymnosperms – selective pressures for the flower smaller & lighter than lighter than gymnosperm microsporophylls o bisexual flowers, flowers, rather than in more occur in o massive, unisexual cones o enabled the evolution of specialized pollination mechanisms

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adaptive significance of the reduced male gametophytes of angiosperm – evolution of a reduced female gametophyte and relatively rapid seed development to deliver sperm cells to the female gametophyte and o effect fertilization very rapidly than rapidly  than in gymnosperms

(4) CARPEL 

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(3) REDUCED MALE GAMETOPHYTE 

pollen tube (angiosperms) tube  (angiosperms) elongates through (and feeds upon) the tissues of the stigma and style of the carpel → reaches the ovule → penetrates the micropyle → transports the two sperm cells directly to the female gametophyte NON-motile sperm – conifers and angiosperms o

reduced, 3-celled male gametophyte microspore - formed by meiosis within the microsporangium  single nucleus divides mitotically to form 2 cells: cells: tube cell & generative cell  microspore → pollen grain pollen grain  – immature, endosporic male o gametophyte  generative cell divides one time – producing two sperm cells

carpel – major apomorphy of angiosperms o constitutes a modified, conduplicate megasporophyll bearing two, adaxial rows of ovules  megasporophyll – a modified leaf that bears megasporangia, which in the seed plants are components of the ovules and seeds Conduplicate – inwardly folded longitudinally  and along the central margin o At maturity the carpel body completely encloses the ovules and seeds, accounting for the name angiosperm (Gr. angio, vessel + sperm, seed) sporophyll-like nature of the carpel develop like a leaf  – o  – initially flattened, dorsiventral shape, with an adaxial (toward the top-center) and abaxial (away from the top-center) surface has veins o dorsal (median) vein or bundle  – one in the  middle, corresponding to the midvein of a leaf ventral (lateral or placental) veins/bundles  –  two others near the two carpel margins

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Pollen Tube siphonogamy – elongate pollen tube grows out of the o pollen grain wall due to germination of pollen grains (In gymnosperms) the pollen tube develops after t he o pollen grains enter the micropyle of the ovule and functions as a haustorial device for a long period of time o (In angiosperms) the pollen tube forms immediately after transfer of pollen to the stigma

ascidiate carpel  – develops from a ring of tissue that grows upward, sometimes assuming a peltate form flowers with 2 or more carpels: apocarpous  (distinct) – separate from one another o syncarpous (connate) – fused together o gynoecium – totality of female reproductive structures in a flower, regardless of their structure o carpel may be alternatively defined as a unit of the gynoecium composed of one or more pistils more pistils with basal ovary, ovary, an o apical style (or style (or styles), which may be absent, and one or more stigmas, stigmas, the tissue receptive to pollen grains  simple pistil – pistil equivalent to one carpel compound pistil  – composed of 2 or more fused  carpel

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adaptive significance of carpels carpels – receivers of pollen  function to selectively control fertilization  incompatibility reaction  – chemicals that are present in the stigma and style may inhibit either pollen germination or pollen tube growth  mediated by incompatibility genes  may inhibit inbreeding, allowing for reproduction only between genetically dissimilar individuals of the species fruit formation and seed dispersal o  fruit – mature ovary or ovaries (made up of one or more carpels) plus any accessory tissue that might be present  pericarp – mature ovary wall  seed dispersal  if the pericarp is fleshy, fruits are dispersed by animals  by wind – aided by the development of wings or trichomes  by water – via various flotation devices  mechanically – by various explosive, hygroscopic, or catapulting methods

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monosporic, 4-nucleate and celled condition  – ancestral condition found in virtually all Nymphaeales and Austrobaileyales Nuphar/Schisandra type Nuphar/Schisandra type o ancestral module  – one polar nucleus in a central ce ll o and 3 cells (the egg apparatus) at the micropylar end  doubled (a third sequence of mitotic divisions) to yield the common Polygonum type  quadrupled to yield something like the 16nucleate Penaea type  Amborella trichopoda – most basal angiosperm  Amborella type  Amborella type – modified type of female o 8-celled via an gametophyte, being 9-nucleate and 8-celled via extra mitotic division in the egg apparatus producing a third synergid cell

(5) TWO INTEGUMENTS 

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bitegmic ovules – growth of two integuments during integuments during ovule development o unique apomorphy of angiosperms unitegmic – all nonflowering seed plants have ovules o with a single integument two integuments typically coalesce during seed coat o development, but may form anatomically different layers (unclear) adaptive significance of two integuments enabled the evolution of specialized seed coat layers

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(6) REDUCED FEMALE GAMETOPHYTE 

reduced female gametophyte  – major apomorphy (seed plants) a single megasporocyte within the megasporangium (nucellus) divides meiotically to form four haploid megaspores  female gametophyte typically generates from only one of these megaspores o (angiosperms) megaspore divides in a sequence of three mitotic divisions, divisions, resulting in a total of eight haploid nuclei o

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Polygonum type Polygonum type – pattern of arrangement of eight nuclei into seven cells region, 3 cells develop: an egg cell in micropylar region, o flanked by two synergid cells  egg apparatus = egg cell + synergid cells In the chalazal region, region, 3 antipodal cells form cells  form o central cell  – remaining volume of female o gamethophyte which contains two polar nuclei “embryo sac” – sac” – female gametophyte of angiosperms o ** Archegonia do not form within the female gametophyte of angiosperms

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adaptive significance of a reduced female gametophyte developmental timing angiosperms have the capacity to more quickly o generate seeds evolution of rapidly spreading annual herbs o

(7) ENDOSPERM FORMATION 

endosperm – major apomorphy product of double fertilization o o pollen tube → micropyle of the ovule, penetrates one of the synergid cells and releases the two sperm cells into the central cell of the female gametophyte  One sperm cell migrates toward and fuses with the egg cell to produce a diploid zygote  other sperm cell fuses with the two polar nuclei to produce a triploid, or 3n, endosperm cell  endosperm – mass of tissue that generally envelopes the embryo of the seed from the repeated mitotic division of endosperm cell

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endosperm – replaces the female gametophyte as the primary nutritive tissue for the embryo in all angiosperms containing cells rich in carbohydrates, oil, or protein o adaptive significance of endosperm – developmental timing o endospermous nutritive tissue of angiosperms does not begin to develop until after fertilization is achieved  in gymnosperms, considerable female gametophytic nutritive tissue is deposited after pollination, even if the ovules are never ultimately fertilized conservation of resources, resources , such that seed storage o compounds are not formed unless fertilization i s assured triploid - Having three sets of chromosomes (one o from the male and two from the female) may enable the endosperm to develop more rapidly (correlated with rapid overall seed development) and may also provide greater potential for chemical variation in nutritive contents

(8) SIEVE TUBE MEMBERS 

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sieve tube members – specialized sugar-conducting cells Sieve cells (and associated albuminous cells) – o primitive sugar-conducting cells and are found in all nonflowering vascular plants  evolutionarily modified from sieve cells and are found only in flowering plants  difference: pores at the end walls are differentiated, being much larger than those on the side walls  sieve plates – collections of differentiated pores at the end walls  compound (composed of two or more aggregations of pores) simple (composed of one pore region)  companion cells  – Parenchyma cells associated with o sieve tube members  function to load and unload sugars into the cavity of sieve tube members

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ORIGIN OF ANGIOSPERMS 

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adaptive signifi cance of sieve tube members over sieve cells - may provide more efficient sugar conduction

One angiosperm specialization concerns water and mineral conductive cells vessels – two ends of the cells have openings, termed perforation plates Amborellales, some Nymphaeales – vesselless, having o only tracheids (which lack perforation plates) Specializations of vessels modification of the perforation plate from scalariform to one with fewer, less transversely oriented openings, to a simple perforation plate (having a single opening

adaptive advantage of vessels over tracheids more efficient solute conduction due to greater rate and lower flow resistance under conditions of low water potential or freezing o temperatures, wider vessels may increase the chance of cavitation (formation of a gas bubble in the water stream), which interrupts water fl ow and can result in plant death if not self-repaired. This may explain why tracheid-bearing conifers dominate over angiosperms in cold climatic regions o

VESSELS 

modification from tapering end walls to perpendicular ones modifIcation from long, narrow cells to short, wide cells

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Charles Darwin: “abominable mystery”- relatively rapid diversification of the “higher plants” (presumed to mean angiosperms) fossil records dispersed pollen grains from Cretaceous period, 140 o million years ago – earliest definitive fossil earliest definitive flowers – as early as 130 million o years ago  Amborella trichopoda of trichopoda of the Amborellales – most basal angiosperm lineage o lacks vessels unisexual flowers with a spiral perianth, laminar o stamens, and separate carpels “pteridosperms” – a paraphyletic assemblage of extinct plants that possessed seeds and had generally fernlike foliage possible angiosperm progenitors o Caytonia of Caytonia of the Caytoniales – fossil taxon that o exemplifi es a putative transition to angiosperms  possessed reproductive structures similar to those of the angiosperms  male reproductive structures resemble anthers in anthers in consisting of a fusion product (synangium) of 3 or 4 micr osporangia;  radially (not bilaterally) symmetric  female reproductive structures consist of a spikelike arrangement of units, cupules  cupule = carpel  ancestral carpel morphology, a conduplicate megasporophyll bearing ovules along two margins  cupule did not function as a carpel in terms of a site for pollen germination Glossopteridales or Glossopteridales or Glossopterids – trees with simple o leaves having a midrib giving rise to an extensive reticulate venation system  open, leaf-like capitulum may represent a partially closed megasporophyll, transitional to an angiospermous carpel in which the megasporophyll encloses ovules

genus Archaefructus genus Archaefructus – have bona fide carpels,