Chapter 38AngiospermReproductionandBiotechnology

Concept 38.1Pollination enables gametes tocome together within a flower

Diploid (2n) sporophytes produce spores by meiosis;these grow into haploid (n) gametophytesGametophytes produce haploid (n) gametes bymitosis; fertilization of gametes produces asporophyte

Angiosperm Life CycleIn angiosperms, the sporophyte is the dominantgeneration, the large plant that we seeThe gametophytes are reduced in size and dependon the sporophyte for nutrients

Fs flowers, double fertilization, and fruits

Flower Structure and FunctionFlowers are the reproductive shoots of the angiospermsporophyte; they attach to a part of the stem called thereceptacleFlowers consist of four floral organs: sepals, petals,stamens, and carpelsA stamen consists of a filament topped by an anther withpollen sacs that produce pollenA carpel has a long style with a stigma on which pollenmay landAt the base of the style is an ovary containing one ormore ovulesA single carpel or group of fused carpels is called a pistil

An Overview of AngiospermReproduction

Figure 38.2a, b

Anther attip of stamen

Filament

AntherStamen

Pollen tube

Germinated pollen grain(n) (male gametophyte)on stigma of carpel

Ovary (base of carpel)

Ovule

Embryo sac (n)(female gametophyte)

FERTILIZATIONEgg (n)

Sperm (n)

PetalReceptacle

Sepal

Style

Ovary

Key

Haploid (n)Diploid (2n)

(a) An idealized flower.

(b) Simplified angiosperm life cycle.See Figure 30.10 for a more detailedversion of the life cycle, including meiosis.

Mature sporophyteplant (2n) withflowers

Seed(developsfrom ovule)

Zygote(2n)

Embryo (2n)(sporophyte)

Simple fruit(develops from ovary)

Germinatingseed

Seed

CarpelStigma

Development of Gametophytesand Pollination

Pollen develops from microspores withinthe microsporangia, or pollen sacs, of anthersIf pollination succeeds, a pollen grain produces a pollentube that grows down into the ovary and dischargessperm near the embryo sacThe pollen grain consists of the two-celled malegametophyte and the spore wallWithin an ovule, megaspores are produced by meiosisand develop into embryo sacs, the femalegametophytesIn angiosperms, pollination is the transfer of pollen froman anther to a stigmaPollination can be by wind, water, bee, moth andbutterfly, fly, bird, bat, or water

Development of a male gametophyte(pollen grain)

(a)

Ovule

Ovule

Integuments

Embryosac

Mega-sporangium

Mega-sporocyte

Integuments

Micropyle

Survivingmegaspore

AntipodelCells (3)

PolarNuclei (2)

Egg (1)

Synergids (2)

Development of a female gametophyte(embryo sac)

(b)

megasporangiumis a large diploidcell called themegasporocyte(megasporemother cell).

1

Three mitotic divisionsof the megaspore formthe embryo sac, amulticellular femalegametophyte. Theovule now consists ofthe embryo sac alongwith the surroundinginteguments (protectivetissue).

3

Female gametophyte(embryo sac)

The megasporocytedivides by meiosis andgives rise to fourhaploid cells, but in mostspecies only one ofthese survives as themegaspore.

2

3 A pollen grain becomes amature male gametophytewhen its generativenucleus divides and formstwo sperm.This usually occurs after apollen grain lands on thestigma of a carpel and thepollen tube begins togrow. (See Figure 38.2b.)

2Each microsporo-cyte divides bymeiosis to producefour haploidmicrospores,each of whichdevelops intoa pollen grain.

Pollen sac(microsporangium)

Micro-sporocyte

Micro-spores (4)

Each of 4microspores

Generativecell (willform 2sperm)

MaleGametophyte(pollen grain)

Nucleusof tube cell

Each one of themicrosporangiacontains diploidmicrosporocytes(microsporemother cells).

1

75 m

20 m

Ragweedpollengrain

MEIOSIS

MITOSIS

KEYto labels

Haploid (2n)Diploid (2n)

Fig. 38-3(a) Development of a male

gametophyte (in pollen grain)

Microsporangium(pollen sac)

Microsporocyte (2n)

4 microspores (n)

Each of 4microspores (n)

Malegametophyte

Generative cell (n)

Ovule

(b) Development of a femalegametophyte (embryo sac)

Megasporangium (2n)

Megasporocyte (2n)

Integuments (2n)

Micropyle

MEIOSIS

Survivingmegaspore (n)

3 antipodal cells (n)

2 polar nuclei (n)

1 egg (n)

2 synergids (n)

Ovule

Embryosac

Integuments (2n)

Ragweedpollengrain

Nucleus oftube cell (n)

MITOSIS

20 µm

75 µm

Double FertilizationDouble fertilization results fromthe discharge of two spermfrom the pollen tube into theembryo sacOne sperm fertilizes the egg,and the other combines withthe polar nuclei, giving rise tothe triploid (3n) food-storingendospermAfter double fertilization, eachovule develops into a seedThe ovary develops into a fruitenclosing the seed(s)

Stigma

Pollen tube

2 sperm

StyleOvary

Ovule

Micropyle

OvulePolar nucleiEggSynergid2 sperm

Endospermnucleus (3n)(2 polar nucleiplus sperm)

Zygote (2n)(egg plus sperm)

Egg

Pollen grain

Polar nuclei

OvuleEndospermnucleus

Integuments

Zygote

Zygote

Terminal cellBasal cell

Basal cell

ProembryoSuspensor

Cotyledons

Shootapex

Rootapex Seed coat

EndospermSuspensor

Endosperm and EmbryoDevelopment

Endosperm developmentusually precedes embryodevelopmentIn most monocots and someeudicots, endosperm storesnutrients that can be used bythe seedlingIn other eudicots, the foodreserves of the endospermare exported to thecotyledonsThe first mitotic division of thezygote is transverse, splittingthe fertilized egg into a basalcell and a terminal cell

Structure of the Mature SeedThe embryo and its food supply are enclosed by a hard,protective seed coatThe seed enters a state of dormancyIn some eudicots, such as the common garden bean, theembryo consists of the embryonic axis attached to two thickcotyledons (seed leaves)Below the cotyledons the embryonic axis is called thehypocotyl and terminates in the radicle (embryonic root);above the cotyledons it is called the epicotylThe seeds of some eudicots, such as castor beans, have thincotyledonsA monocot embryo has one cotyledonGrasses, such as maize and wheat, have a special cotyledoncalled a scutellumTwo sheathes enclose the embryo of a grass seed: acoleoptile covering the young shoot and a coleorhizacovering the young root

Epicotyl

Hypocotyl

Cotyledons

Radicle

Seed coat

Seed coat

Endosperm

(a) Common garden bean, a eudicot with thick cotyledons

Cotyledons

Epicotyl

Hypocotyl

Radicle

(b) Castor bean, a eudicot with thin cotyledons

(c) Maize, a monocot

Scutellum(cotyledon)

Pericarp fusedwith seed coat

EndospermEpicotylHypocotyl

Coleoptile

RadicleColeorhiza

Seed Dormancy and GerminationSeed dormancy increases the chances that germination willoccur at a time and place most advantageous to theseedlingThe breaking of seed dormancy often requires environmentalcues, such as temperature or lighting changesGermination depends on imbibition, the uptake of water dueto low water potential of the dry seedThe radicle (embryonic root) emerges firstNext, the shoot tip breaks through the soil surfaceIn many eudicots, a hook forms in the hypocotyl, and growthpushes the hook above groundThe hook straightens and pulls the cotyledons and shoot tip upIn maize and other grasses, which are monocots, thecoleoptile pushes up through the soil

(a) Common garden bean

Seed coatRadicle

Hypocotyl

Hypocotyl

Cotyledon

Cotyledon

Cotyledon

Hypocotyl

Epicotyl

Foliage leaves

(b) Maize

Radicle

Foliage leaves

ColeoptileColeoptile

Fruit Form and FunctionA fruit develops from the ovaryIt protects the enclosed seeds and aids in seed dispersalby wind or animalsA fruit may be classified as dry, if the ovary dries out atmaturity, or fleshy, if the ovary becomes thick, soft, andsweet at maturityFruits are also classified by their development:

Simple, a single or several fused carpelsAggregate, a single flower with multiple separatecarpelsMultiple, a group of flowers called an inflorescence

An accessory fruit contains other floral parts in additionto ovariesFruit dispersal mechanisms include wind, water, andanimals

FlowerStamenCarpels

Ovary

Stigma

Pea flowerOvule

Seed

Carpel(fruitlet)

Raspberry flower

Stigma

Ovary

Stamen

Stamen

Pineapple inflorescence Apple flower

Stigma

Stamen

Ovule

Each segmentdevelopsfrom thecarpelof oneflower

Pea fruit Raspberry fruit Pineapple fruit Apple fruit(a) Simple fruit (b) Aggregate fruit (c) Multiple fruit (d) Accessory fruit

Sepal

Petal Style

Ovary(in receptacle)

Sepals

SeedReceptacle

Remains ofstamens and styles