chap39

For many people, pollen means sneezing and misery because pol-len grains of many plant species are potent allergens. However,pollen is not part of nature simply to annoy human beings. What is apollen grain? It is a tiny, haploid male plant. To the stigma (the pollen“landing pad”) of a flower, pollen grains represent an opportunity

for mate selection. That is, the stigma may allow some pollen grains to germinate,but not others. If a pollen grain survives the mate selection process and germinates,it may eventually deliver male gametes to a microscopic, haploid female plant em-bedded in the flower.

Why do angiosperms expend energy and resources to produce flowers and thatsometimes obnoxious pollen? The answer is simple: Flowers are sexual reproductivestructures, and reproduction is the most important goal in a plant’s—or any organ-ism’s—life.

In this chapter we will look at several aspects of plant reproduction, includingsome that are still not well understood. We will contrast sexual and asexual repro-duction, and we will consider sexual reproduction in detail. In doing so, we will lookat angiosperm gametophytes, pollination, double fertilization, embryonic develop-ment, and the roles of fruits in seed dispersal. The tran-sition from the vegetative state to the flowering state is akey event in plant development, and we’ll see howchanging seasons trigger flowering in some plants—andspeculate on the existence of a flowering hormone. Wewill conclude the chapter with an examination of asex-ual reproduction in nature and in agriculture.

Many Ways to Reproduce

Plants have many ways of reproducing—and humanshave developed even more ways of reproducing them.Flowers contain the sex organs of plants; it is thus no sur-prise that almost all angiosperms reproduce sexually. Butsome angiosperms reproduce asexually as well; someeven reproduce asexually most of the time. What are theadvantages and disadvantages of these two kinds of re-production? The answers to this question involve geneticrecombination. As we have seen, sexual reproductionproduces new combinations of genes and diverse phe-notypes. Asexual reproduction, in contrast, produces aclone of genetically identical individuals.

Reproduction in Flowering Plants

Key Players in Sexual ReproductionEach species’ pollen has a characteristicsize, shape, and cell wall structure. Thesestructures are the male gametophytes andare essential for sexual reproduction inseed plants.

39

750 CHAPTER THIRT Y-NINE

Both sexual and asexual reproduction are important inagriculture. Many important annual crops are grown fromseeds, which are the products of sexual reproduction. Seed-grown crops include the great grain crops, all of which aregrasses—wheat, rice, corn, sorghum, and millet—as well asplants in other families, such as soybeans and safflower.Other crops, such as strawberries, potatoes, and bananas, areproduced asexually.

Orange trees, which have been under cultivation for cen-turies, can be grown from seed—except for the navel orange,which has no seeds. This plant apparently arose only once inhistory. Early in the nineteenth century, on a plantation onthe Brazilian coast, a single orange seed gave rise to one treethat had aberrant flowers. Parts of the flowers aborted, andseedless fruits formed. Asexual reproduction is the only wayof propagating this plant, and every navel orange in theworld comes from a tree that has been derived asexuallyfrom that original Brazilian navel orange tree.

Unlike navel oranges, strawberries are capable of formingseeds and need not be propagated asexually. Nonetheless,asexual propagation of strawberries is common because vastnumbers of plants that are genetically and phenotypicallyidentical to a plant humans find particularly desirable can beproduced in this way.

We will treat asexual reproduction in greater detail at theend of this chapter. We will begin, however, by consideringsexual reproduction.

Sexual Reproduction in Plants

Sexual reproduction provides genetic diversity through re-combination (see Chapter 9). Meiosis and mating betweendifferent plants shuffle genes into new combinations, givinga population a variety of genotypes in each generation, someof which may be superior to those of their parents. This ge-netic diversity may serve the population well as the envi-ronment changes or as the population expands into new en-vironments. The adaptability resulting from genetic diversityis the major advantage of sexual reproduction over asexualreproduction, although sexual reproduction can also breakup well-adapted combinations of alleles through the sameprocess of recombination.

The flower is an angiosperm’s device for sexual reproductionA complete flower consists of four groups of organs that aremodified leaves: the carpels, stamens, petals, and sepals (seeFigure 30.7). The carpels and stamens are, respectively, the fe-male and male sex organs. A pistil is a structure composed ofone or more carpels. The base of the pistil, called the ovary,contains one or more ovules, each of which contains a megas-

porangium, within which a female gametophyte may de-velop. The stalk of the pistil is the style, and the end of thatstalk is the stigma. Each stamen is composed of a filamentbearing a two-lobed anther, which consists of four microspo-rangia fused together. Male gametophytes begin their devel-opment within the microsporangia.

The petals and sepals of many flowers are arranged inwhorls (circles) or spirals around the carpels and stamens. To-gether, the petals constitute the corolla. Below them, the sepalsconstitute the calyx. The petals are often colored, attractingpollinating animals; the sepals are often green and photosyn-thetic. All the parts of the flower are borne on a stem tip, thereceptacle. Flower parts are very diverse in form, in contrast tothe microscopic gametophytes that develop within them.

Flowering plants have microscopic gametophytesBefore reading this section, you may wish to review the sec-tion in Chapter 29 entitled “Life cycles of plants feature al-ternation of generations” (pages 571–572). Central to under-standing plant reproduction is the concept of alternation ofgenerations, in which a multicellular diploid generation al-ternates with a multicellular haploid generation.

In angiosperms, the diploid sporophyte generation is thelarger and more conspicuous one. The sporophyte generationproduces flowers. The flowers produce spores, which developinto tiny gametophytes that begin and, in the case of themegagametophyte, end their development enclosed by sporo-phyte tisue.

The haploid gametophytes—the gamete-producing gen-eration—of flowering plants develop from haploid spores insporangia within the flower (Figure 39.1):

� Female gametophytes (megagametophytes), which arecalled embryo sacs, develop in megasporangia.

� Male gametophytes (microgametophytes), which arecalled pollen grains, develop in microsporangia.

Within the ovule, a megasporocyte—a cell within the megas-porangium—divides meiotically to produce four haploidmegaspores. In most plants, all but one of these megasporesthen degenerate. The surviving megaspore usually undergoesthree mitotic divisions, producing eight haploid nuclei, all ini-tially contained within a single cell—three nuclei at one end,three at the other, and two in the middle. Subsequent cell wallformation leads to an elliptical, seven-celled megagameto-phyte with a total of eight nuclei (see Figure 39.1):

� At one end of the elliptical megagametophyte are threetiny cells: the egg and two cells called synergids. Theegg is the female gamete, and the synergids participateindirectly in fertilization by attracting and accepting thepollen tube.

� At the opposite end of the megagametophyte are threeantipodal cells, which eventually degenerate.

� In the large central cell are two polar nuclei.

The embryo sac (megagametophyte) is the entire seven-cell,eight-nucleus structure. You can review the development ofthe embryo sac in Figure 39.1.

The pollen grain (microgametophyte) consists of fewercells and nuclei than the embryo sac. The development of apollen grain begins when a microsporocyte within the antherdivides meiotically. Each resulting haploid microspore de-velops a spore wall, within which it normally undergoes onemitotic division before the anthers open and release these

REPRODUCTION IN FLOWERING PLANTS 751

The pollen grain is transferred to the stigma.

DIPLOID (2n)

HAPLOID (n)MeiosisDouble Fertilization

Microsporocyte

AntherOvary

Ovule

Polar nuclei (2)

Mega-gametophyte(n)

Synergids (2)

Egg

Antipodal cells (3)

Endospermnucleus (3n)

Zygote (2n)

Pollen grains(microgametophytes, n)

Pollen grain(microgametophyte)

Tube cell nucleusof pollen grain

Pollen tube

Sperm (2)

Megasporocyte (2n)

Megasporangium

Ovary

Ovule

Survivingmegaspore (n)

Seed

In the ovule, three of the four meiotic products degenerate.

The pollen tube grows toward the embryo sac (see Figure 39.5).

The embryo sac is the female gametophyte. After three mitotic divisions, it contains eight haploid nuclei.

1

2

3

4

The second sperm nucleus fuses with the two polar nuclei.

4

One sperm nucleus fuses with the egg.

3

39.1 Development of Gametophytes and Nuclear Fusion The embryo sac isthe female gametophyte; the pollen grain is the male gametophyte. The male andfemale nuclei meet and fuse within the embryo sac. Most angiosperms have dou-ble fertilization, in which a zygote and an endosperm nucleus form from separate

fusion events—the zygote from one sperm and the egg and the endosperm from theother sperm and two polar nuclei.

two-celled pollen grains. The two cells are the tube celland the generative cell. Further development of thepollen grain, which we will describe shortly, is delayeduntil the pollen arrives at a stigma. In angiosperms, thetransfer of pollen from the anther to the stigma is re-ferred to as pollination.

Pollination enables fertilization in the absence of liquid waterGymnosperms and angiosperms do not require external wa-ter as a medium for gamete travel and fertilization—a free-dom not shared by other plant groups. The male gametes ofgymnosperms and angiosperms travel within pollen grains.But how do angiosperm pollen grains travel from an antherto a stigma?

Many different mechanisms have evolved for pollen trans-port. In some plants, such as peas and their relatives, self-pol-lination is accomplished before the flower bud opens. Pollenis transferred by the direct contact of anther and stigmawithin the same flower, resulting in self-fertilization.

Wind is the vehicle for pollen transport in many species.Wind-pollinated flowers have sticky or featherlike stigmas,and they produce pollen grains in great numbers (Figure 39.2).

Some aquatic angiosperms are pollinated by water carryingpollen grains from plant to plant. Animals, including insects,birds, and bats, carry pollen among the flowers of manyplants.

Some plants practice “mate selection”In our discussion of Mendel’s work (see Chapter 10), we sawthat some plants can reproduce sexually either by cross-pol-lination or by self-pollination. But not all plants have thisflexibility. Many plants reject pollen from their own flowers.This phenomenon, known as self-incompatibility, promotesgenetic variation.

A single gene, the S gene, is responsible for self-incompati-bility in most plants. The S gene has dozens of alleles. A pollengrain is haploid and possesses a single S allele; the recipient pis-til is diploid. In self-incompatible plants, pollen fails to germi-nate, or the pollen tube fails to traverse the style, if the S allele ofthe pollen matches one of the two S alleles in the pistil (Figure39.3).

The stigma plays an important role in “mate selection” byflowering plants. The stigmas of most plants are exposed to thepollen of many other species as well as their own. Pollen fromthe same species binds strongly to the stigma due to cell–cell sig-naling between the stigma and the cell walls of the pollen grains.In contrast, foreign pollen falls off readily or fails to germinate.

A pollen tube delivers male cells to the embryo sacWhen a pollen grain lands on the stigma of a compatible pis-til, it germinates. Germination, for a pollen grain, is the de-


39.2 Wind Pollination The numerous anthers on theseinflorescences (groups of flowers) of a hazelnut tree all pointaway from the stalk and stand free of the plant, promotingdispersal of the pollen by wind.

S3 and S4 pollen are compatible with an S1S2 pistil…

Pollen

Stigma

Style

Pollentube

Ovary

Pollen

…but S1 and S2 pollen do not germinate. They are self-incompatible.

S1S2S1S2 S1S2

S1 S1S2S3 S4 S3Pistil

39.3 Self-Incompatibility Pollen grains do not germinate normallyif their S allele matches one of the S alleles of the stigma. Thus, theegg cannot be fertilized by a sperm from the same plant.

velopment of a pollen tube (Figure 39.4). The pollen tube ei-ther traverses the spongy tissue of the style or, if the style ishollow, grows downward on the inner surface of this femaleorgan until it reaches an ovule. The pollen tube may growmillimeters or even centimeters in the process.

The rapid growth of the pollen tube requires calcium ions,which are taken up by the growing tip of the tube, as well ascell adhesion proteins. The downward growth of the pollentube is believed to be guided by a long-distance chemical sig-nal from the synergids within the ovule. If one synergid isdestroyed, the ovule still attracts pollen tubes, but destruc-tion of both synergids renders the ovule unable to attractpollen tubes, and fertilization does not occur.

Angiosperms perform double fertilizationIn most angiosperm species, the mature pollen grain con-sists of two cells, the tube cell and the generative cell. Thelarger tube cell encloses the much smaller generative cell.Guided by the tube cell nucleus, the pollen tube eventuallygrows through the megasporangial tissue and reaches theembryo sac. The generative cell meanwhile has undergoneone mitotic division and cytokinesis to produce two hap-loid sperm cells.

Both of the sperm cells enter the embryo sac, where theyare released into the cytoplasm of one of the synergids. Thissynergid degenerates, releasing the sperm cells (Figure 39.5).Each sperm cell then fuses with a different cell of the embryosac. One sperm cell fuses with the egg cell, producing thediploid zygote. The other fuses with the central cell, and thatsperm cell nucleus and the two polar nuclei unite to form atriploid (3n) nucleus. While the zygote nucleus begins mitoticdivision to form the new sporophyte embryo, the triploid nu-cleus undergoes rapid mitosis to form a specialized nutritivetissue, the endosperm. The endosperm will later be digestedby the developing embryo, as we saw in the previous chap-ter. The antipodal cells and the remaining synergid eventu-ally degenerate, as does the pollen tube nucleus.

This process is known as double fertilization because itinvolves two nuclear fusion events:

� One sperm cell fuses with the egg cell.� The other sperm cell fuses with the two polar nuclei.

REPRODUCTION IN FLOWERING PLANTS 753Pollen grain Pollen tube

Stigma

39.4 Pollen Tubes Begin to Grow These pollen grains have landedon hairlike structures on the stigma of an Arabidopsis flower, andpollen tubes have penetrated the stigma.

Egg

Initially the pollen tube contains two haploid cells, the generative cell and the tube cell.

1

Three antipodal cells

Generative cell

Tube cell

Tube cell nucleusPolar

nuclei

Synergids

The generative cell divides mitotically, producing two haploid sperm cells.

2 The sperm cells enter the cytoplasm of a synergid.

3

The synergid breaks down; one sperm nucleus unites with the two polar nuclei, forming the first cell of the 3n endosperm generation.

4

5 The other sperm nucleus fertilizes the egg, forming the zygote, the first cell of the 2n sporophyte generation.

39.5 Sperm Nuclei and Double Fertilization The sperm nucleicontribute to the formation of the diploid zygote and the triploidendosperm. Double fertilization is a characteristic feature ofangiosperm reproduction.

The fusion of a sperm cell nucleus with the two polar nucleito form endosperm takes place only in angiosperms. The fu-sion of these three nuclei, the possession of flowers, and theformation of fruit are the three most definitive characteristicsshared by angiosperms.

Embryos develop within seedsShortly after fertilization, highly coordinated growth and de-velopment of embryo, endosperm, integuments, and carpelensues. The integuments—protective tissue layers immedi-ately surrounding the megasporangium—develop into theseed coat, and the carpel ultimately becomes the wall of thefruit that encloses the seed.

The first step in the formation of the embryo is a mitoticdivision of the zygote that gives rise to two daughter cells.These two cells face different fates. An asymmetrical (un-even) distribution of cytoplasm within the zygote causes onedaughter cell to produce the embryo proper and the otherdaughter cell to produce a supporting structure, the suspen-sor (Figure 39.6). The suspensor pushes the embryo againstor into the endosperm and provides one route by which nu-trients pass from the endosperm into the embryo.

With the asymmetrical division of the zygote, polarity hasbeen established, as has the longitudinal axis of the newplant. A long, thin suspensor and a more spherical or globu-lar embryo are distinguishable after just four mitotic divi-sions. The suspensor soon ceases to elongate. However, celldivisions continue, the primary meristems form, and the firstorgans begin to form within the embryo.

In eudicots (monocots are somewhat different), the initiallyglobular embryo takes on a characteristic heart stage form asthe cotyledons (“seed leaves”) start to grow. Further elonga-tion of the cotyledons and of the main axis of the embryo givesrise to what is called the torpedo stage, during which some of

the internal tissues begin to differentiate (see Figure 39.6). Be-tween the cotyledons is the shoot apex; at the other end is theroot apex. Between the shoot and root apices is the hypocotyl.Each of the apical regions contains an apical meristem whosedividing cells will give rise to the organs of the mature plant.

During seed formation, large amounts of nutrients aremoved in from other parts of the plant, and the endospermaccumulates starch, lipids, and proteins. In many species, thecotyledons absorb the nutrient reserves from the surround-ing endosperm and grow very large in relation to the rest ofthe embryo (Figure 39.7a). In others, the cotyledons remainthin (Figure 39.7b); they draw on the reserves in the en-dosperm as needed when the seed germinates.

Suspensor

Embryo

Heart-stageembryo

Endospermnucleus

Embryosac

Zygote

The zygote nucleus divides mitotically, one daughter cell giving rise to the embryo proper and the other to the suspensor.

Shootapex

The tissues surrounding the embryo sac develop into the seed coat.

Endosperm

Hypocotyl

Root apex

Suspensor

Torpedo-stageembryo

Seed coat

Cotyledons

39.6 Early Development of a Eudicot The embryo develops throughintermediate stages, including a characteristic heart-shaped stage, toreach the torpedo stage.

In other eudicots, the endosperm remains separate and the cotyledons remain thin.

In monocots, the single cotyledon is pressed against the endosperm.

(a) Kidney bean (b) Castor bean (c) Corn

Seed coat Seed coat

Endosperm

Shoot apex

Root apex

Cotyledon Cotyledon

Cotyledon

Shoot apex

Root apex

Endosperm

In some eudicots, the cotyledons absorb much of the endosperm and fill most of the seed.

39.7 Variety in Angiosperm Seeds In some seeds, such as kidneybeans (a), the nutrient reserves of the endosperm are absorbed bythe cotyledons. In others, such as castor beans (b) and corn (c), thereserves in the endosperm will be drawn upon after germination.

In the late stages of embryonic development, the seedloses water—sometimes as much as 95 percent of its originalwater content. In this desiccated state, the embryo is inca-pable of further development; it remains quiescent until in-ternal and external conditions are right for germination. (Re-call from Chapter 38 that a necessary early step in seedgermination is the massive imbibition of water.) In additionto embryo and endosperm development, the structures of theovary are also undergoing developmental changes to form aseed and fruit.

Some fruits assist in seed dispersalAfter fertilization, the ovary wall of a flowering plant—to-gether with its seeds—develops into a fruit. A fruit may con-sist of only the mature ovary and the seeds it contains, or itmay include other parts of the flower or structures that areclosely related to it. In some species, this process producesfleshy, edible fruits such as peaches and tomatoes, while inother species the fruits are dry or inedible. Some major vari-ations on this theme are illustrated in Figure 30.12, whichshows only fleshy, edible fruits. Whatever its form, the fruitserves to assure seed dispersal.

Some fruits help disperse seeds over substantial distances,improving the chances that at least a few of the many seedsproduced by a plant will find suitable conditions for germi-nation and growth to sexual maturity. Various trees, includ-ing ash, elm, maple, and tree of heaven, produce a dry,winged fruit that may be blown some distance from the par-ent tree by the wind (Figure 39.8a). Water disperses somefruits; coconuts have been spread in this way from island toisland in the Pacific Ocean (Figure 39.8b). Still other fruitstravel by hitching rides with animals—either inside or out-side them. Fleshy fruits such as berries provide food formammals or birds; seeds that are swallowed whole travelsafely through the animal’s digestive tract and are depositedsome distance from the parent plant. In some species, seedsmust pass through an animal to break dormancy.

We have now traced the sexual life cycle of angiospermsfrom the flower to the fruit to the dispersal of seeds. Seed ger-mination and the vegetative development of the seedlingwere presented in Chapter 38. Now let’s complete the cycleby considering the transition from the vegetative to the flow-ering state, and how this transition is regulated.

The Transition to the Flowering State

If we view a plant as something produced by a seed for thepurpose of bearing more seeds, then the act of flowering isone of the supreme events in a plant’s life. The transition tothe flowering state marks the end of vegetative growth forsome plants. In other plants, vegetative growth may accom-

pany flowering or resume after flowering is completed. Butwhatever the specific pattern, flowering always entails ma-jor developmental changes.

Apical meristems can become inflorescence meristemsThe first visible sign of the transition to the flowering state maybe a change in one or more apical meristems in the shoot sys-tem. During vegetative growth, an apical meristem continuallyproduces leaves, lateral buds, and internodes (Figure 39.9a).This unrestricted growth is indeterminate (see Chapter 35).

Flowers may appear singly or in an orderly cluster thatconstitutes an inflorescence. If a vegetative apical meristembecomes an inflorescence meristem, it ceases production ofleaves, lateral buds, and internodes and produces other struc-tures: smaller leafy structures called bracts, as well as newmeristems in the angles between the bracts and the intern-odes (Figure 39.9b). These new meristems may also be inflo-rescence meristems, or they may be floral meristems, eachof which gives rise to a flower.

Each floral meristem typically produces four consecutivewhorls or spirals of organs—the sepals, petals, stamens, andcarpels—separated by very short internodes, keeping theflower compact (Figure 39.9c). In contrast to vegetative api-cal meristems and some inflorescence meristems, floralmeristems are responsible for determinate growth—the lim-ited growth of the flower to a particular size and form.


(a)

(b)

39.8 Dispersal of Fruits(a) A samara is a winged fruit characteristic of the maple family.(b) A coconut seed germinateswhere it washed ashore on abeach in the South Pacific.

A cascade of gene expression leads to flowering

How do apical meristems become inflorescence meristems,and how do inflorescence meristems give rise to floral meris-tems? How does a floral meristem give rise, in short order, tofour different floral organs? How does each flower come tohave the correct number of each of the floral organs? Nu-merous genes collaborate to produce these results. We’ll re-fer here to some of the genes whose actions have been mostthoroughly studied in Arabidopsis and snapdragons.

� Expression of a group of meristem identity genes initiatesa cascade of further gene expression.

� This cascade begins with cadastral genes, which partici-pate in pattern formation—the spatial organization of thewhorls of organs.

� Cadastral genes trigger the expression of floral organ iden-tity genes, which work in concert to specify the successivewhorls (see Figure 19.12)

Floral organ identity genes are homeotic genes, and theirproducts are transcription factors that mediate the expressionof still other genes.

Having seen how flowering occurs, we will nowconsider how the transition from the vegetative to theflowering state is initiated.

Photoperiodic Control of Flowering

Environmental cues trigger the transition to the flow-ering state in many cases, but such environmental con-trol is also subject to genetic modification. The life cy-cles of flowering plants fall into three categories:annual, biennial, and perennial. Annuals, such asmany food crops, complete their life cycle (seed toflower) in one growing season. Biennials, such as car-rots and cabbage, grow vegetatively for all or part ofone growing season and live on into a second growingseason, during which they flower, form seeds, and die.Perennials, such as oak trees, live for a few to manygrowing seasons, during which both vegetative growthand flowering occur. What control systems give rise tothese and other differences in flowering behavior?

In 1920, W. W. Garner and H. A. Allard of the U.S.Department of Agriculture studied the behavior of anewly discovered mutant tobacco plant. The mutant,named ‘Maryland Mammoth,’ had large leaves and ex-ceptional height. When the other plants in the field

flowered, ‘Maryland Mammoth’ plants continued to grow.Garner and Allard took cuttings of ‘Maryland Mammoth’ intotheir greenhouse, and the plants that grew from those cuttingsfinally flowered in December.

Garner and Allard guessed that this flowering pattern hadsomething to do with the mutant’s response to some envi-ronmental cue. They tested several likely environmental vari-ables, such as temperature, but the key variable proved to beday length. By moving plants between light and dark roomsat different times to vary the day length artificially, they wereable to establish a direct link between flowering and daylength. We now know that the key variable is the length ofthe night, rather than the day, but Garner and Allard did notmake that distinction.

The ‘Maryland Mammoth’ plants did not flower if the lightperiod they were exposed to was longer than 14 hours per day,but flowering commenced after the days became shorter than14 hours. Thus, the critical day length for ‘Maryland Mam-moth’ tobacco is 14 hours (Figure 39.10). The phenomenon ofcontrol by the length of day or night is called photoperiodism.

There are short-day, long-day, and day-neutral plantsPlants that flower in response to photoperiodic stimuli fallinto several classes. Poinsettias, chrysanthemums, and ‘Mary-land Mammoth’ tobacco are short-day plants (SDPs), whichflower only when the day is shorter than a critical maximum.


A floral meristem gives rise to a flower.

A bract is a modified, usually reduced leaflike structure.

Inflorescence meristems give rise to floral meristems, bracts, and more inflorescence meristems.

Vegetativeapicalmeristem

Internode

Leaf

(a)

Inflorescencemeristem

Floralmeristem

Carpel

Stamen Modifiedleaflikestructuresof the flower

Petal

Sepal

(b)

(c)

A vegetatively growing apical meristem continues to produce leaves and internodes.

Floralmeristem

39.9 Flowering and the Apical Meristem A vegetative apicalmeristem (a) grows without producing flowers. Once the transition to the flowering state is made, inflorescence meristems (b) give riseto bracts and to floral meristems (c), which become the flowers.

Spinach and clover are examples of long-day plants (LDPs),which flower only when the day is longer than a critical min-imum. Generally, LDPs are triggered to flower in midsummerand SDPs in late summer, fall, or sometimes in the spring. Be-cause short days occur both before and after midsummer,there is a degree of ambiguity in this signal. Could there bea more precise way for plants to regulate flowering?

Some plants require photoperiodic signals that are morecomplex than just short or long days. One group, the short-long-day plants, must experience first short days and then longones in order to flower. Accordingly, white clover and othershort-long-day plants flower during the long days beforemidsummer. Another group, the long-short-day plants, cannotflower until the long days of summer have been followed byshorter ones, so they bloom only in the fall. Kalanchoe, seenin Figure 39.16b, is a long-short-day plant.

Other processes besides flowering are also under pho-toperiodic control. We have learned, for example, that shortdays trigger the onset of winter dormancy in plants. (Ani-mals, too, show a variety of photoperiodic behaviors, as we’llsee in Chapter 52.)

The flowering of some angiosperms, such as corn, roses,and tomatoes, is not photoperiodic. In fact, there are more ofthese day-neutral plants than there are short-day and long-day plants. Some plants are photoperiodically sensitive onlywhen young and become day-neutral as they grow older.Others require specific combinations of day length and otherfactors—especially temperature—to flower.

The length of the night determines whether a photoperiodic plant will flowerThe terms “short-day plant” and “long-day plant” becameentrenched before scientists learned that photoperiodicallysensitive plants actually measure the length of the night, orof a period of darkness, rather than the length of the day. Thisfact was demonstrated by Karl Hamner of the University ofCalifornia at Los Angeles and James Bonner of the CaliforniaInstitute of Technology (Figure 39.11).

Working with cocklebur, an SDP, Hamner and Bonner rana series of experiments using two sets of conditions:


‘Maryland Mammoth’ tobacco flowers only when days are shorter than 14 hours, its critical day length.

Henbane flowers only when days are longer than 14 hours, its critical day length.

Long days; plant remains

vegetative

Long days;plant flowers

Short days; plant flowers

Short days; plant remains

vegetative

‘Maryland Mammoth’ tobacco

(short-day plant)

Light

14 hours

Dark

Henbane, Hyoscyamus niger (long-day plant)

Light

14 hours

Dark

39.10 Day Length and Flowering By artificially varying the lengthof the day, Garner and Allard showed that the flowering of ‘MarylandMammoth’ tobacco is initiated when the days become shorter than acritical length. ‘Maryland Mammoth’ tobacco is thus called a short-day plant. Henbane, a long-day plant, shows an inverse pattern offlowering.

Plants were moved between light and dark rooms for specified numbers of hours.

METHOD

Only plants given9 or more hoursof dark flowered.

Only plants given 10 hours of dark flowered.

16

Light constant/Darkness varied

Light varied/8 or 10 hours of darkness

16

16

16

16

16

8

10

12

8 8

8

7

6

9

8

8

10

12

10

10

10

10

11

No flowering

No flowering

Time (hours)

RESULTS

EXPERIMENT

Conclusion: Short-day plants measure the length of the night and could more accurately be called long-night plants.

Question: Do short-day plants measure day length or night length?

39.11 Night Length and Flowering The length of the dark period,not the length of the light period, determines flowering.

� For one group of plants, the light period was kept con-stant—either shorter or longer than the critical daylength—and the dark period was varied.

� For another group of plants, the dark period was keptconstant and the light period was varied.

The plants flowered under all treatments in which the darkperiod exceeded 9 hours, regardless of the length of the lightperiod. Thus, Hamner and Bonner concluded that it is thelength of the night that matters; for cocklebur, the critical nightlength is about 9 hours. Thus, it would be more accurate tocall cocklebur a “long-night plant” than a short-day plant.

In cocklebur, a single long night is sufficient photoperiodicstimulus to trigger full flowering some days later, even if theintervening nights are short ones. Most plants are less sensi-tive than cocklebur and require from two to several nights ofappropriate length to induce flowering. For some plants, asingle shorter night in a series of long ones, even one day be-fore flowering would have commenced, inhibits flowering.

By means of other experiments, Hamner and Bonnergained some insight into how plants measure night length.They grew SDPs and LDPs under a variety of light condi-tions. In some experiments, the dark period was interruptedby a brief exposure to light; in others, the light period was in-terrupted briefly by darkness. Interruptions of the light pe-riod by darkness had no effect on the flowering of eithershort-day or long-day plants. Even a brief interruption of thedark period by light, however, completely nullified the effectof a long night (Figure 39.12a). An SDP flowered only if thelong nights were uninterrupted. An LDP experiencing longnights flowered if those nights were interrupted by exposureto light. Thus, the investigators concluded, these plants musthave a timing mechanism that measures the length of a con-tinuous dark period.

The nature of this timing mechanism has been partiallyrevealed, beginning with the determination of the effectivewavelengths of light and the identity of the photoreceptors.In the interrupted-night experiments, the most effectivewavelengths of light were in the red range (Figure 39.12b),and the effect of a red-light interruption of the night could befully reversed by a subsequent exposure to far-red light, in-dicating that a phytochrome is the photoreceptor. Phy-tochromes and blue-light receptors, which affect several as-pects of plant development (see Chapter 38), also participatein the photoperiodic timing mechanism.

What might that mechanism consist of? It was once hy-pothesized that the timing mechanism might simply be theslow conversion of a phytochrome during the night from thePfr form—produced during the light hours—to the Pr form.Such phytochrome conversion would function as an “hour-glass,” and the effect of a night would depend simply uponwhether all the phytochrome had been converted. However,

this suggestion is inconsistent with many experimental ob-servations, such as the fact that when a plant is subjected to adark period several days in duration, the plant’s sensitivity toa light flash during the long night varies on a roughly 24-hourcycle. Such data suggest instead that the phytochrome is onlya photoreceptor, and that the timekeeping role is played by abiological clock that is linked to the phytochrome (which setsthe clock) and also to the production of flowers.

Circadian rhythms are maintained by a biological clockIt is clear that organisms have some way of measuring time,and that they are well adapted to the 24-hour day–night cy-cle of our planet. A biological clock resides within the cells of


EXPERIMENT A

Conclusion: Photoperiodic plants measure the length of the night, not the day. Interrupting a long night with a brief period of light inhibits flowering in short-day plants. Long-day plants flower when the night is short, but interrupting their long day has no effect.

Question: How does interrupting a long day or night affect flowering?

FloweringNo flowering


No floweringFlowering


Long-day plants

Short-dayplants Experimental conditions

EXPERIMENT B

Conclusion: When plants are exposed to red (R) and far-red (FR) light in alternation, the final treatment determines the effect of the light interruption, suggesting that phytochrome participates in photoperiodic responses.

Question: Does phytochrome participate in the photoperiodic timing mechanism?

R

R

R R

FR

FR

FR

R RFR FR

Long-day plants

Short-dayplants

Flowering

No floweringFloweringFloweringNo floweringFlowering

No flowering

FloweringNo floweringNo floweringFloweringNo flowering

39.12 The Effect of Interrupted Days and Nights(a) Experiments suggest that plants are able to measure thelength of a continuous dark period and use this informationto trigger flowering. (b) Phytochromes seem to be involved inthe photoperiodic timing mechanism.

all eukaryotes and some prokaryotes. The major outwardmanifestations of this clock are known as circadian rhythms(from the Latin circa, “about,” and dies, “day”).

We can characterize circadian rhythms, as well as otherregular biological cycles, in two ways: The period is thelength of one cycle, and the amplitude is the magnitude ofthe change over the course of a cycle (Figure 39.13).

The circadian rhythms of cyanobacteria, protists, animals,fungi, and plants have been found to share some importantcharacteristics:

� The period is remarkably insensitive to temperature, al-though lowering the temperature may drastically reducethe amplitude of the rhythmic effect.

� Circadian rhythms are highly persistent; they may contin-ue for days even in an environment in which there are noenvironmental cues, such as light–dark periods.

� Circadian rhythms can be entrained, within limits, bylight–dark cycles that differ from 24 hours. That is, theperiod an organism expresses can be made to coincidewith that of the light–dark cycle to which it is exposed.

� A brief exposure to light can shift the peak of the cycle—it can cause a phase shift.

Plants provide innumerable examples of circadian rhy-thms. The leaflets of plants such as clover normally hangdown and fold at night and rise and unfold during the day.The flowers of many plants show similar “sleep movements,”closing at night and opening during the day. They continueto open and close on an approximately 24-hour cycle evenwhen the light and dark periods are experimentally modified.

The period of circadian rhythms in nature is approxi-mately 24 hours. If a clover plant, for example, were to be

placed in light on a day–night cycle totaling exactly 24 hours,it would express a rhythm with a period of exactly 24 hours.However, if an experimenter used a day–night cycle of, say,22 hours, then over time the rhythm would change—itwould be entrained to a 22-hour period.

If an organism is maintained under constant darkness, itwill express a circadian rhythm with an approximately 24-hour period. However, a brief exposure to light under thesecircumstances can cause a phase shift—that is, it can make thenext peak of activity appear either later or earlier than ex-pected, depending on when the exposure is given. Moreover,the organism does not then return to its old schedule if it re-mains in darkness. If the first peak is delayed by 6 hours, thesubsequent peaks are all 6 hours late. Such phase shifts are per-manent—until the organism receives more exposures to light.

Photoreceptors set the biological clockPhytochromes and blue-light receptors are known to affectthe period of the biological clock, with the different pigmentsreporting on different wavelengths and intensities of light.This diversity of photoreceptors could be an adaptation tothe changes in the light environment that a plant experiencesin the course of a day or a season. How do these photore-ceptors interact with a plant’s biological clock?

The biological clock of Arabidopsis is based on the activi-ties of at least three “clock genes.” The clock genes encoderegulatory proteins that interact to produce a circadian oscil-lation. How does this oscillating clock interact with photore-ceptors and the environment?

Arabidopsis is an LDP. Its clock controls the activity ofCONSTANS (a gene that is not part of the clock mechanism)in such a way that the CONSTANS product, CO protein, ac-cumulates in one phase of the clock’s cycle—the phase inwhich night falls. Under long nights (short days), CO proteinis found at night. Under short nights (long days), CO is alsorelatively abundant at dawn and dusk. When CO proteinlevels are high, light absorbed by phytochrome A and theblue-light receptor cryptochrome 2 leads to flowering (Fig-ure 39.14). Thus, Arabidopsis flowering results from the coin-cidence of light (detected by the two photoreceptors) with aclock-determined phase of the circadian oscillation.

Where is this coincidence-based photoperiodic mechanismlocated in relation to where flowering occurs? Is the timingdevice for flowering located in a particular plant part, or areall parts able to sense the length of the night? This questionwas resolved by “blindfold” experiments, as described next.

Is there a flowering hormone?It quickly became apparent that each leaf is capable of tim-ing the night. If a cocklebur plant—an SDP—is kept under a


Amplitude

Period (about 24 hours)

Effect

Time

…and on the basis of the magnitude of the rhythmic effect, measured by the cycle‘s amplitude.

Circadian rhythms are characterized on the basis of time, measured in periods of about 24 hours…

39.13 Features of Circadian Rhythms Circadian rhythms, like allbiological rhythms, can be characterized in two ways: by period andby amplitude.

regime of short nights and long days, but a leaf is covered soas to give it the needed long nights, the plant will flower (ex-periment A in Figure 39.15). This type of experiment worksbest if only one leaf is left on the plant. If one leaf is given aphotoperiodic treatment conducive to flowering—called aninductive treatment—other leaves kept under noninductiveconditions will tend to inhibit flowering.

Although it is the leaves that sense an inductive photope-riod, the flowers form elsewhere on the plant. Thus, somekind of signal must be sent from the leaf to the site of flowerformation. Three lines of evidence suggest that this signal isa chemical substance—a flowering hormone.

� If a photoperiodically induced leaf is immediately re-moved from a plant after the inductive dark period, the

plant does not flower. If, however, the induced leaf re-mains attached to the plant for several hours, the plantflowers. This result suggests that something must be syn-thesized in the leaf in response to the inductive dark peri-od and then move out of the leaf to induce flowering.

� If two cocklebur plants are grafted together, and if oneplant is exposed to inductive long nights and its graftpartner exposed to noninductive short nights, bothplants flower (experiment B in Figure 39.15).

� In at least one species, if an induced leaf from one plantis grafted onto another, noninduced plant, the host plantflowers.


Rel

ativ

e co

ncen

trat

ion

of C

O

Rel

ativ

e co

ncen

trat

ion

of C

O

40 8 12 16 18 24Time (hours)

0.5

40

1.0

40 8 12 16 18 24Time (hours)

0.5

40

1.0

Under short days, CO protein level remains low throughout the light period, and the plant does not flower.

Light LightDark Dark

Under long days, CO protein levels are high enough at both dawn and dusk so that light absorption by pigments leads to flowering.

No flowering FloweringNo flowering Flowering

39.14 Photoreceptors and theBiological Clock Interact inPhotoperiodic Plants One ofthe genes regulated by the circa-dian clock in Arabidopsis encodesthe CO protein. Floweringdepends on enough CO beingpresent when photoreceptorshave light available to them.

EXPERIMENT A

Conclusion: The leaves measure the dark period. Therefore, some signal must move from the induced leaf to the flowering parts of the plant.

Question: What part of the plant measures the dark period?

Cocklebur, a short-day plant, will not flower if kept under long days and short nights.

If even one leaf is masked for part of the day—thus shifting that leaf to short days and long nights—the plant will flower; note the burrs. If a leaf on a plant at one end of the chain is

subjected to long nights, all of the plants will flower.

Burrs

Masked leaf

EXPERIMENT B

Conclusion: The very stable flowering signal can even travel across multiple grafts.

Question: How stable is the flowering hormone?

Graft

Hypothetical flowering hormone

Masked leaf

A leaf is induced by long nights/short days.

2

3

Five cocklebur plants are grafted together and kept under long days and short nights, with most leaves removed.

1

39.15 Evidence for a Flowering Hormone If even a single leaf isexposed to inductive conditions, a signal travels to the entire plant(and even to other plants, in grafting experiments), inducing it toflower.

Jan A. D. Zeevaart, a plant physiologist at Michigan StateUniversity, performed this last experiment. He exposed a sin-gle leaf of the SDP Perilla to a short-day/long-night regime,inducing the plant to flower. Then he detached this leaf andgrafted it onto another, noninduced, Perilla plant—which re-sponded by flowering. The same leaf grafted onto successivehosts caused each of them to flower in turn. As long as 3months after the leaf was exposed to the short-day/long-night regime, it could still cause plants to flower.

Experiments such as Zeevaart’s led to the conclusion thatthe photoperiodic induction of a leaf causes a more or lesspermanent change in the leaf, causing it to start and continueproducing a flowering hormone that is transported to otherparts of the plant, where the hormone initiates the develop-ment of reproductive structures. Biologists have named thishypothetical hormone florigen, even though, after decadesof active searching, it has not been isolated or characterized.

An elegant experiment suggested that the florigen of SDPsis identical to that of LDPs, even though SDPs produce itonly under long nights and LDPs only under short nights.An SDP and an LDP were grafted together, and both flow-ered, as long as the photoperiodic conditions were inductivefor one of the partners. Either the SDP or the LDP could bethe one induced, but both would always flower. These resultssuggest that a flowering hormone—the elusive florigen—wasbeing transferred from one plant to the other.

The direct demonstration of florigen activity remains acherished goal of plant physiologists. For a long time it wasthought that florigen could be neither a protein nor an RNAbecause those molecules were too large to pass from one liv-ing plant cell to another. However, we now know that suchmacromolecules can be transferred by way of plasmodes-mata, and biologists are reexamining the possibility that anRNA or a protein is the long-sought florigen.

We have considered the photoperiodic regulation of flow-ering, from photoreceptors in a leaf to the biological clock tothe need for a signal that travels from the induced leaf to thesites of flower formation. However, light is not the only en-vironmental variable that affects flowering. In some plants,low temperatures are an essential cue that eventually triggersflowering.

Vernalization and Flowering

Certain cereal grains serve as classic examples of the controlof flowering by temperature. In both wheat and rye, we dis-tinguish two categories of flowering behavior. Spring wheat,for example, is sown in the spring and flowers in the sameyear. It is an annual plant. Winter wheat is biennial and mustbe sown in the fall; it flowers in the following summer. Ifwinter wheat is not exposed to cold after its first year, it willnot flower normally the next year.

The implications of this finding were of great agriculturalinterest in Russia because winter wheat is a better producerthan spring wheat, but it cannot be grown in some parts ofRussia because the winters are too cold for its survival. Sev-eral studies performed in Russia during the early 1900sdemonstrated that if seeds of winter wheat were premoist-ened and prechilled, they could be sown in the spring andwould develop and flower normally the same year. Thus,high-yielding winter wheat could be grown even in previ-ously hostile regions.

This induction of flowering by low temperatures is calledvernalization. Vernalization may require as many as 50 daysof low temperatures (in the range from about –2° to +12°C).Some plant species require both vernalization and long daysto flower. There is a long wait from the cold days of winterto the long days of summer, but because the vernalized stateeasily lasts at least 200 days, these plants do flower whenthey experience the appropriate night length.

Asexual Reproduction

Although sexual reproduction takes up most of the space inthis chapter, asexual reproduction is responsible for many ofthe new plant individuals appearing on Earth. This fact sug-gests that in some circumstances, asexual reproduction mustbe advantageous.

At the beginning of this chapter, we saw that one of theadvantages of sexual reproduction is genetic recombination.Self-fertilization is a form of sexual reproduction, but whena plant self-fertilizes, there are fewer opportunities for geneticrecombination than there are with cross-fertilization. Adiploid, self-fertilizing plant that is heterozygous for a cer-tain locus can produce both kinds of homozygotes for thatlocus plus the heterozygote among its progeny, but it cannotproduce any progeny that carry alleles that it does not itselfpossess. Yet many plants continue to be self-compatible, un-dergo self-fertilization, and produce viable offspring.

Asexual reproduction goes even further than self-fertiliza-tion: It eliminates genetic recombination altogether. When aplant reproduces asexually, it produces a clone of progeny thatare genetically identical to the parent. If a plant is well adaptedto its environment, asexual reproduction may spread its geno-type throughout that environment. This ability to exploit a par-ticular environment is an advantage of asexual reproduction.

There are many forms of asexual reproductionWe call stems, leaves, and roots vegetative organs to distin-guish them from flowers, the reproductive parts of the plant.The modification of a vegetative organ is what makes vege-tative reproduction—asexual reproduction in plants—pos-sible. In many cases, the stem is the organ that is modified.


Strawberries and some grasses, for example, produce hori-zontal stems, called stolons or runners, that grow along thesoil surface, form roots at intervals, and establish potentiallyindependent plants (see Figure 35.4b). Tip layers are uprightbranches whose tips sag to the ground and develop roots, asin blackberry and forsythia.

Some plants, such as potatoes, form enlarged fleshy tipsof underground stems, called tubers (see Figure 35.4a). Rhi-zomes are horizontal underground stems that can give rise tonew shoots. Bamboo is a striking example of a plant that re-produces vegetatively by means of rhizomes. A single bam-boo plant can give rise to a stand—even a forest—of plantsconstituting a single, physically connected entity.

Whereas stolons and rhizomes are horizontal stems, bulbsand corms are short, vertical, underground stems. Lilies andonions form bulbs (Figure 39.16a), short stems with manyfleshy, highly modified leaves that store nutrients. These stor-age leaves make up most of the bulb. Bulbs are thus large un-derground buds. They can give rise to new plants by divid-ing or by producing new bulbs from lateral buds. Crocuses,gladioli, and many other plants produce corms, undergroundstems that function very much as bulbs do. Corms aredisclike and consist primarily of stem tissue; they lack thefleshy modified leaves that are characteristic of bulbs.

Not all vegetative organs modified for reproduction arestems. Leaves may also be the source of new plantlets, as in thesucculent plants of the genus Kalanchoe (Figure 39.16b). Manykinds of angiosperms, ranging from grasses to trees such as as-pens and poplars, form interconnected, genetically homoge-neous populations by means of suckers—shoots produced byroots. What appears to be a whole stand of aspen trees, for ex-ample, may be a clone derived from a single tree by suckers.

Plants that reproduce vegetatively often grow in physi-cally unstable environments, such as eroding hillsides. Plants

with stolons or rhizomes, such as beach grasses, rushes, andsand verbena, are common pioneers on coastal sand dunes.Rapid vegetative reproduction enables these plants, once in-troduced, not only to multiply but also to survive burial bythe shifting sand; in addition, the dunes are stabilized by theextensive network of rhizomes or stolons that develops. Veg-etative reproduction is also common in some deserts, wherethe environment is not often suitable for seed germinationand the establishment of seedlings.

Dandelions, citrus trees, and some other plants reproduceby the asexual production of seeds, called apomixis. As wehave seen, meiosis reduces the number of chromosomes ingametes, and fertilization restores the sporophytic numberof chromosomes in the zygote. Some plants can skip overboth meiosis and fertilization and still produce seeds.Apomixis produces seeds within the ovary without the min-gling and segregation of chromosomes and without theunion of gametes. The ovule simply develops into a seed,and the ovary wall develops into a fruit. An apomictic em-bryo has the sporophytic number (2n) of chromosomes. Theresult of apomixis is a fruit with seeds that are geneticallyidentical to the parent plant.

Apomixis sometimes requires pollination. In someapomictic species, a sperm nucleus must combine with thepolar nuclei in order for the endosperm to form. In otherapomictic species, the pollen provides the signals for embryoand endosperm formation, although neither sperm nucleusparticipates in fertilization. This observation emphasizes thatpollination and fertilization are not the same thing.

Asexual reproduction is important in agricultureFarmers and gardeners take advantage of some naturalforms of vegetative reproduction. They have also developed


(a)

(b)

Storage leaves grow from the stem of this onion.

The short stem is visible at the bottom of the bulb.

The plantlets forming on the margin of this Kalanchoe leaf will fall to the ground and start independent lives.

Allium sp.

39.16 Vegetative Organs Modified for Reproduction (a) Bulbs areshort stems with large buds that store nutrients and can give rise to newplants. (b) In Kalanchoe, new plantlets can form on leaves.

new types of asexual reproduction by manipulating plants.One of the oldest methods of vegetative reproduction usedin agriculture consists of simply making cuttings of stems,inserting them in soil, and waiting for them to form roots andthus become autonomous plants. The cuttings are usually en-couraged to root by treatment with a plant hormone, auxin,as described in Chapter 38.

Horticulturists reproduce many woody plants by graft-ing—attaching a bud or a piece of stem from one plant to theroot or root-bearing stem of another plant. The part of the re-sulting plant that comes from the root-bearing “host” iscalled the stock; the part grafted on is the scion (Figure 39.17).

In order for a graft to succeed, the vascular cambium of thescion must become associated with that of the stock. By celldivision, both cambia form masses of wound tissue. If the twomasses meet and fuse, the resulting continuous cambium canproduce xylem and phloem, allowing transport of water andminerals to the scion and of photosynthate to the stock. Graftsare most often successful when the stock and scion belong tothe same or closely related species. Most fruit grown for mar-ket in the United States is produced on grafted trees.

Scientists in universities and industrial laboratories havebeen developing new ways to produce useful plants via tis-sue culture. Because many plant cells are totipotent (see Fig-ure 19.3), cultures of undifferentiated tissue can give rise toentire plants, as can small pieces of tissue cut directly from aparent plant. Tissue cultures are used commercially to pro-duce numerous new plants rapidly without resorting to seeds.

Culturing tiny bits of apical meristem can produce plantsfree of viruses. Because apical meristems lack developed vas-cular tissues, viruses tend not to enter them. Treatment withhormones causes a single apical meristem to give rise to 20or more shoots; thus, a single plant can give rise to millionsof genetically identical plants within a year by repeatedmeristem culturing. Using this approach, strawberry and po-tato producers are able to start each year’s crop from virus-free plants.

Recombinant DNA techniques applied to tissue culturescan provide plants with increased resistance to pests or in-creased nutritive value to humans. There is also interest inmaking certain valuable, sexually reproducing plants capableof apomixis. By causing cells of different types to fuse, one canobtain plants with exciting new combinations of properties.

Chapter Summary

Many Ways to Reproduce� Almost all flowering plants reproduce sexually, and manyalso reproduce asexually. Both sexual and asexual reproductionare important in agriculture.

Sexual Reproduction in Plants� Sexual reproduction promotes genetic diversity in a popula-tion, which may give the population an advantage under chang-ing environmental conditions or in exploiting new territory.� The flower is an angiosperm’s device for sexual reproduction.� Flowering plants have microscopic gametophytes that devel-op within the flowers of the sporophytes. The megagameto-phyte is the embryo sac, which typically contains eight nuclei ina total of seven cells. The microgametophyte is the pollen grain,which usually contains two cells. Review Figure 39.1. SeeWeb/CD Tutorial 39.1� Pollination enables fertilization in the absence of external water.� In self-incompatible species, the stigma or style rejects pollenfrom the same plant. Review Figure 39.3� The pollen grain delivers sperm cells to the embryo sac bymeans of a pollen tube.� Most angiosperms perform double fertilization: One spermnucleus fertilizes the egg, forming a zygote, and the othersperm nucleus unites with the two polar nuclei to form atriploid endosperm. Review Figure 39.5� The zygote develops into an embryo (with an attached sus-pensor), which remains quiescent in the seed until conditionsare right for germination. The endosperm supplies the nutritivereserve upon which the embryo depends at germination.Review Figures 39.6, 39.7. See Web/CD Activity 39.1� Flowers develop into seed-bearing fruits, which often playimportant roles in the dispersal of the species.

The Transition to the Flowering State� For a vegetatively growing plant to flower, an apical meri-stem in the shoot system must become an inflorescence meri-stem, which gives rise to bracts as well as more meristems. Themeristems it produces may become floral meristems or addi-tional inflorescence meristems. Review Figure 39.9� Flowering results from a cascade of gene expression. Floralorgan identity genes are expressed in floral meristems that giverise to sepals, petals, stamens, and carpels.

Photoperiodic Control of Flowering� Photoperiodic plants regulate their flowering by measuringthe length of light and dark periods.� Short-day plants flower when the days are shorter than aspecies-specific critical day length; long-day plants flower whenthe days are longer than a critical day length. Review Figure 39.10� Some angiosperms have more complex photoperiodicrequirements than short-day or long-day plants have, but mostare day-neutral.


Scion

Stock

In grafting, the scion is aligned so that its vascular cambium is adjacent to the vascular cambium in the stock.

39.17 Grafting Grafting—attaching a piece of a plant to the root orroot-bearing stem of another plant—is a common horticultural tech-nique.The “host”root or stem is the stock; the upper grafted piece isthe scion.

� The length of the night is what actually determines whether aphotoperiodic plant will flower. Review Figure 39.11� Interruption of the nightly dark period by a brief exposure tolight undoes the effect of a long night. Review Figure 39.12. SeeWeb/CD Tutorial 39.2� The mechanism of photoperiodic control involves phy-tochromes and a biological clock. Review Figures 39.13, 39.14� Evidence suggests that there is a flowering hormone, calledflorigen, but the substance has yet to be isolated from any plant.Review Figure 39.15

Vernalization and Flowering� In some plant species, exposure to low temperatures—vernal-ization—is required for flowering.

Asexual Reproduction� Asexual reproduction allows rapid multiplication of organ-isms that are well suited to their environment.� Vegetative reproduction involves the modification of a vege-tative organ—usually the stem—for reproduction. Stolons, tiplayers, tubers, rhizomes, bulbs, corms, and suckers are meansby which plants may reproduce vegetatively. � Some plant species produce seeds asexually by apomixis.� Agriculturalists use natural and artificial techniques of asexu-al reproduction to reproduce particularly desirable plants.� Horticulturists often graft different plants together to takeadvantage of favorable properties of both stock and scion.Review Figure 39.17� Tissue culture techniques, made possible by the totipotencyof many plant cells, are used to propagate plants asexually, toproduce virus-free clones of crop plants, and to manipulateplants by recombinant DNA technology.

Self-Quiz1. Sexual reproduction in angiosperms

a. is by way of apomixis.b. requires the presence of petals.c. can be accomplished by grafting.d. gives rise to genetically diverse offspring.e. cannot result from self-pollination.

2. The typical angiosperm female gametophytea. is called a megaspore.b. has eight nuclei.c. has eight cells.d. is called a pollen grain.e. is carried to the male gametophyte by wind or animals.

3. Pollination in angiospermsa. never requires external water.b. never occurs within a single flower.c. always requires help by animal pollinators.d. is also called fertilization.e. makes most angiosperms independent of external water

for reproduction.

4. Which statement about double fertilization is not true?a. It is found in most angiosperms.b. It takees place in the microsporangium.c. One of its products is a triploid nucleus.d. One sperm nucleus fuses with the egg nucleus.e. One sperm nucleus fuses with two polar nuclei.

5. The suspensora. gives rise to the embryo.b. is heart-shaped in eudicots.

c. separates the two cotyledons of eudicots.d. ceases to elongate early in embryonic development.e. is larger than the embryo.

6. Which statement about photoperiodism is not true?a. It is related to the biological clock.b. A phytochrome plays a role in the timing process.c. It is based on measurement of the length of the night.d. Most plant species are day-neutral.e. It is limited to the plant kingdom.

7. Although florigen has never been isolated, we think it existsbecausea. night length is measured in the leaves, but flowering

occurs elsewhere.b. it is produced in the roots and transported to the shoot

system.c. it is produced in the coleoptile tip and transported to the

base.d. we think that gibberellin and florigen are the same

compound.e. it may be activated by prolonged (more than a month)

chilling.

8. Which statement about vernalization is not true?a. It may require more than a month of low temperatures.b. The vernalized state generally lasts for about a week.c. Vernalization makes it possible to have two winter wheat

crops each year.d. It is accomplished by subjecting moistened seeds to chilling.e. It was of interest to Russian scientists because of their

native climate.

9. Which of the following does not participate in asexualreproduction?a. Stolonb. Rhizomec. Zygoted. Tubere. Corm

10. Apomixis involvesa. sexual reproduction.b. meiosis.c. fertilization.d. a diploid embryo.e. no production of a seed.

For Discussion1. For a crop plant that reproduces both sexually and asexually,

which method of reproduction might the farmer prefer? Why?

2. Thompson Seedless grapes are produced by vines that aretriploid. Think about the consequences of this chromosomalcondition for meiosis in the flowers. Why are these grapesseedless? Describe the role played by the flower in fruit for-mation when no seeds are being formed. How do you sup-pose Thompson Seedless grapes are propagated?

3. Poinsettias are popular ornamental plants that typically bloomjust before Christmas. Their flowering is photoperiodicallycontrolled. Are they long-day or short-day plants? Explain.

4. You plan to induce the flowering of a crop of long-day plantsin the field by using artificial light. Is it necessary to keep thelights on continuously from sundown until the point at whichthe critical day length is reached? Why, or why not?


chap39

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