Chapter 42: Plant Reproduction

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42Plant Reproduction

Concept Outline

42.1 Angiosperms have been incredibly successful, inpart, because of their reproductive strategies.

Rise of the Flowering Plants. Animal and winddispersal of pollen increases genetic variability in a species.Seed and fruit dispersal mechanisms allow offspring tocolonize distant regions. Other features such as shortenedlife cycles may also have been responsible for the rapiddiversification of the flowering plants.Evolution of the Flower. A complete flower has fourwhorls, containing protective sepals, attractive petals, malestamens, and female ovules.

42.2 Flowering plants use animals or wind to transferpollen between flowers.

Formation of Angiosperm Gametes. The malegametophytes are the pollen grains, and the femalegametophyte is the embryo sac.Pollination. Evolutionary modifications of flowers haveenhanced effective pollination.Self-Pollination. Self-pollination is favored in stableenvironments, but outcrossing enhances genetic variability. Fertilization. Angiosperms use two sperm cells, one tofertilize the egg, the other to produce a nutrient tissuecalled endosperm.

42.3 Many plants can clone themselves by asexualreproduction.

Asexual Reproduction. Some plants do without sexualreproduction, instead cloning new individuals from parts ofthemselves.

42.4 How long do plants and plant organs live?

The Life Span of Plants. Clonal plants can liveindefinitely through their propagules. Parts of plantssenesce and die. Some plants reproduce sexually only onceand die.

The remarkable evolutionary success of flowering plantscan be linked to their reproductive strategies (figure

42.1). The evolution and development of flowers has beendiscussed in chapters 37 and 41. Here we explore reproduc-tive strategies in the angiosperms and how their unique fea-tures, flowers and fruits, have contributed to their success.This is, in part, a story of coevolution between plants andanimals that ensures greater genetic diversity by dispersingplant gametes widely. However, in a stable environment,there are advantages to maintaining the status quo geneti-cally. Asexual reproduction is a strategy to clonally propa-gate individuals. An unusual twist to sexual reproduction insome flowering plants is that senescence and death of theparent plant follows.

FIGURE 42.1Reproductive success in flowering plants. Unique reproductivesystems and strategies have coevolved between plants and animals,accounting for almost 250,000 flowering plants inhabiting all butthe harshest environments on earth.

form varies from cacti, grasses, and daisies to aquaticpondweeds. Most shrubs and trees (other than conifers andGinkgo) are also in this phylum. This chapter focuses on re-production in angiosperms (figure 42.2) because of theirtremendous success and many uses by humans. Virtually all

838 Part XI Plant Growth and Reproduction

Rise of the Flowering PlantsMost of the plants we see daily are angiosperms. The250,000 species of flowering plants range in size from al-most microscopic herbs to giant Eucalyptus trees, and their

42.1 Angiosperms have been incredibly successful, in part, because of theirreproductive strategies.

Anther

Microsporemother cell (2n)

Megasporemother cell (2n)

MEIOSIS Pollen grains(microgametophyte) (n)

Pollen grain

Stigma

Tube cell nucleus

Spermcells

Formation ofpollen tube (n)

Pollentube

Egg

DOUBLEFERTILIZATION

Endosperm (3n)

Seed (2n)

Seed coatEmbryo

Adultsporophyte (2n)with flowers

Carpel

Ovary

OvuleMEIOSIS

Eight-nucleate embryo sac(megagametophyte) (n)

Pollen sac

Youngembryo (2n)

Pollentubecell

FIGURE 42.2Angiosperm life cycle. Eggs form within the embryo sac inside the ovules, which, in turn, are enclosed in the carpels. The pollen grains,meanwhile, are formed within the sporangia of the anthers and are shed. Fertilization is a double process. A sperm and an egg cometogether, producing a zygote; at the same time, another sperm fuses with the polar nuclei to produce the endosperm. The endosperm isthe tissue, unique to angiosperms, that nourishes the embryo and young plant.

of our food is derived, directly or indirectly, from floweringplants; in fact, more than 90% of the calories we consumecome from just over 100 species. Angiosperms are alsosources of medicine, clothing, and building materials.While the other plant phyla also provide resources, theyare outnumbered seven to one by the angiosperms. For ex-ample, there are only about 750 extant gymnospermspecies!

Why Are the Angiosperms Successful?

When flowering plants originated, Africa and South Amer-ica were still connected to each other, as well as to Antarc-tica and India, and, via Antarctica, to Australia and NewZealand (figure 42.3). These landmasses formed the greatcontinent known as Gondwanaland. In the north, Eurasiaand North America were united, forming another super-continent called Laurasia. The huge landmass formed bythe union of South America and Africa spanned the equatorand probably had a climate characterized by extreme tem-peratures and aridity in its interior. Similar climates occurin the interiors of major continents at present. Much of theearly evolution of angiosperms may have taken place inpatches of drier and less favorable habitat found in the inte-rior of Gondwanaland. Many features of flowering plantsseem to correlate with successful growth under arid andsemiarid conditions.

The transfer of pollen between flowers of separateplants, sometimes over long distances, ensures outcrossing(cross-pollination between individuals of the samespecies) and may have been important in the early successof angiosperms. The various means of effective fruit dis-persal that evolved in the group were also significant inthe success of angiosperms (see chapter 40). The rapidlife cycle of some of the angiosperms (Arabidopsis can gofrom seed to adult flowering plant in 24 days) was an-other factor. Asexual reproduction has given many inva-sive species a competitive edge. Xylem vessels and otheranatomical and morphological features of the an-giosperms correlate with their biological success. As earlyangiosperms evolved, all of these advantageous featuresbecame further elaborated and developed, and the pace oftheir diversification accelerated.

The Rise to Dominance

Angiosperms began to dominate temperate and tropicalterrestrial communities about 80 to 90 million years ago,during the second half of the Cretaceous Period. We candocument the relative abundance of different groups ofplants by studying fossils that occur at the same time andplace. In rocks more than 80 million years old, the fossilremains of plant phyla other than angiosperms, includ-ing lycopods, horsetails, ferns, and gymnosperms, are

most common. Angiosperms arose in temperate andtropical terrestrial communities in a relatively shorttime.

At about the time that angiosperms became abundant inthe fossil record, pollen, leaves, flowers, and fruits of somefamilies that still survive began to appear. For example,representatives of the magnolia, beech, and legume fami-lies, which were in existence before the end of the Creta-ceous Period (65 million years ago), are alive and flourish-ing today.

A number of insect orders that are particularly associ-ated with flowers, such as Lepidoptera (butterflies andmoths) and Diptera (flies), appeared or became moreabundant during the rise of angiosperms. Plants and in-sects have clearly played a major role in each other’s pat-terns of evolution, and their interactions continue to beof fundamental importance. Additional animals, includ-ing birds and mammals, now assist in pollination andseed dispersal.

By 80 to 90 million years ago, angiosperms weredominant in terrestrial habitats throughout the world.

Chapter 42 Plant Reproduction 839

Equator

G o n d w a n a l a n d

L a u r a s i a

FIGURE 42.3The alignment of the continents when the angiosperms firstappeared in the fossil record about 130 million years ago.Africa, Madagascar, South America, India, Australia, andAntarctica were all connected and part of the huge continent ofGondwanaland, which eventually separated into the discrete landmasses we have today.

Evolution of the FlowerPollination in angiosperms does not involve direct contactbetween the pollen grain and the ovule. Pollen matureswithin the anthers and is transported, often by insects,birds, or other animals, to the stigma of another flower.When pollen reaches the stigma, it germinates, and apollen tube grows down, carrying the sperm nuclei to theembryo sac. After double fertilization takes place, develop-ment of the embryo and endosperm begins. The seed ma-tures within the ripening fruit; the germination of the seedinitiates another life cycle.

Successful pollination in many angiosperms depends onthe regular attraction of pollinators such as insects, birds,and other animals, so that pollen is transferred betweenplants of the same species. When animals disperse pollen,they perform the same functions for flowering plants thatthey do for themselves when they actively search out mates.The relationship between plant and pollinator can be quiteintricate. Mutations in either partner can block reproduc-tion. If a plant flowers at the “wrong” time, the pollinatormay not be available. If the morphology of the flower orpollinator is altered, there may be physical barriers to polli-nation. Clearly floral morphology has coevolved with polli-nators and the result is much more complex and diversethan the initiation of four distinct whorls of organs de-scribed in chapter 40.

Characteristics of Floral Evolution

The evolution of the angiosperms is a focus of chapter 37.Here we need to keep in mind that the diversity of an-giosperms is partly due to the evolution of a great variety offloral phenotypes that may enhance the effectiveness ofpollination. All floral organs are thought to have evolvedfrom leaves. In early angiosperms, these organs maintainthe spiral phyllotaxy often found in leaves. The trend hasbeen toward four distinct whorls. A complete flower has fourwhorls of parts (calyx, corolla, androecium, and gynoe-cium), while an incomplete flower lacks one or more of thewhorls (figure 42.4).

In both complete and incomplete flowers, the calyx usu-ally constitutes the outermost whorl; it consists of flattenedappendages, called sepals, which protect the flower in thebud. The petals collectively make up the corolla and maybe fused. Petals function to attract pollinators. While thesetwo outer whorls of floral organs are sterile, they can en-hance reproductive success.

Androecium (from the Greek andros, “man”, + oikos,“house”) is a collective term for all the stamens (malestructures) of a flower. Stamens are specialized structuresthat bear the angiosperm microsporangia. There are simi-lar structures bearing the microsporangia in the pollencones of gymnosperms. Most living angiosperms have sta-mens whose filaments (“stalks”) are slender and oftenthreadlike, and whose four microsporangia are evident at

the apex in a swollen portion, the anther. Some of themore primitive angiosperms have stamens that are flattenedand leaflike, with the sporangia producing from the upperor lower surface.

The gynoecium (from the Greek gyne, “woman,” +oikos, “house”) is a collective term for all the female parts ofa flower. In most flowers, the gynoecium, which is uniqueto angiosperms, consists of a single carpel or two or morefused carpels. Single or fused carpels are often referred toas the simple or compound pistils, respectively. Most flow-ers with which we are familiar—for example, those oftomatoes and oranges—have a single compound pistil. Inother mostly primitive flowers—for example, buttercupsand stonecups—there may be several to many separate pis-tils, each formed from a single carpel. Ovules (which de-velop into seeds) are produced in the pistil’s swollen lowerportion, the ovary, which usually narrows at the top into aslender, necklike style with a pollen-receptive stigma at itsapex. Sometimes the stigma is divided, with the number ofstigma branches indicating how many carpels are in theparticular pistil. Carpels are essentially inrolled floral leaveswith ovules along the margins. It is possible that the firstcarpels were leaf blades that folded longitudinally; the mar-gins, which had hairs, did not actually fuse until the fruitdeveloped, but the hairs interlocked and were receptive topollen. In the course of evolution, there is evidence thehairs became localized into a stigma, a style was formed,and the fusing of the carpel margins ultimately resulted in apistil. In many modern flowering plants, the carpels havebecome highly modified and are not visually distinguish-able from one another unless the pistil is cut open.

Trends of Floral Specialization

Two major evolutionary trends led to the wide diversityof modern flowering plants: (1) separate floral parts have


Anther


Megasporemother cell (2n)

MEIO

Ovary

OvuleMEIOSIS

FIGURE 42.4Structure of an angiosperm flower.

grouped together, or fused, and (2) floral parts have beenlost or reduced (figure 42.5). In the more advanced an-giosperms, the number of parts in each whorl has oftenbeen reduced from many to few. The spiral patterns of at-tachment of all floral parts in primitive angiosperms have,in the course of evolution, given way to a single whorl ateach level. The central axis of many flowers has short-ened, and the whorls are close to one another. In someevolutionary lines, the members of one or more whorlshave fused with one another, sometimes joining into atube. In other kinds of flowering plants, different whorlsmay be fused together. Whole whorls may even be lostfrom the flower, which may lack sepals, petals, stamens,carpels, or various combinations of these structures. Mod-ifications often relate to pollination mechanisms and, insome cases like the grasses, wind has replaced animals forpollen dispersal.

While much floral diversity is the result of natural se-lection related to pollination, it is important to recognizethe impact breeding (artificial selection) has had onflower morphology. Humans have selected for practicalor aesthetic traits that may have little adaptive value tospecies in the wild. Maize (corn), for example, has beenselected to satisfy the human palate. Human interventionensures the reproductive success of each generation;while in a natural setting modern corn would not havethe same protection from herbivores as its ancestors, andthe fruit dispersal mechanism would be quite different(see figure 21.13). Floral shops sell heavily bred specieswith modified petals, often due to polyploidy, that en-hance their economic value, but not their ability to at-tract pollinators. In making inferences about symbiosesbetween flowers and pollinators, be sure to look at nativeplants that have not been genetically altered by humanintervention.

Trends in Floral Symmetry

Other trends in floral evolution have affected the symmetryof the flower (figure 42.6). Primitive flowers such as thoseof buttercups are radically symmetrical; that is, one coulddraw a line anywhere through the center and have tworoughly equal halves. Flowers of many advanced groups arebilaterally symmetrical; that is, they are divisible into twoequal parts along only a single plane. Examples of suchflowers are snapdragons, mints, and orchids. Such bilater-ally symmetrical flowers are also common among violetsand peas. In these groups, they are often associated withadvanced and highly precise pollination systems. Bilateralsymmetry has arisen independently many times. In snap-dragons, the cyclodia gene regulates floral symmetry, and inits absence flowers are more radial (figure 42.7). Here theevolutionary introduction of a single gene is sufficient tocause a dramatic change in morphology. Whether the samegene or functionally similar genes arose in parallel in otherspecies is an open question.

The first angiosperms likely had numerous free, spirallyarranged flower parts. Modification of floral partsappears to be closely tied to pollination mechanisms.More recently, horticulturists have bred plants foraesthetic reasons resulting in an even greater diversityof flowers.


FIGURE 42.5Trends in floralspecialization.Wild geranium,Geraniummaculatum. Thepetals are reducedto five each, thestamens to ten.

FIGURE 42.6Bilateralsymmetry in anorchid. Whileprimitive flowersare usually radiallysymmetrical,flowers of manyadvanced groups,such as the orchidfamily(Orchidaceae), arebilaterallysymmetrical.

(a) (b)

FIGURE 42.7Genetic regulation of asymmetry in flowers. (left) Snapdragonflowers normally have bilateral symmetry. (right) The cyclodia generegulates floral symmetry, and cyclodia mutant snapdragons haveradially symmetrical flowers.

Formation of Angiosperm GametesReproductive success depends on uniting the gametes (eggand sperm) found in the embryo sacs and pollen grains offlowers. As mentioned previously, plant sexual life cyclesare characterized by an alternation of generations, in whicha diploid sporophyte generation gives rise to a haploid ga-metophyte generation. In angiosperms, the gametophytegeneration is very small and is completely enclosed withinthe tissues of the parent sporophyte. The male gameto-phytes, or microgametophytes, are pollen grains. The fe-male gametophyte, or megagametophyte, is the embryosac. Pollen grains and the embryo sac both are produced inseparate, specialized structures of the angiosperm flower.

Like animals, angiosperms have separate structures forproducing male and female gametes (figure 42.8), but thereproductive organs of angiosperms are different fromthose of animals in two ways. First, in angiosperms, bothmale and female structures usually occur together in thesame individual flower (with exceptions noted in chapter38). Second, angiosperm reproductive structures are notpermanent parts of the adult individual. Angiosperm flow-ers and reproductive organs develop seasonally, at times ofthe year most favorable for pollination. In some cases, re-productive structures are produced only once and the par-ent plant dies. It is significant that the germ line for an-giosperms is not set aside early in development, but formsquite late, as detailed in chapter 40.


42.2 Flowering plants use animals or wind to transfer pollen between flowers.

Anther


Meiosis

Microspores (n)

Megaspores (n)

Mitosis

Mitosis

Pollen grains (n)(microgametophyte)

Tube cellnucleus

Generative cell

Ovule

Megasporemother cell (2n) Surviving

megasporeAntipodals

Polarnuclei

Degeneratedmegaspores 8-nucleate embryo sac

(megagametophyte) (n)

Synergids Eggcell

Meiosis

FIGURE 42.8Formation of pollen grains and the embryo sac. Diploid (2n) microspore mother cells are housed in the anther and divide by meiosis toform four haploid (n) microspores. Each microspore develops by mitosis into a pollen grain. The generative cell within the pollen grainwill later divide to form two sperm cells. Within the ovule, one diploid megaspore mother cell divides by meiosis to produce four haploidmegaspores. Usually only one of the megaspores will survive, and the other three will degenerate. The surviving megaspore divides bymitosis to produce an embryo sac with eight nuclei.

Pollen Formation

Pollen grains form in the two pollen sacs located in theanther. Each pollen sac contains specialized chambers inwhich the microspore mother cells are enclosed and pro-tected. The microspore mother cells undergo meiosis toform four haploid microspores. Subsequently, mitotic di-visions form four pollen grains. Inside each pollen grainis a generative cell; this cell will later divide to producetwo sperm cells.

Pollen grain shapes are specialized for specific flowerspecies. As discussed in more detail later in the chapter,fertilization requires that the pollen grain grow a tube thatpenetrates the style until it encounters the ovary. Mostpollen grains have a furrow from which this pollen tubeemerges; some grains have three furrows (figure 42.9).

Embryo Sac Formation

Eggs develop in the ovules of the angiosperm flower.Within each ovule is a megaspore mother cell. Eachmegaspore mother cell undergoes meiosis to producefour haploid megaspores. In most plants, only one ofthese megaspores, however, survives; the rest are ab-sorbed by the ovule. The lone remaining megaspore un-dergoes repeated mitotic divisions to produce eight hap-loid nuclei that are enclosed within a seven-celledembryo sac. Within the embryo sac, the eight nuclei arearranged in precise positions. One nucleus is located nearthe opening of the embryo sac in the egg cell. Two arelocated in a single cell in the middle of the embryo sacand are called polar nuclei; two nuclei are contained incells called synergids that flank the egg cell; and theother three nuclei reside in cells called the antipodals, lo-cated at the end of the sac, opposite the egg cell (figure42.10). The first step in uniting the sperm in the pollengrain with the egg and polar nuclei is to get pollen ger-minating on the stigma of the carpel and growing towardthe embryo sac.

In angiosperms, both male and female structures oftenoccur together in the same individual flower. Thesereproductive structures are not a permanent part of theadult individual and the germ line is not set aside earlyin development.


FIGURE 42.9Pollen grains. (a) In the Easter lily,Lilium candidum, the pollen tubeemerges from the pollen grain throughthe groove or furrow that occurs on oneside of the grain. (b) In a plant of thesunflower family, Hyoseris longiloba, threepores are hidden among theornamentation of the pollen grain. Thepollen tube may grow out through anyone of them.

(a) (b)

2 Antipodals(3rd antipodalnot visible)

2 Polar nuclei

Egg

Synergids

FIGURE 42.10A mature embryo sac of a lily. The eight haploid nucleiproduced by mitotic divisions of the haploid megaspore arelabeled.

PollinationPollination is the process by which pollen isplaced on the stigma. Pollen may be carriedto the flower by wind or by animals, or itmay originate within the individual floweritself. When pollen from a flower’s antherpollinates the same flower’s stigma, theprocess is called self-pollination.

Pollination in Early Seed Plants

Early seed plants were pollinated passively,by the action of the wind. As in present-dayconifers, great quantities of pollen were shedand blown about, occasionally reaching thevicinity of the ovules of the same species. In-dividual plants of any given species mustgrow relatively close to one another for sucha system to operate efficiently. Otherwise, the chance thatany pollen will arrive at the appropriate destination is verysmall. The vast majority of windblown pollen travels lessthan 100 meters. This short distance is significant com-pared with the long distances pollen is routinely carried bycertain insects, birds, and other animals.

Pollination by Animals

The spreading of pollen from plant to plant by pollinatorsvisiting flowers of specific angiosperm species has played animportant role in the evolutionary success of the group. Itnow seems clear that the earliest angiosperms, and perhapstheir ancestors also, were insect-pollinated, and the coevo-lution of insects and plants has been important for bothgroups for over 100 million years. Such interactions havealso been important in bringing about increased floral spe-cialization. As flowers become increasingly specialized, sodo their relationships with particular groups of insects andother animals.

Bees. Among insect-pollinated angiosperms, the mostnumerous groups are those pollinated by bees (figure42.11). Like most insects, bees initially locate sources offood by odor, then orient themselves on the flower orgroup of flowers by its shape, color, and texture. Flowersthat bees characteristically visit are often blue or yellow.Many have stripes or lines of dots that indicate the locationof the nectaries, which often occur within the throats ofspecialized flowers. Some bees collect nectar, which is usedas a source of food for adult bees and occasionally for lar-vae. Most of the approximately 20,000 species of bees visitflowers to obtain pollen. Pollen is used to provide food incells where bee larvae complete their development.

Only a few hundred species of bees are social or semi-social in their nesting habits. These bees live in colonies, asdo the familiar honeybee, Apis mellifera, and the bumble-

bee, Bombus. Such bees produce several generations a yearand must shift their attention to different kinds of flowersas the season progresses. To maintain large colonies, theyalso must use more than one kind of flower as a food sourceat any given time.

Except for these social and semi-social bees and about1000 species that are parasitic in the nests of other bees, thegreat majority of bees—at least 18,000 species—are soli-tary. Solitary bees in temperate regions characteristicallyhave only a single generation in the course of a year. Oftenthey are active as adults for as little as a few weeks a year.

Solitary bees often use the flowers of a given group ofplants almost exclusively as sources of their larval food.The highly constant relationships of such bees with thoseflowers may lead to modifications, over time, in both theflowers and the bees. For example, the time of day whenthe flowers open may correlate with the time when the beesappear; the mouthparts of the bees may become elongatedin relation to tubular flowers; or the bees’ pollen-collectingapparatuses may be adapted to the pollen of the plants thatthey normally visit. When such relationships are estab-lished, they provide both an efficient mechanism of pollina-tion for the flowers and a constant source of food for thebees that “specialize” on them.

Insects Other Than Bees. Among flower-visiting in-sects other than bees, a few groups are especially promi-nent. Flowers such as phlox, which are visited regularly bybutterflies, often have flat “landing platforms” on whichbutterflies perch. They also tend to have long, slender flo-ral tubes filled with nectar that is accessible to the long,coiled proboscis characteristic of Lepidoptera, the order ofinsects that includes butterflies and moths. Flowers likejimsonweed, evening primrose, and others visited regularlyby moths are often white, yellow, or some other pale color;they also tend to be heavily scented, thus serving to makethe flowers easy to locate at night.


FIGURE 42.11Pollination by abumblebee. As thisbumblebee, Bombus,squeezes into thebilaterally symmetrical,advanced flower of amember of the mintfamily, the stigmacontacts its back andpicks up any pollen thatthe bee may haveacquired during a visit toa previous flower.

Birds. Several interesting groups of plants are regularlyvisited and pollinated by birds, especially the humming-birds of North and South America and the sunbirds ofAfrica (figure 42.12). Such plants must produce largeamounts of nectar because if the birds do not find enoughfood to maintain themselves, they will not continue to visitflowers of that plant. Flowers producing large amounts ofnectar have no advantage in being visited by insects becausean insect could obtain its energy requirements at a singleflower and would not cross-pollinate the flower. How arethese different selective forces balanced in flowers that are“specialized” for hummingbirds and sunbirds?

Ultraviolet light is highly visible to insects. Carotenoids,yellow or orange pigments frequently found in plants, areresponsible for the colors of many flowers, such as sunflow-ers and mustard. Carotenoids reflect both in the yellowrange and in the ultraviolet range, the mixture resulting ina distinctive color called “bee’s purple.” Such yellow flow-ers may also be marked in distinctive ways normally invisi-ble to us, but highly visible to bees and other insects (figure42.13). These markings can be in the form of a bull’s-eyeor a landing strip.

Red does not stand out as a distinct color to most in-sects, but it is a very conspicuous color to birds. To mostinsects, the red upper leaves of poinsettias look just like theother leaves of the plant. Consequently, even though theflowers produce abundant supplies of nectar and attracthummingbirds, insects tend to bypass them. Thus, the redcolor both signals to birds the presence of abundant nectarand makes that nectar as inconspicuous as possible to in-sects. Red is also seen again in fruits that are dispersed bybirds.

Other Animals. Other animals including bats and smallrodents may aid in pollination. The signals here also arespecies specific. These animals also assist in dispersing theseeds and fruits that result from pollination. Monkeys areattracted to orange and yellow and will be effective in dis-persing those fruits.

Wind-Pollinated Angiosperms

Many angiosperms, representing a number of differentgroups, are wind-pollinated—a characteristic of early seedplants. Among them are such familiar plants as oaks,birches, cottonwoods, grasses, sedges, and nettles. Theflowers of these plants are small, greenish, and odorless;their corollas are reduced or absent (see figures 42.14 and42.15). Such flowers often are grouped together in fairlylarge numbers and may hang down in tassels that waveabout in the wind and shed pollen freely. Many wind-pollinated plants have stamen- and carpel-containing flow-ers separated among individuals or on a single individual. Ifthe pollen-producing and ovule-bearing flowers are sepa-rated, it is certain that pollen released to the wind willreach a flower other than the one that sheds it, a strategy

that greatly promotes outcrossing. Some wind-pollinatedplants, especially trees and shrubs, flower in the spring, be-fore the development of their leaves can interfere with thewind-borne pollen. Wind-pollinated species do not dependon the presence of a pollinator for species survival.

Bees are the most frequent and characteristicpollinators of flowers. Insects often are attracted by theodors of flowers. Bird-pollinated flowers arecharacteristically odorless and red, with the nectar notreadily accessed by insects.


FIGURE 42.12Hummingbirds and flowers. A long-tailed hermit hummingbirdextracts nectar from the flowers of Heliconia imbricata in theforests of Costa Rica. Note the pollen on the bird’s beak.Hummingbirds of this group obtain nectar primarily from long,curved flowers that more or less match the length and shape oftheir beaks.

FIGURE 42.13How a bee sees a flower. (a) The yellow flower ofLudwigia peruviana (Onagraceae) photographed in normallight and (b) with a filter that selectively transmitsultraviolet light. The outer sections of the petals reflectboth yellow and ultraviolet, a mixture of colors called“bee’s purple”; the inner portions of the petals reflectyellow only and therefore appear dark in the photographthat emphasizes ultraviolet reflection. To a bee, thisflower appears as if it has a conspicuous central bull’s-eye.

(a) (b)

Self-PollinationAll of the modes of pollination that we have consideredthus far tend to lead to outcrossing, which is as highly ad-vantageous for plants as it is for eukaryotic organisms gen-erally. Nevertheless, self-pollination also occurs among an-giosperms, particularly in temperate regions. Most of theself-pollinating plants have small, relatively inconspicuousflowers that shed pollen directly onto the stigma, some-times even before the bud opens. You might logically askwhy there are many self-pollinated plant species if out-crossing is just as important genetically for plants as it is foranimals. There are two basic reasons for the frequent oc-currence of self-pollinated angiosperms:

1. Self-pollination obviously is ecologically advanta-geous under certain circumstances because self-pollinators do not need to be visited by animals toproduce seed. As a result, self-pollinated plants expendless energy in the production of pollinator attractantsand can grow in areas where the kinds of insects orother animals that might visit them are absent or veryscarce—as in the Arctic or at high elevations.

2. In genetic terms, self-pollination produces progeniesthat are more uniform than those that result from out-crossing. Remember that because meiosis is involved,there is still recombination and the offspring will notbe identical to the parent. However, such progeniesmay contain high proportions of individuals well-adapted to particular habitats. Self-pollination in nor-mally outcrossing species tends to produce large num-bers of ill-adapted individuals because it bringstogether deleterious recessive genes; but some of thesecombinations may be highly advantageous in particu-lar habitats. In such habitats, it may be advantageousfor the plant to continue self-pollinating indefinitely.This is the main reason many self-pollinating plantspecies are weeds—not only have humans made weedhabitats uniform, but they have also spread the weedsall over the world.

Factors That Promote Outcrossing

Outcrossing, as we have stressed, is of critical importancefor the adaptation and evolution of all eukaryotic organ-isms. Often flowers contain both stamens and pistils, whichincrease the likelihood of self-pollination. One strategy topromote outcrossing is to separate stamens and pistils.

In various species of flowering plants—for example, wil-lows and some mulberries—staminate and pistillate flowersmay occur on separate plants. Such plants, which produceonly ovules or only pollen, are called dioecious, from theGreek words for “two houses.” Obviously, they cannot self-pollinate and must rely exclusively on outcrossing. In otherkinds of plants, such as oaks, birches, corn (maize), andpumpkins, separate male and female flowers may both beproduced on the same plant. Such plants are called monoe-

cious, meaning “one house” (figure 42.14). In monoeciousplants, the separation of pistillate and staminate flowers,which may mature at different times, greatly enhances theprobability of outcrossing.

Even if, as usually is the case, functional stamens andpistils are both present in each flower of a particular plantspecies, these organs may reach maturity at different times.Plants in which this occurs are called dichogamous. If thestamens mature first, shedding their pollen before the stig-mas are receptive, the flower is effectively staminate at thattime. Once the stamens have finished shedding pollen, thestigma or stigmas may then become receptive, and theflower may become essentially pistillate (figures 42.15 and42.16). This has the same effect as if the flower completelylacked either functional stamens or functional pistils; itsoutcrossing rate is thereby significantly increased.


FIGURE 42.14Staminate andpistillate flowersof a birch, Betula.Birches aremonoecious; theirstaminate flowershang down in long,yellowish tassels,while their pistillateflowers mature intoclusters of small,brownish, conelikestructures.

FIGURE 42.15Wind-pollinatedflowers. The largeyellow anthers,dangling on veryslender filaments, arehanging out, about toshed their pollen to thewind; later, theseflowers will becomepistillate, with long,feathery stigmas—wellsuited for trappingwindblown pollen—sticking far out of them.Many grasses, like thisone, are thereforedichogamous.

Many flowers are constructed suchthat the stamens and stigmas do notcome in contact with each other.With such an arrangement, there is anatural tendency for the pollen to betransferred to the stigma of anotherflower rather than to the stigma of itsown flower, thereby promotingoutcrossing.

Even when a flower’s stamens andstigma mature at the same time, geneticself-incompatibility, which is wide-spread in flowering plants, increasesoutcrossing. Self-incompatibility resultswhen the pollen and stigma recognizeeach other as being genetically relatedand fertilization is blocked (figure42.17). Self-incompatibility is con-trolled by the S (self-incompatibility)locus. There are many alleles at the Slocus that regulate recognition re-sponses between the pollen and stigma.There are two types of self-incompati-bility. Gametophytic self-incompatibil-ity depends on the haploid S locus ofthe pollen and the diploid S locus of the stigma. If either ofthe S alleles in the stigma match the pollen S allele, pollentube growth stops before it reaches the embryo sac. Petuniashave gametophytic self-incompatibility. In the case of sporo-phytic self-incompatibility, such as in broccoli, both S allelesof the pollen parent are important; if the alleles in the stigmamatch with either of the pollen parent S alleles, the haploidpollen will not germinate.

Much is being learned about the cellular basis of thisrecognition and the signal transduction pathways thatblock the successful growth of the pollen tube. Thesepollen recognition mechanisms may have had their ori-gins in a common ancestor of the gymnosperms. Fossilswith pollen tubes from the Carboniferous are consistent

with the hypothesis that they had highly evolved pollen-recognition systems. These may have been systems thatrecognized foreign pollen that predated self-recognitionsystems.

Self-pollinated angiosperms are frequent where there isa strong selective pressure to produce large numbers ofgenetically uniform individuals adapted to specific,relatively uniform habitats. Outcrossing in plants maybe promoted through dioecism, monoecism, self-incompatibility, or the physical separation or differentmaturation times of the stamens and pistils.Outcrossing promotes genetic diversity.


(a) (b)

FIGURE 42.16Dichogamy, as illustrated by the flowers of fireweed, Epilobium angustifolium. Morethan 200 years ago (in the 1790s) fireweed, which is outcrossing, was one of the first plantspecies to have its process of pollination described. First, the anthers shed pollen, and thenthe style elongates above the stamens while the four lobes of the stigma curl back andbecome receptive. Consequently, the flowers are functionally staminate at first, becomingpistillate about two days later. The flowers open progressively up the stem, so that thelowest are visited first. Working up the stem, the bees encounter pollen-shedding,staminate-phase flowers and become covered with pollen, which they then carry to thelower, functionally pistillate flowers of another plant. Shown here are flowers in (a) thestaminate phase and (b) the pistillate phase.

S1S2pollen parent

S1S1

S1

S1

S2

S2 S2

S2

S2S3carpel of

pollen recipient

S1S2pollen parent

S2S3carpel of

pollen recipient

(a) Gametophytic self-incompatibility (b) Sporophytic self-incompatibility

X

X X

FIGURE 42.17Self-pollination can be geneticallycontrolled so self-pollen is not recognized.(a) Gametophytic self-incompatibility isdetermined by the haploid pollen genotype. (b)Sporophytic self-incompatibility recognizes thegenotype of the diploid pollen parent, not justthe haploid pollen genotype. In both cases, therecognition is based on the S locus, which hasmany different alleles. The subscript numbersindicate the S allele genotype. In gametophyticself-incompatibility, the block comes afterpollen tube germination. In sporophytic self-incompatibility, the pollen tube fails togerminate.

FertilizationFertilization in angiosperms is a complex, somewhat un-usual process in which two sperm cells are utilized in aunique process called double fertilization. Double fertil-ization results in two key developments: (1) the fertiliza-tion of the egg, and (2) the formation of a nutrient sub-stance called endosperm that nourishes the embryo. Oncea pollen grain has been spread by wind, by animals, orthrough self-pollination, it adheres to the sticky, sugarysubstance that covers the stigma and begins to grow apollen tube that pierces the style (figure 42.18). Thepollen tube, nourished by the sugary substance, growsuntil it reaches the ovule in the ovary. Meanwhile, thegenerative cell within the pollen grain tube cell divides toform two sperm cells.

The pollen tube eventually reaches the embryo sac inthe ovule. At the entry to the embryo sac, the tip of thepollen tube bursts and releases the two sperm cells. Simul-

taneously, the two nuclei that flank the egg cell disinte-grate, and one of the sperm cells fertilizes the egg cell,forming a zygote. The other sperm cell fuses with the twopolar nuclei located at the center of the embryo sac, form-ing the triploid (3n) primary endosperm nucleus. The pri-mary endosperm nucleus eventually develops into the en-dosperm.

Once fertilization is complete, the embryo develops bydividing numerous times. Meanwhile, protective tissues en-close the embryo, resulting in the formation of the seed.The seed, in turn, is enclosed in another structure calledthe fruit. These typical angiosperm structures evolved inresponse to the need for seeds to be dispersed over longdistances to ensure genetic variability.

In double fertilization, angiosperms utilize two spermcells. One fertilizes the egg, while the other helps forma substance called endosperm that nourishes theembryo.


Generativecell

Tubecell

Stigma

Style

Ovary

Ovule

Carpel

Pollination

Embryosac

Tube cell

Sperm cells

Tube cell nucleus

Growth of

pollen tube

Pollen tube

Double fertilizationRelease of sperm cells

Pollen grain

Zygote(2n)

Antipodals

Polar nuclei

Egg cell

Synergids

Endosperm(3n)

FIGURE 42.18The formation of the pollen tube and double fertilization. When pollen lands on the stigma of a flower, the pollen tube cell growstoward the embryo sac, forming a pollen tube. While the pollen tube is growing, the generative cell divides to form two sperm cells. Whenthe pollen tube reaches the embryo sac, it bursts through one of the synergids and releases the sperm cells. In a process called doublefertilization, one sperm cell nucleus fuses with the egg cell to form the diploid (2n) zygote, and the other sperm cell nucleus fuses with thetwo polar nuclei to form the triploid (3n) endosperm nucleus.

Asexual ReproductionWhile self-pollination reduces genetic variability, asexualreproduction results in genetically identical individuals be-cause only mitotic cell divisions occur. In the absence ofmeiosis, individuals that are highly adapted to a relativelyunchanging environment persist for the same reasons thatself-pollination is favored. Should conditions change dra-matically, there will be less variation in the population fornatural selection to act upon and the species may be lesslikely to survive. Asexual reproduction is also used in agri-culture and horticulture to propagate a particularly desir-able plant whose traits would be altered by sexual repro-duction, even self-pollination. Most roses and potatoes forexample, are vegetatively propagated.

Vegetative Reproduction

In a very common form of asexual reproduction called veg-etative reproduction, new plant individuals are simplycloned from parts of adults (figure 42.19). The forms ofvegetative reproduction in plants are many and varied.

Stolons. Some plants reproduce by means of runners, orstolons—long, slender stems that grow along the surface ofthe soil. In the cultivated strawberry, for example, leaves,flowers, and roots are produced at every other node on therunner. Just beyond each second node, the tip of the run-ner turns up and becomes thickened. This thickened por-tion first produces adventitious roots and then a new shootthat continues the runner.

Rhizomes. Underground stems, or rhizomes, are alsoimportant reproductive structures, particularly in grassesand sedges. Rhizomes invade areas near the parent plant,and each node can give rise to a new flowering shoot. Thenoxious character of many weeds results from this type ofgrowth pattern, and many garden plants, such as irises, arepropagated almost entirely from rhizomes. Corms, bulbs,and tubers are rhizomes specialized for storage and repro-duction. White potatoes are propagated artificially fromtuber segments, each with one or more “eyes.” The eyes, or“seed pieces,” of potato give rise to the new plant.

Suckers. The roots of some plants—for example, cherry,apple, raspberry, and blackberry—produce “suckers,” orsprouts, which give rise to new plants. Commercial vari-eties of banana do not produce seeds and are propagated bysuckers that develop from buds on underground stems.When the root of a dandelion is broken, as it may be if oneattempts to pull it from the ground, each root fragmentmay give rise to a new plant.

Adventitious Leaves. In a few species, even the leavesare reproductive. One example is the house plant Kalanchoë

daigremontiana, familiar to many people as the “maternityplant,” or “mother of thousands.” The common names ofthis plant are based on the fact that numerous plantletsarise from meristematic tissue located in notches along theleaves. The maternity plant is ordinarily propagated bymeans of these small plants, which, when they mature, dropto the soil and take root.

Apomixis

In certain plants, including some citruses, certain grasses(such as Kentucky bluegrass), and dandelions, the em-bryos in the seeds may be produced asexually from theparent plant. This kind of asexual reproduction is knownas apomixis. The seeds produced in this way give rise toindividuals that are genetically identical to their parents.Thus, although these plants reproduce asexually bycloning diploid cells in the ovule, they also gain the ad-vantage of seed dispersal, an adaptation usually associatedwith sexual reproduction. As you will see in chapter 43,embryos can also form via mitosis when plant tissues arecultured. In general, vegetative reproduction, apomixis,and other forms of asexual reproduction promote theexact reproduction of individuals that are particularlywell suited to a certain environment or habitat. Asexualreproduction among plants is far more common in harshor marginal environments, where there is little marginfor variation. There is a greater proportion of asexualplants in the arctic, for example, than in temperateregions.

Plants that reproduce asexually clone new individualsfrom portions of the root, stem, leaves, or ovules ofadult individuals. The asexually produced progeny aregenetically identical to the parent individual.


42.3 Many plants can clone themselves by asexual reproduction.

FIGURE 42.19.Vegetative reproduction. Small plants arise from notches alongthe leaves of the house plant Kalanchoë daigremontiana.

The Life Span of PlantsPlant Life Spans Vary Greatly

Once established, plants live for highlyvariable periods of time, depending onthe species. Life span may or may notcorrelate with reproductive strategy.Woody plants, which have extensivesecondary growth, nearly always livelonger than herbaceous plants, whichhave limited or no secondary growth.Bristlecone pine, for example, can liveupward of 4000 years. Some herba-ceous plants send new stems above theground every year, producing themfrom woody underground structures.Others germinate and grow, floweringjust once before they die. Shorter-livedplants rarely become very woody be-cause there is not enough time for theaccumulation of secondary tissues. De-pending on the length of their life cy-cles, herbaceous plants may be annual, biennial, orperennial, while woody plants are generally perennial(figure 42.20). Determining life span is even more com-plicated for clonally reproducing organisms. Aspen treesform huge clones from asexual reproduction of theirroots. Collectively, an aspen clone may form the largest“organism” on earth. Other asexually reproducing plantsmay cover less territory but live for thousands of years.Creosote bushes in the Mojave Desert have been identi-fied that are up to 12,000 years old!

Annual Plants

Annual plants grow, flower, and form fruits and seedswithin one growing season; they then die when theprocess is complete. Many crop plants are annuals, includ-ing corn, wheat, and soybeans. Annuals generally growrapidly under favorable conditions and in proportion tothe availability of water or nutrients. The lateral mer-istems of some annuals, like sunflowers or giant ragweed,do produce poorly developed secondary tissues, but mostare entirely herbaceous. Annuals typically die after flower-ing once, the developing flowers or embryos using hor-monal signaling to reallocate nutrients so the parent plantliterally starves to death. This can be demonstrated bycomparing a population of bean plants where the beansare continually picked with a population where the beansare left on the plant. The frequently picked populationwill continue to grow and yield beans much longer thanthe untouched population. The process that leads to thedeath of a plant is called senescence.

Biennial Plants

Biennial plants, which are much less common than an-nuals, have life cycles that take two years to complete.During the first year, biennials store photosynthate inunderground storage organs. During the second year ofgrowth, flowering stems are produced using energystored in the underground parts of the plant. Certaincrop plants, including carrots, cabbage, and beets, are bi-ennials, but these plants generally are harvested for foodduring their first season, before they flower. They aregrown for their leaves or roots, not for their fruits orseeds. Wild biennials include evening primroses, QueenAnne’s lace, and mullein. Many plants that are consideredbiennials actually do not flower until they are three ormore years of age, but all biennial plants flower onlyonce before they die.

Perennial Plants

Perennial plants continue to grow year after year andmay be herbaceous, as are many woodland, wetland, andprairie wildflowers, or woody, as are trees and shrubs.The majority of vascular plant species are perennials.Herbaceous perennials rarely experience any secondarygrowth in their stems; the stems die each year after a pe-riod of relatively rapid growth and food accumulation.Food is often stored in the plants’ roots or undergroundstems, which can become quite large in comparison totheir less substantial aboveground counterparts.


42.4 How long do plants and plant organs live?

FIGURE 42.20Annual and perennial plants. Plants livefor very different lengths of time. (a)Desert annuals complete their entire lifespan in a few weeks. (b) Some trees, suchas the giant redwood (Sequoiadendrongiganteum), which occurs in scatteredgroves along the western slopes of theSierra Nevada in California, live 2000years or more.

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Chapter 42: Plant Reproduction