Chapter 24: Reproduction in Plants - Polson Schools...

652 REPRODUCTION IN PLANTS

Reproduction in Plants

What You’ll Learn ■ You will compare and con-

trast the life cycles of mosses,ferns, and conifers.

■ You will sequence the lifecycle of a flowering plant.

■ You will describe the charac-teristics of flowers, seeds, andfruits.

Why It’s Important Plants are essential to Earth’sbiosphere. The fruits and seedsproduced by flowering plantsare a major food source forhumans and animals, and criticalfor the survival of many species.

Carefully look at the differentlife cycle figures that appear inthe chapter. Pick one andsketch it in your notebook. Adddefinitions of any new vocabu-lary words that are used in thediagrams.

To find out more about plants,visit the Glencoe Science Web site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

24ChapterChapter

Animals often play animportant role in polli-nating flowering plants.Insects, including bees,transport pollen fromflower to flower. Mostnonflowering plants,such as mosses, rely onwind or water for the dis-persal of spores.

BIOLOGY

http://science.glencoe.com

Section

Alternation ofGenerations

As you learned earlier, plants gothrough an alternation of generationsduring their life cycles. Rememberthat the two phases of the plant lifecycle are the gametophyte stage andthe sporophyte stage.

The cells of the sporophyte are alldiploid. Certain cells of the sporo-phyte undergo meiosis and producehaploid spores. These spores grow,by mitotic division, into the gameto-phyte. The multicellular gameto-phyte that is formed is composed ofhaploid cells. Some cells of the game-tophyte will differentiate and formhaploid gametes. The female gameteis the egg, and the male gamete is thesperm. When a sperm fertilizes anegg, a diploid zygote is formed. Thiszygote divides by mitosis, producinga tiny sporophyte or embryo. Thedevelopment of the embryo into a

mature sporophyte allows the lifecycle to begin again. Figure 24.1illustrates alternation of generations.

653

You may have seen the fine yellow dustthat covers everything when pine treesrelease their pollen. As annoying as

this pollen may seem, it has a valuable func-tion. It is an important stage in the life cycleof pine trees. Other plants have even moredramatic stages of their life cycles, such asexploding moss capsules and fern sporangia.

SECTION PREVIEW

ObjectivesReview the steps ofalternation of genera-tion.Describe the life cyclesof mosses, ferns, andconifers.

Vocabularyvegetative reproductionprotonemamegasporemicrosporemicropyle

24.1 Life Cycles of Mosses,Ferns, and Conifers

Male pine conereleasing pollen

Figure 24.1 All plants exhibit an alternation of generations.The gametophyte (n) stage produces gametes.The sporophyte (2n) produces spores.

GAMETOPHYTE (n)

Spores (n)

Meiosis

SPOROPHYTE (2n)

Mitosis

Fertilization

Femalegamete (n)

Malegamete (n)

Growing Plants Asexually Plants are capable of reproduc-ing asexually. Reproductive cells such as egg or sperm are notneeded in asexual reproduction. Plants are able to use structuressuch as roots, stems, and even leaves to produce new offspring.

Procedure! Prepare three different plant parts for study using diagrams

A, B, and C as a guide. @ Observe any changes that occur to your plants over the

next two weeks.# Design a data table that will provide enough room for

diagrams of your observations, the number of days since the start of the experiment should be included.

$ Make your initial diagrams of the plant parts today and label these diagrams as “Day 1.”

% Observations should be made every three days. Replace any lost water as needed.

Analysis 1. What experimental evidence do you have that:

a. plants use a variety of structures for asexual reproduc-tion?

b. asexual reproduction is a rapid process?c. asexual reproduction requires only one parent?

2. Describe several advantages of asexual reproduction in plants.

MiniLab 24-1MiniLab 24-1 Experimenting

even seen a female gametophyte of aflowering plant. Botanists usually referto the bigger, more obvious plant asthe dominant generation. The domi-nant generation lives longer and cansurvive independently of the othergeneration. In most plant species thesporophyte is the dominant plant.

Asexual reproductionMost plants can also reproduce by

a process called vegetative reproduc-tion. Vegetative reproduction isasexual reproduction in plants wherea new plant is produced from anexisting vegetative structure. Forinstance, liverworts produce asexualstructures called gemmae that falloff and develop into new plants,Figure 24.2. The new plants havethe same genetic make-up as theoriginal plant, as if they werecloned. You can learn more aboutasexual reproduction in the MiniLabshown here.

Life Cycle of MossesMosses belong to one of the few

plant divisions in which the gameto-phyte plant is the dominant genera-tion. A haploid spore germinates to


Figure 24.2Small cups filled with tiny gemmae haveformed on the thallus of this liverwort.

Water

Beaker

Potato with eye(stem & buds)

Toothpick

Carrot(root)Water

Water

Test tube

Garlic clove(storage leaves)

AA BB CC

The basic pattern of this life cycleis the same for all plants. However,there are many variations on this pat-tern within the plant kingdom. Forinstance, recall that in mosses thegametophyte is bigger than the sporo-phyte. In others, such as floweringplants, the gametophyte is tiny, evenmicroscopic. Most people have never

Meiosis

Mitosis

Fertilization

Sporophyte (2n)

Developingsporophyte

Gametophyte (n)

Capsule

Germinatingspores

Sperm

Egg

Zygote (2n)

ProtonemaFemalegametophyte(n)

Malegametophyte(n)

Open capsule

ArchegoniumAntheridium

SPOROPHYTEGENERATION

2n

GAMETOPHYTEGENERATION

n

Rhizoids

form a structure called a protonema.The protonema (proht uh NEE muh)is a small green filament of cells thatdevelops into either a male or afemale gametophyte. In somemosses, the gametophyte can pro-duce both kinds of reproductivestructures. Remember that thearchegonium is the female reproduc-tive structure in which eggs are pro-duced and that sperm are producedin the antheridium.

The motile sperm are released fromthe antheridium and swim through acontinuous film of rainwater or dew tothe archegonium. The sperm fertilizesthe egg inside the archegonium, form-

ing a diploid zygote. The zygotedivides by mitosis to form a newsporophyte. The sporophyte is a stalkwith a capsule at the top. It grows outof the archegonium and remainsattached to the gametophyte. Thesporophyte receives much of its nutri-tion from the gametophyte. Meioticdivision within the capsule produceshaploid spores.

The capsule ripens, bursts, andreleases the spores, which can be car-ried great distances by air currents. Ifthe spore lands in a favorable envi-ronment, it germinates, completingthe life cycle. Review the moss lifecycle as you examine Figure 24.3.

24.1 LIFE CYCLE OF MOSSES, FERNS, AND CONIFERS 655

Figure 24.3 The leafy green gametophyte of amoss produces gametes that fuseto form a zygote. The zygotedevelops into the sporophyte. The sporophyte produces spores. The spores germi-nate and grow into a gametophyte, com-pleting the life cycle of a moss.

Figure 24.4Fern sporophytes are easily seen by a hikerwalking through a forest. However, only thevery observant person would be able to finda fern gametophyte.

Most fern sporophytesgrow 25 cm or taller.

AA The heart-shapedfern gametophyteis usually less thana centimeter across.

CC

The clusters of sporangia on theunderside of a fern frond arecalled sori. Each sporangium con-tains spores that are released,sometimes in dramatic fashion.

BB

Some mosses also reproduce asex-ually by vegetative reproduction.They can break up into pieces whenthe plant is dry and brittle. With thearrival of wetter conditions, thesepieces each become a whole plant.

Life Cycle of FernsUnlike mosses, the dominant stage

of the fern life cycle is the sporo-phyte plant. The fern sporophytesinclude the familiar fronds you see inFigure 24.4. The fronds of the ferngrow from the rhizome, which is theunderground stem. On the undersideof some fronds are the sori, which areclusters of sporangia. Meiotic divi-

sion within the sporangia producesthe spores. When environmentalconditions are right, the sporangiaburst to release haploid spores.

A spore germinates to form aheart-shaped gametophyte called aprothallus, as shown in Figure 24.5.The prothallus produces botharchegonia and antheridia on its sur-face. The flagellated sperm releasedby the antheridium swim through afilm of water to the archegoniumwhere the egg is fertilized. Thediploid zygote that is the product ofthis fertilization develops into thesporophyte. Initially, this developingsporophyte depends upon the game-tophyte for its nutrition. However,


Meiosis

Meiosis

Spores (n)

Rhizome

Fronds

Sporophytes(2n)

Sporangium

Pinna

Sorus

Roots


2n


2n

Fertilization

Mitosis

Sperm

Rhizoids

Archegonia

Archegonium

Egg

Antheridia

Antheridium

Zygote

once the sporophyte produces itsgreen fronds, it can carry on photo-synthesis and survive on its own. Theprothallus disintegrates as the sporo-phyte matures, producing a strongrhizome that can support the fronds.If pieces of rhizome break away, new

fern plants will develop from them byvegetative reproduction. New spo-rangia develop on the pinnae of thefronds, spores will be released, andthe cycle will begin again. The lifecycle of the fern is summarized inFigure 24.5.

24.1 LIFE CYCLE OF MOSSES, FERNS, AND CONIFERS 657

Figure 24.5In the life cycle of afern, the sporophytegeneration becomesindependent of thegametophyte.

The Life Cycle of Conifers

The dominant stage in conifers isthe sporophyte generation. One ofthe more familiar conifer sporo-phytes is shown in Figure 24.6. Theadult conifer produces male andfemale cones on separate branches ofthe tree. The cones contain spore-producing structures, or sporangia,on their scales. The female cones,which are larger than the male cones,develop two ovules on the upper sur-face of each cone scale. Each ovulecontains a sporangium with a diploidcell that produces, by meiosis, fourmegaspores. A megaspore is afemale spore that eventually becomesthe female gametophyte. One of thefour megaspores will survive andgrow by mitotic cell divisions into

the female gametophyte. The femalegametophyte consists of hundreds ofcells but is still dependent on thesporophyte for protection and nutri-tion. Within the female gametophyteare two or more archegonia, eachcontaining an egg. The male coneshave sporangia that undergo meiosisto produce male spores calledmicrospores. Each microspore willdevelop into a male gametophyte, orpollen grain. Each pollen grain, withits hard, water-resistant outer cover-ing, is a male gametophyte. Look atFigure 24.6 to see examples of maleand female conifer gametophytes.

In conifers, pollination is thetransfer of the pollen grain from themale cone to the female cone.Pollination occurs when a wind-borne pollen grain falls near theopening in one of the ovules of the


Figure 24.6In conifers, the sporo-phyte is immensecompared with themicroscopic gameto-phytes.

OriginWORDWORD

micropyleFrom the Greekwords mikros,meaning “small,”and pyle, meaning“gate.” The micro-pyle is the smallopening at one endof the embryo sac.

This pine sporophytecan grow more than25 meters tall.

AA

The femalegametophytein this pineovule is lessthan 0.01 mmlong.

BB

A pollen grainis so small itcan be carriedby the wind.

CC

What traits do mosses, ferns, and conifers share?Sometimes it helps to organize information in a table. Theadvantage of a table is that it summarizes traits and showssimilarities and differences in a simple format.

AnalysisCopy the following data table. Complete the table using

“yes” and “no” answers.

Thinking Critically1. Which two plant groups share the most characteristics?

Which two share the fewest?2. While on a woodland trail, would you easily observe:

a. A pine gametophyte? Sporophyte? Explain.b. A fern gametophyte? Sporophyte? Explain.

3. Using information from your table, summarize the repro-ductive similarities and differences among mosses, ferns,and conifers.

Problem-Solving Lab 24-1Problem-Solving Lab 24-1 Making and Using Tables

female cone. The pollen grainadheres to a sticky drop of fluid thatcovers the opening of the ovule. Asthe fluid evaporates, the pollen grainis drawn closer to an opening of theovule called the micropyle (Mi kruhpile). Although pollination hasoccurred, fertilization will not takeplace for at least a year. The pollengrain and the female gametophytewill mature during this time.

As the pollen grain matures, it pro-duces a pollen tube that grows throughthe micropyle and into the ovule. Asperm cell from the male gametophyteis transported by the pollen tube tothe egg, where fertilization occurs.The zygote, which is nourished bythe female gametophyte, developsinside the ovule into an embryo withseveral cotyledons. The cotyledonsnourish the developing sporophyte.The ovule provides the seed coat asthe mature seed is produced.

The seed is released when thefemale cone opens. When conditionsare favorable, the seed germinatesinto a new, young sporophyte—apine tree seedling, Figure 24.7. Seeif you can identify the stages of thelife cycle in Figure 24.8. Use theProblem-Solving Lab on this page tofurther explore the life cycles ofmosses, ferns, and conifers.

659

Figure 24.7Conifer seeds germinateinto new, young sporo-phytes such as the pinetree seedling shown here.

Trait ConiferFernMoss

Has alteration of generations

Film of water needed for fertilization

Dominant gametophyte

Dominant sporophyte

Sporophyte is photosynthetic

Produces seeds

Produces sperm

Produces pollen grains

Produces eggs

Data Table

Male cone

Megaspores

Ovule Microspores

Microspore mother cells

Female cone

Pollengrain

Pollen grain

Femalegametophyte

Egg

Two archegoniawith egg cells

Sperm nucleus

Seed

Cotyledons

Youngseedling

Adultsporophyte

Seed coat

Embryo

Storedfood

Malegametophyte

Germinating pollen

One egg is fertilized


2n


n

Fertilization

Meiosis


Figure 24.8The life cycle of a coniferincludes the productionof two types of spores bythe sporophyte. Thesespores develop into the male and female gametophytes.

Section AssessmentSection AssessmentUnderstanding Main Ideas1. Explain how vegetative reproduction can pro-

duce a new plant. Provide an example.2. In what way is the sporophyte generation of a

moss dependent on the gametophyte generation?3. Describe the formation of the male gametophyte

in a conifer.4. What are two differences between the life cycle

of a fern and that of a conifer?

Thinking Critically5. Why is the term alternation of generations

appropriate to describe the life cycle of a plant?

6. Sequencing Sequence the events in the life of afern, beginning with the prothallus. For morehelp, refer to Organizing Information in the SkillHandbook.

SKILL REVIEWSKILL REVIEW

Section

24.2 FLOWERS AND FLOWERING 661

What Is a Flower?The process of sexual reproduction

in flowering plants takes place in theflower, which is a complex structuremade up of several parts. Some partsof the flower are directly involved infertilization and seed production.Other floral parts have functions inpollination. There are probably asmany different shapes, sizes, colors,and configurations of flower parts asthere are species of flowering plants.In fact, features of the flower areoften used in plant identification.

The structure of a flowerEven though there is an almost

limitless variation in flower shapesand colors, all flowers share a simple,basic structure. A flower is usuallymade up of four kinds of organs:sepals, petals, stamens, and pistils.The flower parts you are probablymost familiar with are the petals.Petals are leaflike, usually colorfulstructures arranged in a circle aroundthe top of a flower stem. Sepals arealso leaflike, usually green, and encir-cle the flower stem beneath thepetals.

How would you choose flowersfor a garden or a bouquet?Perhaps you would start with

fragrant roses, jasmine, or gardenias.You might add color with tall spikes ofgladioli, cushions of marigolds, brightdaisies, or irises. Grasses would con-tribute a graceful shape, though theirflowers may be so small they are easyto overlook. All of these flowers are beautiful to look at and some have delicate scents as well. In what other ways are all of these flowers alike?

SECTION PREVIEW

ObjectivesIdentify the structuresof a flower.Examine the influenceof photoperiodism onflowering

Vocabularypetalssepalsstamenantherpistilovaryphotoperiodismshort-day plantlong-day plantday-neutral plant

24.2 Flowers and Flowering

Flowers display a varietyof shapes and colors.

Inside the circle of petals are thestamens. A stamen is the male repro-ductive structure of a flower. At thetip of the stamen is the anther. Theanther produces pollen that containssperm.

At the center of the flower,attached to the top of the flowerstem, lie one or more pistils. Thepistil is the female structure of theflower. The bottom portion of thepistil enlarges to form the ovary, astructure with one or more ovules,each containing one egg. As you readin the previous section, the femalegametophyte develops inside theovule. You can learn more about

floral structure and practice your labskills in the BioLab at the end of thischapter.

Modifications in flower structureA flower that has all four organs—

sepals, petals, stamens, and pistils—iscalled a complete flower. The morn-ing glory and tiger lily shown inFigure 24.9 are examples of com-plete flowers. A flower that lacks oneor more organs is called an incom-plete flower. For example, squashplants have separate male and femaleflowers. The male flowers have sta-mens but no pistils; the female flow-ers bear pistils but no stamens.


Figure 24.9The diversity offlower forms is evi-dence of the successof flowering plants.

The male flowers of the walnuttree form long catkins.

BB

The petals of the morningglory are fused together toform a bell shape.

CC

Thistles bearclusters of tiny,tubular flowerswithin a massof spiny bracts.

DD

The location of corn tasselsat the top of the plant aidsin wind pollination.

EE

The spotted petals ofthe tiger lily curl awayfrom the reproductivestructures at the cen-ter of the flower.

AA

How do flowers differ? There is considerable variation inflower shape. This variation occurs when certain flower partsare fused together, parts are rearranged, or when parts maybe totally missing. However, with all the variation seen inflower shape, there are certain general patterns. Almost alldicot plants will have flower parts that are in fours or fives ormultiples of these numbers. For example, a plant havingeight or ten petals, sepals, and stamens would be a dicot.Almost all monocot plants have flower parts in threes or mul-tiples of three.

AnalysisDiagram A shows a flower with all of its parts labeled.

Imagine that the flower has been cut along the dashed line.Diagram B is a diagrammatic cross-section view seen whenlooking down onto the cut edge of the bottom half. DiagramB is shown a little larger than A so that any details can bemore clearly seen. The flower is from a dicot plant becausethere are five sepals, petals, and stamens.

Thinking Critically1. Diagrams C, D, and E are diagrammatic cross-section views

of flowers. Determine if diagram:a. C is a monocot or dicot. Explain.b. D is a monocot or dicot. Explain.c. E is a monocot or dicot. Explain.

2. Do diagrams A, C, D, and E show flowers that are com-plete or incomplete? Explain. Note: Complete flowershave sepals, petals, stamens, and pistils whereas incom-plete flowers lack one or more of these parts.

3. Which flowers are capable of self-pollination? Explain.4. Which flowers require cross-pollination? Explain.

Problem-Solving Lab 24-2Problem-Solving Lab 24-2 Interpreting ScientificIllustrations

Plants such as sweet corn that areadapted for pollination by windrather than animal pollinators haveno petals. Figure 24.9 shows someexamples of the variety in flowerforms. Study the structure of a typi-cal flower in the Inside Story. You canexplore flower adaptations further inthe Problem-Solving Lab shown here.

There is an amazing amount ofdiversity in the structures of antho-phyte flowers, seeds, fruits, and vege-tative structures. Anthophytes aredivided into different divisions andclasses based on these differences.The relationships among the differ-ent classes and divisions of antho-phytes are shown in Figure 24.10.See if you can recognize any of thedifferent divisions of plants.

PhotoperiodismThe relative length of day and

night has a significant effect on therate of growth and the timing offlower production in many species offlowering plants. For example,chrysanthemums produce flowersonly during the fall, when the daysare getting shorter and the nightslonger. A grower who wants to pro-duce chrysanthemum flowers in themiddle of summer drapes black clothover the plants to artificially increasethe length of night. The response offlowering plants to the difference inthe duration of light and dark periodsin a day is called photoperiodism.

Plant biologists originally thoughtthat day length controlled flowering.However, they now know that it is thelength of the night, or dark period,that controls flowering. Plants can beplaced in three categories dependingon the conditions they require forflower production. Plants are eithershort-day plants, long-day plants, orday-neutral plants.


AA BBPistil

Stamen

Petal

Sepal

AA BBPistil

Stamen

Petal

Sepal

CC DD EE


OrchidOrchidaceae

GrassPoaceae

LilyLiliaceae

PalmPalmae

RaspberryRosaceae

ColeusLamiaceae

CarawayApiaceae

ChicoryAsteraceae

CocoaSterculiaceae

CactusCactaceae

MagnoliaMagnoliaceae

OakFagaceae

MilkweedAsclepiadaceae

Dicots

Protists

Monocots

PRESENT CENOZOIC PALEOZOIC PRECAMBRIANMESOZOIC

Figure 24.10There are two classes of antho-phytes—monocots and dicots. Withineach class there are many differentfamilies, which show a great amountof variation in their vegetative andreproductive structures.

PLANTS


Parts of a Flower

Of the four major organs of a flower, only two—the stamens and pistils—are fertile structures directly

involved in seed development. Sepals and petals supportand protect the fertile structures and help attract pollina-tors. The structure of a typical flower is illustrated here bya phlox flower.

Critical Thinking How are different flower shapesimportant to a plant’s survival?

Blue phlox

INSIDESSTORTORYY

INSIDE

Petals These are usually brightly coloredand often have perfume or nectar at their bases to attract pollinators. In many flowers, the petal also provides a surface for insect pollinators to rest on while feeding. Petals may be fused to form a tube, or shaped in ways that make the flower more attractive to pollinators.

11

Stigma At the top of thepistil is this sticky or featherysurface on which pollengrains land and grow. Thestyle is the slender stalk ofthe pistil that connects thestigma to the ovary. Thepollen tube grows down thelength of the style to reachthe ovary. The ovary, whichwill eventually become thefruit, contains the ovules.Each ovule, if fertilized, willbecome a seed.

22

Stamen The pollen-producinganther at the tip and a thin fila-ment that attaches the antherto the flower stem make up thestamen. When the pollen grainsdeveloping inside the antherreach maturity, the anther splitsopen to release them.

44Sepals A ring of sepals makes up the outermost portion of the flower. The sepals serve as a protective coveringfor the flower bud, helping to protect itfrom insect damage and prevent it fromdrying out. Sepals sometimes are colored and resemble petals.

33

Ovary

Style

Pistil

Ovule

Stigma

Stamen

Filament

Corolla

Sepals

Calyx

Petals

Anther

Pollengrains

Short-day plants are induced toflower by exposure to a long night.These are plants that usually formflower buds in the fall when the daysare getting shorter and the nights arelong, as shown in Figure 24.11A.Flowering occurs in the spring, as incrocuses, or in the fall as in strawberryplants. Long-day plants flower whendays are longer than the nights, asshown in Figure 24.11B. Examples ofthese are carnations, petunias, potatoes,

and garden peas. Most plant speciesare day-neutral plants, which meanstemperature, moisture or environ-mental factors other than day lengthcontrol their flowering times, asshown in Figure 24.11C. The pho-toperiodism of flowers may ensurethat a plant produces its flowers at atime when there is an abundant popu-lation of pollinators. This is importantbecause pollination is a critical eventin the life cycle of a flowering plant.


Section AssessmentSection AssessmentUnderstanding Main Ideas 1. Compare and contrast sepals and petals.2. Describe the male and female parts of a flower.3. Explain why squash flowers are considered

incomplete flowers.4. How does photoperiodism influence flowering?

Thinking Critically5. In the middle of the summer a florist receives a

large shipment of short-day plants. What mustthe florist do to induce flowering?

6. Comparing and Contrasting Explain why thestructure of wind-pollinated flowers is often dif-ferent from that of insect pollinated flowers. Formore help, refer to Organizing Information inthe Skill Handbook.


Figure 24.11Photoperiodism refers to aplant’s sensitivity to thechanging length of night.

OriginWORDWORD

photoperiodismFrom the Greekwords photos, mean-ing “light,” andperiodos, meaning “a period.” Theflowering responseof a plant to periodsof dark and light isphotoperiodism.

Short-day plants, such as pansies(above) and goldenrod, flower inlate summer and fall or early spring.

AA

Spinach and lettuce(above) are long-day plants thatflower in midsum-mer.

BB

Most plants are day-neutral.Flowering in cucumbers (above),tomatoes, and corn is not influ-enced by a dark period.

CC

Section

The Life Cycle of an Anthophyte

The life cycle of flowering plantsis similar to that of conifers in manyways. In both coniferophytes andanthophytes, the gametophyte gen-eration is contained within thesporophyte. Many of the reproduc-tive structures are also similar.However, anthophytes are the onlyplants that produce flowers andfruits. Figure 24.12 summarizes thelife cycle of flowering plants.

Development of the female gametophyte

In anthophytes, the female game-tophyte is formed in the ovule within

the ovary. In the ovule, a cell under-goes meiosis, producing haploidmegaspores. One of these mega-spores will produce the female game-tophyte. The other three spores die. In most flowering plants, the megaspore divides by mitosis threetimes, producing eight nuclei. Theseeight nuclei are the embryo sac orfemale gametophyte. Six of the nucleiare contained within six haploid cells,one of which is the egg cell. The tworemaining nuclei, which are calledpolar nuclei, are both in one cell.This cell, the central cell, is locatedat the center of the embryo sac. Theegg cell is near the micropyle. Theother five cells are arranged as shownin Figure 24.13.

24.3 THE LIFE CYCLE OF A FLOWERING PLANT 667

Transferring pollen from anther tostigma is just one step in the life cycleof a flowering plant. How does polli-

nation lead to the development of seeds encasedin fruit? How do sperm cells in the pollengrain reach the egg cells in the ovary? Thesesteps in the reproductive cycle of anthophytestake place without water—anevolutionary step thatenabled flowering plants tooccupy nearly every envi-ronment on Earth.

SECTION PREVIEW

ObjectivesDescribe the life cycleof a flowering plant.Outline the processesof seed and fruit formation and seed germination.

Vocabularypolar nucleidouble fertilizationendospermdormancygerminationradiclehypocotyl

24.3 The Life Cycle of aFlowering Plant

Bee orchid Ophrysspeculum and ovary of a flower (inset)


n


2n

Fertilization

Meiosis

Flowers

Youngseedling

Germinatingseed

Seed

Fruit

Pollen tube

Adultsporophyteplant

Zygote

Endospermnucleus

Pollentube

Doublefertilization

Egg

Tubenucleus

Sperm

Malegametophyte

Microsporesin fours

Ovule withmegasporemother cell

Anther Pollen sacwith microsporemother cellsOvary

Fourmegaspores

Female gametophytewith four nuclei

CODE668 REPRODUCTION IN PLANTS

Figure 24.12In the life cycle of a flowering plant, the sporophyte gener-ation nourishes and protects the developing gametophyte.After fertilization, the new sporophyte, which is containedin a seed or fruit, is released from the parent plant.

Female gametophyte

Ovule

Embryo sac

Central cell

Egg cell

Micropyle

Development of the male gametophyte

The formation of the male gameto-phyte begins in the anther, as seen inFigure 24.14. Haploid microsporesare produced by meiosis within theanther. The microspores each divideinto two cells. A thick, protective wallsurrounds these two cells. This two-celled structure is the immature malegametophyte, or pollen grain. The

cells within the pollen grain are thetube cell and the generative cell.When the pollen grains are maturethe anther splits open. Depending onthe type of flower, the pollen may becarried to the pistil by wind, water, oranimals.

PollinationIn anthophytes, pollination is the

transfer of the pollen grain from the


Figure 24.13 The eight nuclei produced by themegaspore form the femalegametophyte of anthophytes.

MEIOSIS

Microsporemother cell (2n)

Tubenucleus

Generativecell

Pollen grain(male gametophyte)

Microspores(n)

ANTHER

Pollen sac

Figure 24.14 Meiotic division ofeach of many cellswithin the anther produces fourmicrospores. Thesemicrospores developinto the male gameto-phyte or pollen grain

anther to the pistil. Plant reproduc-tion is most successful when the pol-lination rate is high, which meansthat the pistil of a flower receivesenough pollen of its own species tofertilize the egg in each ovule. Manyanthophytes have elaborate mecha-nisms that help ensure that pollengrains are deposited in the right placeat the right time. Some of these areshown in Figure 24.15. Although itmay seem wasteful for wind-polli-nated plants to produce such largeamounts of pollen, it does help

ensure pollination. Most anthophytespollinated by animals produce nectar,which serves as a valuable, highlyconcentrated food for visitors to theflowers. Nectar is a liquid made up ofproteins and sugars. It usually col-lects in the cuplike area at the base ofthe petals. Animals such as insectsand birds brush up against theanthers while trying to get to thenectar. The pollen that attaches tothem can be carried to anotherflower, resulting in pollination. Someinsects also gather pollen to use as


Figure 24.15The shape, color, and size ofa flower reflect its relation-ship with a pollinator.

The butterfly uses itslong proboscis to sipnectar that bees andflies cannot reach.

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Flowers polli-nated by hum-mingbirds areoften tubular andcolored bright redor yellow but mayhave little scent.

DD

Bats sip nectarfrom night-blooming flow-ers with astrong, mustyodor, such asbananas andsome cacti.

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The ultraviolet markings ofsome flowers guide insectsto a flower’s nectar.

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The wind-pollinated flowersof this ragweed plant aresmall and green and lackstructures that would blockwind currents.

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food. By producing nectar andattracting animal pollinators, animal-pollinated plants are able to promotepollination without producing largeamounts of pollen.

Some nectar-feeding pollinators areattracted to a flower by its color orscent or both. Some of the bright,vivid flowers attract pollinators such asbutterflies and bees. Some of theseflowers have markings that are invisi-ble to the human eye but are easilyseen by insects. Flowers that are polli-nated by beetles and flies have a strongscent but are often dull in color.

Many flowers have structural adap-tations that favor cross-pollination.This results in greater genetic varia-tion because the sperm from oneplant fertilizes the egg from another.For example, the flowers of certainspecies of orchids resemble femalewasps. The male wasps visit theflower and attempt to mate with itand become covered with pollen,which is deposited on orchids it mayvisit in the future.

FertilizationOnce a pollen grain has reached

the stigma of the pistil, several eventstake place before fertilization occurs.Inside each pollen grain are two hap-loid cells, the tube cell and the gener-ative cell. The tube cell nucleusdirects the growth of the pollen tubedown through the pistil to the ovary,as shown in Figure 24.16. The gen-erative cell divides by mitosis, pro-ducing two haploid sperm cells. Thesperm cells are transported by thepollen tube through a tiny opening inthe ovule called the micropyle.

Within the ovule is the femalegametophyte, composed of eight hap-loid cells. One of the sperm unites withthe egg cell forming a diploid zygote,which begins the new sporophytegeneration. The other sperm cell fuseswith the central cell, which containsthe polar nuclei, to form a cell with atriploid (3n) nucleus. This process, inwhich one sperm fertilizes the egg andthe other sperm joins with the centralcell, is called double fertilization.


Pollen grain

Stigma

Style

Ovary

Central cell(2n)

Ovule

Egg nucleus

Two spermnuclei

Pollen tube

Tube nucleus

Double Fertilization

One spermfertilizes thecentral cell(3n)

One spermfertilizes theegg cell (2n)

Figure 24.16In flowering plants,the male gameto-phyte grows throughthe pistil to reach thefemale gametophyte.Double fertilizationinvolves two spermcell nuclei. One sperm cell uniteswith the egg nucleusand the other spermcell uniteswiththediploid central cell of the female gametophyte.

Ovules

Ovary

Sepals

Fusedpetals

Stamens

Fruit

Double fertilization is unique toanthophytes and is illustrated inFigure 24.16. The triploid nucleuswill divide many times, eventuallyforming the endosperm of the seed.The endosperm is food storage tis-sue that supports development of theembryo in anthophyte seeds.

Many flowers contain more thanone ovule. Pollination of these flowersrequires that at least one pollen grainland on the stigma for each ovule con-tained in the ovary. In a watermelonplant, for example, hundreds of pollengrains are required to pollinate a sin-gle flower if each ovule is to be fertil-ized. You are probably familiar withthe hundreds of watermelon seedsthat are the result of this process.

Seeds and FruitsThe embryo contained within a

seed is the next sporophyte genera-tion. The formation of seeds and thefruits that enclose them help ensurethe survival of the next generation.

Seed formationAfter fertilization takes place, most

of the flower parts die and the seedsbegin to develop. The wall of the ovulebecomes the hard seed coat, whichmay aid in dispersal and helps protectthe embryo until it begins growinginto a new plant. Inside the ovule, thezygote divides and grows into theplant embryo. The triploid centralcell develops into the endosperm.


Figure 24.17A fruit consists of the seeds andthe surrounding mature ovary ofa flowering plant.

OriginWORDWORD

endospermFrom the Greekwords endon, mean-ing “within,” andsperma, meaning“seed.” The endo-sperm is storage tissue found in theseeds of manyanthophytes.

When the eggs in the ovules ofa blueberry flower have beenfertilized, the petals, stamens,and stigma wither and fallaway.

AA

The wall of the ovary in ablueberry becomes fleshyand grows up and around theovary as the seeds develop.

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The remains of thesepals and some driedstamens usually canbe seen at the top.

CC

CAREERS IN BIOLOGY

Greens Keeper

Do you like to work outside? Is golfyour favorite sport? Then you

already know the value of a velvetygreen fairway, which is the firstrequirement in becoming a greenskeeper.

Skills for the JobA greens keeper maintains both

the playing quality and the beauty of agolf course. Beginning keepers usually learnon-the-job and spend their days mowing the greens. Greenskeepers who want to manage large crews will need a two- orfour-year degree in turf management and a certificate ingrounds management. They must be thoroughly familiarwith different types of grasses and know how weather andwear affect them so they can keep fairways smooth and per-fectly green. Other careers in turf management including car-ing for the grounds of shopping centers, schools, sports play-ing fields, cemeteries, office buildings, and other locations.

For more careers in relatedfields, be sure to check theGlencoe Science Web site.science.glencoe.com

Fruit formationAs the seeds develop, the sur-

rounding ovary enlarges andbecomes the fruit. A fruit is the struc-ture that contains the seeds of ananthophyte. Figure 24.17 shows howthe fruit of a blueberry develops fromthe ovary inside the flower.

A fruit is as unique to a plant as itsflower, and many plants can be identi-fied by examining the structure oftheir fruit. You are familiar withplants that develop fleshy fruits, suchas apples, grapes, melons, tomatoes,and cucumbers. Other plants developdry fruits such as peanuts, sunflower“seeds,” and walnuts. In dry fruits, theovary around the seeds hardens as thefruit matures. Some plant foods thatwe call vegetables or grains are actu-ally fruits, as shown in Figure 24.18.Can you think of any vegetables thatare actually fruits? For example,tomatoes are fleshy fruits that areoften referred to as vegetables.


Figure 24.18 A fruit is the ripened ovary of aflower that contains the seeds orseed of the plant. The mostfamiliar fruits are those we con-sume as food.

Fleshy fruits develop a juicy fruitwall full of water and sugars.

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Dry fruits have dry fruit walls. The ovary wallmay start out with a fleshy appearance, as inhickory nuts or bean pods, but when the fruitis fully matured, the ovary wall is dry.

AA

BIOLOGY


Seed dispersalA fruit not only protects the seeds

inside it, but also may aid in dispersingthose seeds away from the parent plantand into new habitats. The dispersalof seeds, Figure 24.19, is importantbecause it reduces competition forsunlight, soil, and water between theparent plant and its offspring. Animalssuch as raccoons, deer, bears, and birdshelp distribute many seeds by eatingdry or fleshy fruits. They may carrythe fruit some distance away from theparent plant before consuming it andspitting out the seeds. Or they may eatthe fruit, seeds and all. Seeds that areeaten may pass through the digestivesystem unharmed and are depositedin the animal’s wastes. Squirrels,birds, and other nut gatherers maydrop and lose some of the seeds theycollect, or even bury them only toforget where. These seeds can thengerminate far from the parent plant.

Plants, such as water lilies andcoconut palms that live in or nearwater produce fruits or seeds with air

pockets in the walls, which enablethem to float and drift away from theparent plant. The ripened fruits ofmany plants split open to releaseseeds designed for dispersal by thewind or by clinging to animal fur.Orchid seeds are so tiny that theyresemble dust grains or feathers andare easily blown about by the wind.The fruit of the poppy flower forms aseed-filled capsule that releases sprin-kles of tiny seeds like a salt shaker asit bobs about in the wind. Tumble-weed seeds are scattered by the windas the whole plant rolls along theground.

Seed germinationAt maturity, seeds are fully formed.

The seed coat dries and hardens,enabling the seed to survive condi-tions that are unfavorable to the par-ent plant. The seeds of some plantspecies must germinate immediatelyor die. However, the seeds of someplant species can remain in the soiluntil conditions are favorable for


Figure 24.19A wide variety of seed-dispersal mechanismshave evolved amongflowering plants.

The ripe pods of violets snap openwith a pop, which sends a showerof small seeds in all directions.

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Clinging fruits, likethose of the cockle-bur and burdock,are covered byhooks that stick tothe fur or feathersof passing animalsor the clothes ofpassing humans.

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Wind-dispersedseeds have adapta-tions that enablethem to be heldaloft while theydrift away fromtheir parent.

CC

growth and development of the newplant. This period of inactivity in amature seed is called dormancy. Thelength of time a seed remains dor-mant can vary from one species toanother. Some seeds, such as willow,magnolia, and maple remain dor-mant for only a few weeks afterthey mature. These seeds cannotsurvive harsh conditions for longperiods of time. Other plantsproduce seeds that can remaindormant for remarkably long peri-ods of time, Figure 24.20. Evenunder harsh conditions, the seeds ofdesert wildflowers and some coniferscan survive dormant periods of 15 to20 years. Scientists discoveredancient seeds of the East IndianLotus, Nelumbo nucifera, in China,which they have radiocarbon dated tobe more than a thousand years old.Imagine their amazement when theseseeds germinated!

Requirements for germinationDormancy ends when the seed is

ready to germinate. Germination isthe beginning of the development ofthe embryo into a new plant. Theabsorption of enough water and thepresence of oxygen and favorabletemperatures usually end dormancy,but there may be other requirements.Water is important because it acti-vates the embryo’s metabolic system.Once metabolism has begun, the seedmust continue to receive water or itwill die. Just before the seed coatbreaks open, the plant embryo beginsto respire rapidly. Many seeds germi-nate best at temperatures between25°C and 30°C. Arctic species germi-nate at lower temperatures than dotropical species. At temperaturesbelow 0°C or above 45°C, most seedswon’t germinate at all.

Some seeds have special require-ments for germination, Figure 24.21.

For example, some germinate morereadily after they have passedthrough the acid environment of ananimal’s digestive system. Othersrequire a period of freezing tempera-tures, as do apple seeds, extensivesoaking in saltwater, as do coconutseeds, or certain day lengths. Recallthat the seeds of some conifers willnot germinate unless they have beenexposed to fire. The same is true of

Figure 24.20Seeds can remaindormant for longperiods of time.Lupine seeds likethese may germinateafter remaining dor-mant for decades.

Figure 24.21The seeds of thedesert tree Cercidiumfloridum have hardseed coats that mustbe cracked open inorder to germinate.This occurs when theseeds tumble downrocky gullies in suddenrainstorms.


Figure 24.23Germination of a bean seed is stimulated bywarm temperatures and water, which softensthe seed coat.

Figure 24.22Many wildflowers require fire for their seeds to germinate.This is especially true in prairie environments where firesare periodically set to induce the germination of prairiewildflower seeds.

certain wildflower species, includinglupines and gentians, Figure 24.22.

The germination of a typical dicotembryo is shown in Figure 24.23.Once the seed coat has been softenedby water, the embryo starts toemerge from the seed. The first partof the embryo to appear is theembryonic root called the radicle(RAD ih kul). The radicle grows downinto the soil and develops into a root.The portion of the stem nearest theseed is called the hypocotyl (HI pohkaht ul). In some plants, the first partof the stem to push above ground isan arched portion of the hypocotyl.As the hypocotyl continues growing,it straightens, bringing with it the

The hypocotylis the first partof the stem toappear.

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As new leavesmature, thecotyledonswither andfall away.

DDAs the hypocotylstraightens, theplant’s first leaves,the cotyledons, are exposed to sunlight.

CC

The radicle willbecome the pri-mary root.

AA Radicle

Cotyledon

Seed coat

Primary root

Secondaryroots

Hypocotyl

Epicotyl

Cotyledon

Withered cotyledons

Hypocotyl

Looking at Germinating Seeds Seeds are made up of a plant embryo, a seed coat, and in some plants, a food-storage tissue. Monocot and dicot seeds differ in their internal structures.

Procedure! Obtain from your teacher a soaked,

ungerminated corn kernel (monocot), a bean seed (dicot), and corn and bean seeds that have begun to germinate.

@ Remove the seed coats from each of the ungerminatedseeds, and examine the structures inside. Use low-powermagnification. Locate and identify each structure of theembryo and any other structures you observe.

# Examine the germinating seeds. Locate and identify thestructures you observed in the dormant seeds.

Analysis1. Diagram the dormant embryos in the soaked seeds, and

label their structures.2. Diagram the germinating seeds, and label their structures.3. List at least three major differences you observed in the

internal structures of the corn and bean seeds.

MiniLab 24-2MiniLab 24-2cotyledons and the plant’s first leaves.As the stem grows larger and theleaves turn green, the plant can pro-duce its own food through photosyn-thesis. To learn more about germinat-ing seeds, try the MiniLab shown here.

Vegetative reproductionThe roots, stems, and leaves of

plants are called vegetative structures.When these structures produce a newplant, it is called vegetative reproduc-tion. Vegetative reproduction is com-mon among anthophytes. Some mod-ified stems of anthophytes, such aspotato tubers, can produce a newplant. Potatoes will grow new stemsfrom their “eyes” or buds. Farmersmake use of this feature when theycut potato tubers into pieces andplant them. The buds on these sec-tions grow new shoots that produceentire new plants.

Although cloning animals is a rela-tively new phenomenon, gardenershave relied for years on cloning toreproduce plants. Using vegetativereproduction to grow numerous plantsfrom one plant it is frequently referredto as vegetative propagation. Someplants, such as geraniums, can bepropagated by planting cuttings, whichare pieces of the stem or a leaf thathas been cut off another plant. Evensmaller pieces of plants can be used

to grow plants by tissue culture. Tinypieces of plants are placed on nutri-ent agar in test tubes or petri dishes.The plants produced by cuttings andtissue cultures contain the samegenetic makeup as the original plants.Therefore, they are botanical clones.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. What is the relationship between the pollina-

tion of a flower and the production of one ormore seeds?

2. What part of an anthophyte flower becomesthe fruit?

3. Describe the process of double fertilization inanthophytes.

4. Explain how the production of nectar couldenhance the pollination of a flowering plant.

Thinking Critically5. Describe the formation of the female gameto-

phyte in a flowering plant.

6. Making and Using Tables Make a table thatindicates whether each structure of a flower isinvolved in pollination, fruit formation, seed pro-duction, or seed dispersal. For more help, refer toOrganizing Information in the Skill Handbook.


Corn seed germination

Observing

Examining the Structure of a Flower

F lowers are the reproductive structures of anthophytes. Seeds thatdevelop within the flower are carried inside a fruit. Seeds provide

an extremely important form of reproduction in flowering plants.Flowers come in many colors and shapes. Often their colors or shapes arerelated to the manner in which pollination takes place. The major organsof a flower include the petals, sepals, stamens, and pistils. Some flowersare incomplete, which means they do not have all four kinds of organs.You will study a complete flower.

INVESTIGATEINVESTIGATE

microscope 2 coverslipssingle-edged dropper

razor blade water

Safety Precautions Always wear goggles in the lab.

Handle the razor blade with extremecaution. Always cut away from you.Use caution when working with amicroscope and slides. Wash yourhands with soap and water after han-dling plant material.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

1. Examine your flower. Locate thesepals and petals. Note theirnumbers, size, color, and arrange-ment on the flower stem.

2. Remove the sepals and petalsfrom your flower by gentlypulling them off the stem. Locatethe stamens, each of which con-sists of a thin filament with a

PROCEDUREPROCEDURE

ProblemWhat do the parts of a flower look

like? How are they arranged?

ObjectivesIn this BioLab, you will:■ Observe the structures of a flower.■ Identify the functions of flower parts.

Materialsflower—any complete flower that is

available locally, such as phlox, lily,or tobacco flower

hand lens (or stereomicroscope)colored pencils (red, green, blue)2 microscope slides

678

pollen-filled anther on the tip.Note the number of stamens.

3. Locate the pistil. The stigma atthe top of the pistil is often sticky.The style is a long, narrow struc-ture that leads from the stigma tothe ovary.

4. Place an anther from one of thestamens onto a microscope slideand add a drop of water. Cut theanther into several pieces withthe razor blade. CAUTION:Always take care when using arazor blade.

5. Examine the anther under lowand high power of your micro-scope. The small, dotlike struc-tures are pollen grains.

6. Slice the ovary in half length-wise with the razor blade.Mount one half, cut side fac-ing up, on a microscopeslide.

7. Examine the ovary sectionwith a hand lens or stere-omicroscope. Themany, small, dotlikestructures that fill thetwo ovary halves areovules. Each ovulecontains an eggcell that is notvisible under lowpower. A tinystalk connects eachovule to the ovary wall.

8. Identify the ovary and ovules.9. Make a diagram of the flower,

labeling all its parts. Color thefemale reproductive parts red.Color the male reproductive partsgreen. Color the remaining partsblue.


1. Observing How many stamens arepresent in your flower? How manypistils, ovaries, sepals, and petals?

2. Comparing and ContrastingMake a reasonable estimate of thenumber of pollen grains in theanther and the number of ovules inan ovary of your flower.

3. Interpreting Data Which pro-duces more? Pollen grains by oneanther? Ovules produced by oneovary? Give a possible explanationfor your answer.

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

Going FurtherGoing Further

Project Use a field guide to identify com-mon wildflowers in your area. Most fieldidentifications are made on the basis ofcolor, shape, numbers, and arrangement offlower parts. If collecting is permitted, pick afew common flowers to press and makeinto a display of local flora.

To find out more about flowers, visit the Glencoe

Science Web site.science.glencoe.com

BIOLOGY


For thousands of years, humans have influ-enced the breeding of plants, especially food

crops and flowers. Today’s plant breeders createhybrid strains with a variety of desired character-istics, such as more colorful or fragrant flowers,tastier fruit, higher yields, or increased resistanceto disease.

The perfect ear of corn The first step in creat-ing a hybrid is the selection of parent plants withdesirable characteristics. A breeder might select acorn plant that ripens earlier in the season or onethat can be sown earlier in the spring because itsseeds germinate well in cool, moist soil.

The next step is to grow several self-polli-nated generations of each plant to form a true-breeding line that always shows the desired char-acteristic. To do this, each plant must be pre-vented from cross-pollinating with other corn.The female flowers, called silks, grow near themiddle of the corn stalk. The breeder coverseach flower to prevent wind-borne pollen fromfertilizing it. The pollen-producing male tasselsare removed and the breeder uses their pollen tohand-pollinate each flower.

Once each true-breeding line has been estab-lished, the real experimentation begins. Breederscross different combinations of true-breedinglines to see what characteristics the resulting F1hybrids will have. These trials show which of thetrue-breeding lines reliably pass their desiredcharacteristic to hybrid offspring, and whichcrosses produce seeds that the breeder can mar-ket as a new, improved variety of corn.

Applications for the futureCell culture and genetic engineering tech-

nologies are new plant breeding techniques.Protoplast fusion removes the cell walls from the

cells of leaves or seedlings, then uses electricityor chemicals to fuse cells of two different species.Some of these fused cells have been successfullycultured in the lab and grown into adult plants,though none have produced seeds.

Recombinant DNA technology has been usedto insert specific genes into the chromosomes of aplant. This technique helps produce plants thatare resistant to frost, drought, or disease.


Hybrid Plants

If you’ve looked through any seed catalogs lately, you may have noticed phrases like “newthis season!” or “improved yield,” or “sweeter-tasting.” Sometimes the designation “F1”

is given beside the names of some plant varieties. All of these plants are hybrids thathave been produced in experiments conducted by plant breeders.

TechnologyTechnologyBIOBIO

Analyzing Concepts Why do seed companiesrecommend not saving seeds from hybrid vari-eties to plant the following year? (Hint: The off-spring of self-pollinated hybrids constitute the F2generation.)

To find out more about hybrid seeds, visit the Glencoe

Science Web site. science.glencoe.com

INVESTIGATING THE TECHNOLOGYINVESTIGATING THE TECHNOLOGY

Technician performing hybridization studies

BIOLOGY


Chapter 24 AssessmentChapter 24 Assessment

SUMMARYSUMMARY

Section 24.1

Section 24.2

Section 24.3

Main Ideas■ In mosses, a gametophyte forms archegonia and

antheridia. The gametophyte is dominant.■ In ferns, the prothallus forms archegonia and

antheridia. The sporophyte is dominant.■ In conifers, cones produce spores that form

male or female gametophytes. The pollen grainproduces sperm, which fertilizes the egg. Theembryo is protected by a seed.

Vocabularymegaspore (p. 658)micropyle (p. 659)microspore (p. 658)protonema (p. 655)vegetative reproduction

(p. 654)

Main Ideas■ Flowers are made up of four organs: sepals,

petals, stamens, and pistils.■ Photoperiodism affects the timing of

flower production.

Vocabularyanther (p. 662)day-neutral plant (p. 666)long-day plant (p. 666)ovary (p. 662)petals (p. 661)photoperiodism (p. 663)pistil (p. 662)sepals (p. 661)short-day plant (p. 666)stamens (p. 662)

CHAPTER 24 ASSESSMENT 681

1. Pollen and nectar produced by flowers pro-vide ________ for butterflies and bees.a. protection c. shelterb. food d. fruit

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS2. Flowers that are dull in color and have no

nectar yet have a strong scent might be polli-nated by ________.a. bees c. beetlesb. butterflies d. hummingbirds

Life Cycles ofMosses, Ferns,and Conifers

Flowers andFlowering

Main Ideas■ The male gametophyte is produced by a

microspore in the anther. The female gameto-phyte is produced by a megaspore in the ovule.

■ Sperm are transported by a pollen tube to theovule, where fertilization takes place.

■ In double fertilization, one sperm joins with theegg to form a zygote. The second sperm joinsthe central cell to form endosperm.

■ The ovary wall becomes the fruit.■ Fruits and seeds are modified for dispersal.■ Seeds can stay dormant for long periods of time.

Vocabularydormancy (p. 675)double fertilization

(p. 671)endosperm (p. 672)germination (p. 675)hypocotyl (p. 676)polar nuclei (p. 667)radicle (p. 676)

The Life Cycleof a FloweringPlant


9. A(n) ________ is one that has all four organs:sepals, petals, stamens, and pistils.a. short-day plant c. incomplete flowerb. long-day plant d. complete flower

10. The response of flowering plants to the dif-ference in the duration of light and dark peri-ods in a day is called ________.a. photoperiodism c. pollinationb. nastic movement d. dormancy

11. A triploid cell resulting from double fertiliza-tion becomes the ________.

12. The structure marked A in the photograph is the ________.

13. The gametophyte produces ________ that arehaploid.

14. In ferns, sperm released by the ________ fer-tilize eggs in the ________.

15. Male pinecones produce ________ that willdevelop into the male gametophyte, or pollengrain.

16. The period of inactivity in a mature seed iscalled ________.

17. After pollination, a pollen tube grows down-ward through the pistil to the ________.

18. A short-day plant is more likely to flowerwhen the days are ________ and the nightsare ________.

19. During fertilization in anthophytes, the twosperm cells enter the ovule through an open-ing called the ________.

20. Growing new plants from cuttings or tissueculture is called ________.

21. You eat peas, beans, corn, peanuts, and cere-als. Why are seeds a good source of food?

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

682 CHAPTER 24 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Plan Your Work and Work Your Plan Set up a study schedule for yourself well in ad–vance of your test. Plan your workload so that youdo a little each day rather than a lot all at once.The key to retaining information is to repeatedlyreview and practice it.

AA

3. Moss gametophytes are ________ and formgametes by ________.a. diploid; meiosis c. diploid; mitosisb. haploid; mitosis d. haploid; meiosis

4. By eating fruit, mammals help ________.a. fertilize flowers c. photoperiodismb. nastic movement d. disperse seeds

5. While feeding, butterflies and bees carrypollen from flower to flower, causing________.a. pollination c. germinationb. dormancy d. photoperiodism

6. The heart-shaped structure formed by adeveloping fern spore is called a ________.a. protonema c. prothallusb. sporophyte d. frond

7. Which of the following plants do NOT pro-duce pollen?a. pine tree c. apple treeb. mosses d. corn

8. Which of the following is a fern gameto-phyte?a. c.

b. d.


CHAPTER 24 ASSESSMENT 683

22. How does dormancy contribute to the sur-vival of a plant species in a desert ecosystem?

23. In what ways can the relationship between thegametophyte and sporophyte in seed plantsbe regarded as good for the gametophyte?

24. Explain why a scientist might hypothesizethat the eating habits of herbivorous mam-mals affected the evolution of fruits in flow-ering plants.

25. Observing and Inferring A plant speciesproduces heavy, spiked pollen grains. Whatconclusion can you draw about the plant’spollination method?

26. Comparing and Contrasting Compare andcontrast the formation of a moss sporophyteand a fern sporophyte.

27. Formulating Hypotheses Form a hypothesisthat explains why the primary root is the firstpart of the plant to emerge from a germinat-ing seed.

28. Concept Mapping Complete the conceptmap by using the following vocabulary terms:pistil, microspore, anther, stamen, ovary,megaspores.

THINKING CRITICALLYTHINKING CRITICALLY

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

The graph below provides data from anexperiment that tests the effects of ionizingradiation on the germination of seeds.

Using a Graph Use the graph to answer thefollowing questions.1. Which group of beans had the highest

percentage of germination?a. controlb. high-exposure levelc. medium-exposure leveld. low-exposure level

2. As the radiation dose increases, germi-nation ________.a. increases c. stopsb. decreases d. is not affected

3. When beans are given a low dose ofradiation, ________ germinate.a. 25 percent c. noneb. 50 percent d. 100 percent

4. When beans are given a medium dose ofradiation, ________ germinate.a. 12.5 percent c. 37.5 percentb. 25 percent d. 50 percent

5. Designing an Experiment Design anexperiment on bean plants in which thefollowing hypothesis is tested: Beanplants exposed to ionizing radiation willnot grow as tall as those that are notexposed.

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web site.science.glencoe.com

CD-ROM

Perc

ent g

erm

inat

ion

Control High

Exposure level

Medium Low

100

50

0

Germination of Beans After Exposure to Radiation

Flowers have

male parts

that producethat produce

in an

female parts

1.

2.

3.

4.

5.

6.

in an


Non-Seed Plants Non-seed plants reproduce by forming spores.

A spore is a haploid (n) reproductive cell, pro-duced by meiosis, which can withstand harsh envi-ronmental conditions. When conditions becomefavorable, a spore can develop into the haploid,gametophyte generation of a plant. A spore willbecome either a female or male gametophyte.

Mosses, Liverworts, andHornworts

Mosses, liverworts, and hornworts are threedivisions of non-seed, nonvascular plants that livein cool, moist habitats. Because they have no

vascular tissues to move water and nutrients fromone part of the plant to another, they cannotgrow more than a few inches tall.

FOCUS ON ADAPTATIONSFOCUS ON ADAPTATIONS

The life cycle of most plant speciesalternates between two stages, or

generations. The sporophyte generationproduces spores, which develop into thegametophyte generation. The gameto-phyte produces gametes. In nonvascularplants, the gametophyte is larger andmore conspicuous than the sporophyte.In vascular plants, the sporophyte domi-nates. The gametophyte of a vascularplant is extremely small and may remainburied in the soil or inside the body ofthe sporophyte.

Gametophyte GenerationA gametophyte is haploid (n) and pro-duces eggs and sperm. In mosses, thegametophyte is the familiar soft, greengrowth that covers rotting logs or moistsoil. The tiny moss gametophytes pro-duce male and female branches. Spermcells produced by the male branchesmust swim through rain or dew to reachthe egg cells produced by the femalebranches. Fertilization takes place insidethe female reproductive organ and adiploid (2n) zygote is produced.

Alternation of Generations

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Plants Earth is virtually covered with plants. Plants provide food

and shelter for multitudes of organisms. Through theprocess of photosynthesis, they transform the radiant energyof sunlight into chemical energy in food and release oxygento the atmosphere. All plants are multicellular eukaryotes.Plant cells are surrounded by a cell wall made of cellulose.

For a preview of the plant unit, study this BioDigest before you read the chapters.After you have studied the plant chapters, you can use the BioDigest to review the unit.

BIODIGESTBIODIGEST

VITAL STATISTICSVITAL STATISTICS

Non-Seed PlantsNumbers of species:Bryophyta—mosses, 20 000 speciesLycophyta—club mosses, 1000 speciesSphenophyta—horsetails, 15 speciesPterophyta—ferns, 12 000 speciesAnthocerophyta—hornworts, 100 speciesHepatophyta—liverworts, 6500 species

Mosses often grow inmasses that form thick

carpets

The leafy gameto-phyte is haploid.The spore stalksand capsules arediploid.

Club Mosses Club mosses are non-seed plants in the divi-

sion Lycophyta. They possess vascular tissue andare found primarily in moist environments.Species that exist today are only a few centime-ters high, but they are otherwise similar to fossilLycophytes that grew as high as 30 m and formeda large part of the vegetation of Paleozoicforests.

Ferns Ferns, division Pterophyta, are the most well-

known and diverse of the non-seed vascularplants. They have leaves called fronds that growup from an underground stem called the rhi-zome. Ferns are found in many different habitats,including shady forests, stream banks, roadsides,and abandoned pastures.

Horsetails Horsetails are non-seed vascular plants in

the division Sphenophyta. They are commonlyfound growing in areas with damp soil, such asstream banks and sometimes along roadsides.Present-day horsetails are small, but theirancestors were treelike.

Plants

Sporophyte Generation Thezygote develops into an embryo, whichgrows into the sporophyte generation ofthe moss. The sporophytes grow out of thetip of the female branches of the gameto-phyte and consist of capsule-topped stalks.Cells inside each capsule undergo meiosisto form haploid (n) spores.

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BIODIGESTBIODIGEST

The hollow,jointed stems of horsetails are surroundedby whorls ofscalelike leaves.

As in all vascularplants, the sporo-phyte of this clubmoss is the domi-nant generation.Spores develop atthe base of specialleaves that formcone-shaped struc-tures called strobili.

Fern spores developin clustered structurescalled sori, usuallyfound on the under-sides of fronds.

Sperm

Capsule

Sporophyte

Egg

Gametophyte

AntheridiumMale reproductive

organ

ArchegoniumFemale

reproductiveorgan

The green, leafy growth of this moss is the gametophyte. The brown stalks toppedwith spore-filled capsulesare the sporophyte.

Seed Plants A seed is a reproductive structure that con-

tains a sporophyte embryo and a food supplyenclosed in a protective coating. The food supplynourishes the young plant during the first stagesof growth. Like spores, seeds can survive harshconditions. The seed develops into the sporophytegeneration of the plant. Seed plants includeconifers and flowering plants.

Conifers Conifers, division Coniferophyta, produce

seeds, usually in woody strobili called cones, andhave needle-shaped or scale-like leaves. Coniferseeds are not enclosed in a fruit. Most conifersare evergreen plants, which means they bearleaves all year round.

Adapted for Coldand Dry Climates

Conifers are common incold or dry habitats through-out the world. Conifer needleshave a compact shape and athick, waxy covering that helpsreduce evaporation and con-serve water. Conifer stems arecovered with a thick layer ofbark that insulates the tissuesinside. These adaptationsenable conifers to carry on lifeprocesses even when tempera-tures are below freezing.

FOCUS ON ADAPTATIONSFOCUS ON ADAPTATIONS

A ll plants probably evolvedfrom filamentous green

algae that lived in the nutrient-rich waters of Earth’s ancientoceans. An ocean-dwelling algacan absorb water and dissolvedminerals directly into its cells. Asland plants evolved, new struc-tures developed for absorbing andtransporting water and mineralsfrom the soil to all the aerial partsof the plant.

Nonvascular PlantsIn nonvascular plants, water andnutrients must travel from one cellto another by the relatively slowprocesses of osmosis and diffusion.As a result, nonvascular plants arelimited to environments whereplenty of water is available.

Moving from Water to Land

Plants

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BIODIGESTBIODIGEST

Seeds of conifers develop at the baseof each woody scale of female cones.

Plants probably evolvedfrom filamentous

green algae.

The leaves and branches of conifersare flexible. They bend under theweight of snow and ice, allowing anybuildup to slide off before it becomesheavy enough to break the branch.


ConifersExamples: Pine, spruce, fir, larch, yew, redwood, juniper.Numbers: 400 species.Size range: Giant sequoias of central California, to 99 mtall, the most massive organisms in the world; coast red-woods of California, to 117 m, the tallest trees in the world.

Stamen

Sepals

Petals

Stigma

Pistil Style

Ovary

Ovules

Pollen

Filament

Anther

Flowering Plants The flowering plants, division Anthophyta,

form the largest and most diverse group of plantson Earth today. They provide much of the foodeaten by humans. Anthophytes produce flowersand develop seeds enclosed in a fruit.

Monocots and Dicots The Anthophytes are classified into two

classes: the monocotyledons and the dicoty-ledons. Cotyledons, or “seed leaves,” are con-tained in the seed along with the plant embryo.Monocots have a single seed leaf that absorbsfood for the embryo. The two seed leaves ofdicots store food for the embryo.

Flowers Flowers are the organs of reproduction in

anthophytes. Sepals enclose the flower bud andprotect it until it opens; petals, which are oftenbrightly colored or perfumed, attract pollinators.Inside the circle of petals are the pistil and stamens.

Plants

Vascular Plants The stems of mostplants contain vascular tissues made up oftubelike, elongated cells through which water,food, and other materials move from one partof the plant to another. One reason vascularplants can grow larger than nonvascular plantsis because vascular tissue is a much more effi-cient method of internal transport than osmo-sis and diffusion. In addition, vascular tissuesinclude thickened fibers that can support tallerupright growth.

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BIODIGESTBIODIGEST

Monocots (left) include grasses,orchids, and palms. Dicots(below) include many flower-ing trees and wildflowers.

An unbroken column of watertravels from the roots in xylemtissues. Sugars formed by photosynthesis travel aroundthe plant in phloem tissues.

The pistil is the female reproductive organ.Inside the ovary at the base of the pistil arethe ovules. Ovules contain the female game-tophyte generation of the plant. Femalegametes—egg cells—form in each ovule.

The stamen is the malereproductive organ of a

flower. Pollen grains con-taining male gametes

form inside the anther.

Water andsugars

Flower

Flower stalk

Leaf

Roots Sugars to sink

H2O

H2O

Sugars

CO2

Pollen tube

Ovary

Sperm nuclei

Ovule

Egg nucleusand endospermnucleus

Pollen In seed plants, the sperm are enclosed in the

thick-coated pollen grains, which are the malegametophyte generation of the plant. Pollen is one of the important adaptations that hasenabled seed plants to live in a wide variety ofland habitats.

Pollinators Flowers can be pollinated by wind, insects,

birds, and even bats. Some flowers have colorfulor perfumed petals that attract pollinators.Flowers may also contain sweet nectar, as well aspollen, which provides pollinators with food.

FruitFollowing fertilization,

the ovary develops into afruit with seeds inside.Some flowering plantsdevelop fleshy fruits,such as apples, melons,tomatoes, and squash.Other flowering plantsdevelop dry fruits, such as peanuts, almonds, or sunflowers. Fruits help protect seeds until they are mature. Fruits also help scatter seeds into new habitats.

Plants

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BIODIGESTBIODIGEST

Plants that depend on thewind to carry pollen fromanther to stigma tend tohave small, inconspicuousflowers. The flowers ofgrasses and this alder arepollinated by the wind.

In a process called double fertilization, one ofthe sperm fertilizes the egg and the other uniteswith the endosperm nucleus.

Pollen is carried to the stigma of a flower. Thepollen grain grows a tube down the style to theovary. Two sperm travel down the tube.

Plants that depend on insects for pollination may be brightlycolored and fragrant. Pollen rubsoff on the bee that visits a flower

to feed onnectar. Whenit moves to anotherflower, someof the pollenmay rub offonto thestigma.

Many plantsproduce fruitsthat are eatenby animals.

Maple tress producefruits with a winglikeshape that can becarried long distancesby the breeze.


Flowering PlantsExamples: Grasses, oaks, maples, palms,irises, orchids, roses, beans.Numbers: 230 000 species (60 000 monocots;170 000 dicots).Size range: A few millimeters to 75 m.

Stems growing up

Roots growing down

Plant Responses Plants respond to changes in

their environment such as light,temperature, and water availabil-ity. Chemicals called hormonescontrol some of these responsesby increasing cell division andgrowth.

Plants

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BIODIGESTBIODIGEST

Roots exhibitpositive geotro-pism. Stems showa negative geo-tropic response.

The phototropicresponse shownhere is the resultof increased cellgrowth on theside of the stemaway from thelight.

BIODIGEST ASSESSMENTBIODIGEST ASSESSMENT

Understanding Main Ideas1. Which of the following is a Bryophyte?

a. moss c. club mossb. horsetail d. conifer

2. The term for a mature fern leaf is ________.a. leaf c. frondb. scale d. needle

3. Nonvascular plants would most likely befound growing ________.a. in sandy desert soilb. on an ocean beachc. on a snowy mountain sloped. in a shady, moist environment

4. Which plant group has leaves adapted forlife in cold environments?a. Anthophyta c. Pterophytab. Sphenophyta d. Coniferophyta

5. Dicots have ________.a. one cotyledonb. two cotyledonsc. needlelike leavesd. spores borne in cone-shaped strobili

6. Reproductive structures of conifers are________.a. flowers c. fruitsb. cones d. sori

7. Mosses, ferns, and club mosses are alikebecause they require ________.a. water for fertilizationb. adaptations for conserving waterc. insects for pollinationd. warm, sunny habitats

8. Lycophytes, sphenophytes, and conifero-phytes have specialized leaves that formreproductive structures known as ________.a. sori c. conesb. flowers d. strobili

9. Vascular plants do not include the ________.a. Lycophytes c. Sphenophytesb. Bryophytes d. Pterophytes

10. Which plant group produces flowers andseeds enclosed in a fruit?a. Anthophyta c. Pterophytab. Coniferophyta d. Lycophyta

Thinking Critically1. Compare the spore-bearing structures of

ferns with the seed-bearing structures ofconifers.

2. Why do vascular plants have an adaptiveadvantage over nonvascular plants?

3. Describe three ways in which seeds may bedispersed.

Chapter 24: Reproduction in Plants - Polson Schools...

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