Chapter 24: Reproduction in Plants - CoconinoHighSchool · on reproduction in plants ... vegetative reproduction protonema megaspore microspore ... gametophyte is the familiar form,

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Reproduction in Plants

Reproduction in Plants

What You’ll Learn ■ You will compare and contrast

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 Much of life on Earth, includingyou, depends on plants. Humanseat plants and some people eatorganisms that eat plants. Ifplants did not reproduce andcontinue their species, the foodsources for much of life on Earthwould disappear.

Pollen grains of seed plants—coniferophytes and antho-phytes—contain malereproductive cells. Botanists andpaleobotanists can use theunique structures of pollengrains to identify living orextinct seed-plant species. Thiscolor-enhanced SEM photographshows pollen grains from severalspecies of living anthophytes.

Understandingthe Photo

RMF/Visuals Unlimited

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24.1SECTION PREVIEWObjectivesReview the steps of alter-nation of generations.Survey and identifymethods of reproductionand the life cycles ofmosses, ferns, and conifers.

Review Vocabularygametophyte: haploid

form of an organism inalternation of genera-tions that producesgametes (p. 516)

New Vocabularyvegetative reproductionprotonemamegasporemicrosporemicropyle

24.1 LIFE CYCLES OF MOSSES, FERNS, AND CONIFERS 633

Life Cycles of Mosses,Ferns, and Conifers

Illustrate and Write As you read Chapter 24, illustrate and write about the life cycles of each group of plants behind the appropriate tab.

Life Cycles Make the following Foldable to help you organize events in the life cycles of mosses, ferns, and conifers.

Fold a sheet of paper in half lengthwise. Make the back part about 5 cm longer than the front part.

Turn the paper so the fold is on the bottom, then fold it into thirds.

Unfold and cut only the top layer along each fold to make three tabs.

Label the Foldable as shown.

STEP 1

STEP 3

STEP 2

STEP 4

Life Cycles

Mosses ConifersFerns

Alternation of GenerationsAs you learned earlier, plant life cycles include an alter-

nation of generations. As shown in Figure 24.1, an alter-nation of generations consists of a sporophyte stage and agametophyte stage.

All cells of a sporophyte are diploid. Certain cells of asporophyte undergo meiosis, which produces haploidspores. These spores undergo cell divisions and form amulticellular, haploid gametophyte. Some cells of a game-tophyte differentiate and form haploid gametes. Thefemale gamete is an egg and the male gamete is a sperm.When a sperm fertilizes an egg, a diploid zygote forms.This is sexual reproduction. The zygote can undergo celldivisions and form an embryo sporophyte. If the embryodevelops to maturity, the cycle can begin again.

This basic life cycle pattern is the same for most plants.However, there are many variations on this pattern withinthe plant kingdom. For instance, recall that in mosses thegametophyte is the familiar form, not the sporophyte.

GAMETOPHYTE (n)Spores (n)

Meiosis

SPOROPHYTE (2n)

Mitosis andcell division

Fertilization

Femalegamete (n)

Malegamete (n)

Figure 24.1 In an alternation of generations, the game-tophyte (n) stage produces gametes, andthe sporophyte (2n) produces spores.

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In others, such as flowering plants,the gametophyte is microscopic.Most people have never even seenthe female gametophyte of a flower-ing plant. Botanists usually refer tothe bigger, more obvious plant as thedominant generation. The dominant

generation lives longer and can sur-vive independently of the other gen-eration. In most plant species thesporophyte is the dominant plant.

Asexual reproductionMost plants also can reproduce asex-

ually by a process called vegetativereproduction. In this type of repro-duction, new plants are produced fromexisting plant organs or parts of organs.The new plants have the same geneticmakeup as the original plant. Forexample, some thallose liverwortgametophytes can produce cuplikestructures, as shown in Figure 24.2.Inside, minute pieces of tissue calledgemmae ( JEH mee) develop. If gemmaefall from the cup, they can grow intoother liverwort gametophytes that aregenetically identical to the thallus. Youcan learn more about asexual reproduc-tion in MiniLab 24.1.

Compare the geneticmakeups of plants produced fromone plant by asexual reproduction toeach other and the original plant.

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

634 REPRODUCTION IN PLANTS

ExperimentGrowing Plants Asexually Plants are capable of reproduc-ing asexually. Reproductive cells such as egg or sperm are notneeded in asexual reproduction. Plant organs such as roots,stems, and even leaves can produce new offspring.

Procedure! Prepare three different plant parts for study using dia-

grams A, B, and C as a guide. @ Design a data table in which you can diagram your obser-

vations and list the number of days since the experimentbegan.

# Diagram the plant parts and label them as Day 1.$ Make and record observations every three days. Replace

any lost water as needed.% Observe any changes that occur to your plants over the

next two weeks.

Analysis 1. Observe What experimental evidence do you have that:

a. plants can use different structures for asexual reproduction?

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

2. List Describe advantages of asexual reproduction in plants.

Water

Beaker

Potato with eye(stem & buds)

Toothpick

Carrot(root)Water

Water

Test tube

Garlic bulb(storage leaves)

AA BB CC

Runk-Schoenberger/Grant Heilman Photography

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Life Cycle of MossesThe gametophyte stage is the domi-

nant generation in mosses. A haploidmoss spore can germinate and grow toform a protonema (proh tuh NEE

muh). It is a small green filament ofcells that can develop into the gameto-phyte. In some mosses, male and femalereproductive structures form on sepa-rate gametophytes, but in others, maleand female reproductive structures areon the same gametophyte. Recall thatthe egg-producing, female reproduc-tive structure is an archegonium, andthe sperm-producing, male reproduc-tive structure is an antheridium.

Motile sperm from an antheridiumswim in a continuous film of water toan egg in an archegonium. If fertiliza-tion occurs, a diploid zygote forms.The zygote undergoes cell divisionsforming the sporophyte that consistsof a stalk with a capsule at the top. Thesporophyte remains attached to anddependent on the gametophyte. Cellsin the capsule undergo meiosis, pro-ducing haploid spores. When the cap-sule matures, it bursts open andreleases spores. If the spores land in afavorable environment, they can ger-minate and the cycle repeats, as shownin Figure 24.3.


Meiosis

Mitosis

Fertilization

Developingsporophyte

Gametophyte (n)

Capsule

Capsule opensreleasing spores

Germinatingspores

Sperm

Egg

Zygote (2n)

ProtonemaFemalegametophyte(n)

Malegametophyte(n)

SPOROPHYTEGENERATION

2n

GAMETOPHYTEGENERATION

n

Sporophyte (2n)

Figure 24.3 The moss gametophyteproduces gametes thatjoin to form a zygote.The zygote developsinto the sporophytethat produces spores.Spores can germinateand grow into a ga-metophyte, completingthe moss’s life cycle.

(inset)Dwight R. Kuhn

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Figure 24.4Fern sporophytes are common in a forest.However, a fern gametophyte is rarely seen.

Most fern sporophytes can grow 25 cm or taller. Differentiate Howis a fern’s method of growthdifferent from a moss’s?

A

Each round cluster of sporangia on theunderside of a fern frond is called a sorus.Each sporangium contains spores that are

released, sometimes in dramatic fashion.

B

The heart-shaped ferngametophyte is usually5 mm–6 mm across.

C

Stained LM Magnification: 25�

Some moss gametophytes also re-produce by vegetative reproduction.They can break into pieces when dryand brittle then, when moisturereturns, each piece can grow and forma protonema then a gametophyte.

Life Cycle of FernsUnlike mosses, the dominant stage

of the fern life cycle is the sporophytestage. The fern sporophyte includesthe familiar fronds you see in Figure24.4A. Fern fronds grow from a rhi-zome, which is an underground stem.On the underside of some fronds aresori, which are clusters of sporangia.Meiosis occurs within the sporangia,

producing haploid spores. When en-vironmental conditions are right, thesporangia open and release haploidspores, as shown in Figure 24.4B.

A spore can germinate to form aheart-shaped gametophyte called aprothallus, as shown in Figure 24.4C.The prothallus produces both arche-gonia and antheridia on its surface.The flagellated sperm released byantheridia swim through a film ofwater to eggs in archegonia. If fertil-ization occurs, the diploid zygote candevelop into the sporophyte. Initially,this developing sporophyte dependsupon the gametophyte for its nutri-tion. However, once the sporophyteproduces green fronds, it can carry on

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photosynthesis and survive on its own.The prothallus dies and decomposesas the sporophyte matures. Themature fern sporophyte consists of arhizome from which roots and frondsgrow. If pieces of rhizome break away,

new fern plants can develop fromthem by vegetative reproduction.Sporangia can develop on the fronds,spores can be released, and the cyclecan begin again. The life cycle of thefern is summarized in Figure 24.5.


Meiosis

Spores (n)

Growingprothallus

Prothallus

Rhizome

Fronds

Sporangium

Sorus

Roots


2n


n

Fertilization

Cell divisions

Sperm

Rhizoids

Archegonium

Egg

Antheridium

Zygote

Figure 24.5In the life cycle of a fern,the sporophyte genera-tion becomes indepen-dent of the gametophyte.

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The Life Cycle of Conifers

The dominant stage in conifers isthe sporophyte generation. One of themore familiar conifer sporophytes isshown in Figure 24.6A. The adultconifer produces male and femalecones on separate branches of oneplant. Cones contain spore-producingstructures, or sporangia, on their scales.

Female cones, which are larger thanthe male cones, develop two ovules onthe upper surface of each cone scale.Each ovule contains a sporangium witha diploid cell that undergoes meiosisand produces four megaspores. Amegaspore is a female spore that even-tually can become the female gameto-phyte. One of the four megasporessurvives and grows by cell divisions

into the female gametophyte. It con-sists of hundreds of cells and is depend-ent on the sporophyte for protectionand nutrition. Within the female ga-metophyte are two or more archego-nia, each containing an egg.

Male cones have sporangia thatundergo meiosis to produce malespores called microspores. Eachmicrospore can develop into a male gametophyte, or pollen grain. Eachpollen grain, with its hard, water-resistant outer covering, is a malegametophyte. Examples of male andfemale conifer gametophytes are shownin Figure 24.7.

In conifers, pollination is the trans-fer of pollen grains from the male coneto the female cone. Pollination canoccur when a wind-borne pollen grainfalls near the opening in one of the


micropyle fromthe Greek wordsmikros, meaning“small,” and pyle,meaning “gate”;The micropyle isthe small openingat one end of theovule.

Figure 24.6In conifers, the sporophyte isimmense compared with themicroscopic gametophytes.

This pine sporophytecan grow more than25 m tall.

A

The female gametophytein this pine ovule is lessthan 0.01 mm long.

B

A pollen grain isso small it can becarried by thewind.

C

Stained LM Magnification: 70�

Stained LM Magnification: unavailable

(tl)Carolina Biological Supply/Visuals Unlimited, (bl)Carolina Biological Supply/Phototake, NYC, (r)Kenneth J. Stein/Phototake, NYC

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ovules of the female cone. The openingof the ovule is called the micropyle(MI kruh pile). The pollen grainadheres to a sticky drop of fluid thatcovers the micropyle. As the fluid evap-orates, the pollen grain is drawn closerto the micropyle. Although pollinationhas occurred, fertilization does not takeplace for at least a year. The pollengrain and the female gametophyte willmature during this time.

As the pollen grain matures, it pro-duces a pollen tube that growsthrough the micropyle and into theovule. A sperm nucleus from the malegametophyte moves through thepollen tube to the egg. If fertilizationoccurs, a zygote forms. It is nourishedby the female gametophyte and candevelop into an embryo with severalcotyledons. The cotyledons will nour-ish the sporophyte after germination.


Male cone

Megaspores

Ovule

MicrosporesMicrospore mother cells

Female cone

(pollen grain)

Pollen grain

Micropyle

Femalegametophyte

Egg

Two archegoniawith egg cells

Sperm nucleus

Seed

Cotyledons

Youngseedling

Adultsporophyte

Seed coat

Embryo

Storedfood

Malegametophyte

Maturing pollen grainOne egg is fertilized


2nGAMETOPHYTEGENERATION

nFertilization

Meiosis

Figure 24.7The life cycle of a coniferincludes the production oftwo types of spores by thesporophyte. These sporesdevelop into the male and female gametophytes.

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Understanding Main Ideas1. Explain how vegetative reproduction can produce

a new plant. Provide an example.2. How is the sporophyte generation of a moss

dependent on the gametophyte generation?3. Interpret the function of a plant’s reproductive

system.4. Compare and contrast the life cycle of a fern and

a conifer.

Thinking Critically5. Why is the term alternation of generations appro-

priate to describe the life cycle of a plant?

6. Sequence Make an events-chain concept map ofthe life cycle of a fern, beginning with the pro-thallus. For more help, refer to Sequence in theSkill Handbook.

SKILL REVIEWSKILL REVIEW


A seed coat forms around the ovule as themature seed is produced. Mature seedsare released when the female cone opens.

When conditions are favorable, areleased seed can germinate and grow intoa new, young sporophyte, as shown inFigure 24.8. Review the stages of aconifer’s life cycle in Figure 24.7. Use theProblem-Solving Lab on this page to furtherexplore the characteristics of the life cyclesof mosses, ferns, and conifers.

Figure 24.8Conifer seeds germinate into new, youngsporophytes such as the pine tree seedlingshown here.

Make and Use TablesWhat characteristics do mosses, ferns, and conifers share?It can help to organize information in a table, because character-istics, similarities, and differences are shown in a simple format.

Solve the ProblemCopy and complete the following data table using “yes” or “no.”

Thinking Critically1. Identify Which two plant groups share the most charac-

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

observe:a. a pine gametophyte? Sporophyte? Explain.b. a fern gametophyte? Sporophyte? Explain.

3. Compare and Contrast Using information from yourtable, compare and contrast reproduction among mosses,ferns, and conifers.

Data Table

Trait Moss Fern Conifer

Has alternation of generations

Film of water needed for fertilization

Dominant gametophyte

Dominant sporophyte

Sporophyte is photosynthetic

Produces seeds

Produces sperm

Produces pollen grains

Produces eggs

Noble Proctor/Photo Researchers

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C.P. George/Visuals Unlimited

24.2SECTION PREVIEWObjectivesIdentify the organs of aflower.Examine how photoperi-odism influences flowering.

Review Vocabularyresponse: an organism’s

reaction to a change inits internal or externalenvironment (p. 9)

New Vocabularypetalsepalstamenantherpistilovaryphotoperiodismshort-day plantlong-day plantday-neutral plant

24.2 FLOWERS AND FLOWERING 641

What is a flower?The process of sexual reproduction in flowering plants takes place in a

flower, which is a complex structure made up of several organs. Someorgans of the flower are directly involved in fertilization and seed pro-duction. Other floral organs function in pollination. There are probablyas many different shapes, sizes, colors, and configurations of flowers asthere are species of flowering plants. In fact, flower characteristics areoften used in plant identification.

The structure of a flowerEven though there is an almost limitless variation in flower shapes and

colors, all flowers share a basic structure. A flower’s structure is genet-ically determined and usually made up of four kinds of organs: sepals,petals, stamens, and pistils. The flower parts you are probably most famil-iar with are the petals. Petals are usually the colorful structures at the topof a flower stem. The flower stem is called the peduncle. Sepals are usu-ally leaflike and encircle the peduncle below the petals.

Inside the petals are the stamens. A stamen is the male reproductiveorgan of a flower. At the tip of the stamen is the anther. The anther produces pollen that eventually contains sperm.

A Flower by Any Other NameUsing Prior Knowledge When you hearthe word flower, what image comes to mind?Is it a fragrant rose, a yellow daisy, or atrumpet-shaped lily? These flowers havecharacteristics that are similar and othersthat are different. You may know that theyall have petals, but what other parts arefound in each of them? In this section youwill read about a flower’s organs, their func-tions, and how they vary from species tospecies. You also will learn how an externalstimulus affects flowering times.Assemble For one week, find images of flow-ers in magazines and other periodicals. With per-mission, cut them out and bring them to class.Try not to duplicate your selections. Create aclass list of the name of each flower and assem-ble a classroom collage of the flower images.

Flowers and Flowering

Flowers display a variety of shapes and colors.

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Identifying Organs of a FlowerFigure 24.9Of the four major organs of a flower, only two—the stamens andpistils—are reproductive organs directly involved in seed develop-ment. Sepals and petals support and protect the reproductiveorgans and help attract pollinators. The structure of a typicalflower is illustrated here. Critical Thinking How are differentflower shapes important to a plant’s survival?

642 REPRODUCTION IN PLANTSInga Spence/Visuals Unlimited

Stigma

Style

Ovary

Pistil

Petals

Sepal

Peduncle

StamenAnther

Filament

Petals These are usually brightly colored andoften have perfume or nectar at their bases toattract pollinators. In many flowers, the petalalso provides a surface for insect pollinators torest on while feeding. All of the petals of aflower are called the corolla.

AA

Pistil The femalereproductive organ of aflower is the pistil. At the topof the pistil is the stigma thatreceives the pollen. The style is a slender stalk that connectsthe stigma to the ovary inwhich ovules grow. Each ovulecan produce an egg. If fertili-zation occurs, the ovuledevelops into the seed. Thenumber of pistils in flowersvaries from none to many.

DD

Stamen Pollen isproduced in the anther atthe tip of a thin stalk calleda filament. Together, theanther and filament makeup the stamen, the malereproductive organ. Thenumber of stamens inflowers varies from none to many, like the flowershown here.

BB

Sepals A ring of sepals makes upthe outermost portion of the flower.Sepals serve as a protective coveringfor the flower bud. They sometimesare colored and resemble petals. Allof the sepals of a flower are calledthe calyx.

CC

St.-John’s-Wort

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At the center of the flower,attached to the top of the peduncle, isusually one or more pistils. The pistilis the female organ of the flower. Thebottom portion of the pistil is theovary, a structure with one or moreovules, each usually contains one egg.As you read in the previous section,the female gametophyte developsinside the ovule. You can learn moreabout floral structure and practiceyour lab skills in the BioLab at the endof this chapter.

Modifications in flower structureA flower that has all four organs—

sepals, petals, stamens, and pistils—iscalled a complete flower, as shown inFigure 24.9. The morning glory,shown in Figure 24.10A, is anotherexample of a complete flower. Aflower that lacks one or more organs iscalled an incomplete flower. Forexample, walnut trees have separate

male and female flowers. The maleflowers, as shown in Figure 24.10B,have stamens but no pistils; the femaleflowers bear pistils but no stamens.

The flowers of plants such as sweetcorn, as shown in Figure 24.10C, andgrasses, have no petals and are adaptedfor pollination by wind rather than byanimals. You can explore flower adap-tations further in the Problem-SolvingLab on the next page.

PhotoperiodismThe relative lengths of daylight

and darkness each day have a signif-icant effect on the rate of growthand the timing of flower productionin many species of flowering plants.For example, some chrysanthemumplants produce flowers only duringthe fall, when the length of daylightis getting shorter and the length of darkness is getting longer daily.

Figure 24.10The diversity of flower forms is evi-dence of the success of floweringplants.

The petals of the morning gloryare fused together to form abell shape.

A The tassels at thetop of a corn plantare male flowers.

C

The maleflowers of thewalnut treeform longstructurescalled catkins.

B

photoperiodismfrom the Greekwords photos,meaning “light,”and periodos,meaning “aperiod”; The flow-ering response of aplant to periods ofdark and light isphotoperiodism.

643

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A grower who wants to producethese chrysanthemum flowers duringthe middle of summer must artifi-cially increase the length of dark-ness. The grower can do this bydraping a black cloth over andaround the chrysanthemum plantsbefore sunset each day, and thenremoving it the following morning.The response of flowering plants todaily daylight-darkness conditions iscalled photoperiodism.

Plant biologists originally thoughtthat the length of daylight controlledflowering. However, they now knowthat it is the length of darkness thatcontrols flowering, and that thedarkness must be uninterrupted.Each plant species has a criticalperiod—specific daylight-darknessconditions that will initiate flower-ing. Plants can be placed in one offour categories depending on thedaylight-darkness conditions thatthey require for flower production.Plants are short-day plants, long-dayplants, day-neutral plants, or inter-mediate day plants.

A short-day plant flowers whenthe number of daylight hours isshorter than that of its critical period.Short-day plants usually flower some-time during late summer, fall, winter,or spring. Examples of short-dayplants include asters, poinsettias,strawberries, ragweed, and pansies, asshown in Figure 24.11A.

Explain why pansiesstop flowering in midsummer.

A long-day plant flowers when thenumber of daylight hours is longerthan that of its critical period. Long-day plants usually flower in summer,but also will flower if lighted continu-ally. Carnations, petunias, potatoes,spinach, and wheat are long-dayplants, as well as the lettuce shown inFigure 24.11B.


Interpret Scientific IllustrationsHow do flowers differ? There is great variation in flowershape. This variation occurs when certain flower parts are fusedtogether, parts are rearranged, or when parts may be totallymissing. Almost all dicot plants will have flower parts that arein multiples of four or five. For example, a plant having eightor ten petals would be a dicot. Almost all monocot plants haveflower parts in multiples of three.

Solve the ProblemDiagram A shows a flower with its parts labeled. Imagine thatthe flower has been cut along the dashed line. Diagram B is anenlarged cross-section view of the bottom half. The flower isfrom a dicot plant because there are five sepals, petals, andstamens.

Thinking CriticallyUse diagrams B, C, D, and E to answer the following questions.1. Determine Diagrams C, D, and E also are diagrammatic

cross-section views of flowers. Is diagram:a. C a monocot or dicot? Explain.b. D a monocot or dicot? Explain.c. E a monocot or dicot? Explain.

2. Infer Which diagrams are complete flowers, and whichare incomplete flowers?

3. Predict Which flowers might be capable of self-pollination?Explain.

4. Predict Which flowers require another flower for pollina-tion? Explain.

CC DD EE

AA BBPistil

Stamen

Petal

Sepal

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Understanding Main Ideas 1. Compare and contrast sepals and petals.

2. Describe the male and female reproductive organsof a flower.

3. Explain why walnut flowers are considered incomplete flowers.

4. Discuss how photoperiodism influences flowering.5. Infer why a gardener’s holly plant flowers every

spring but never produces holly berries.

Thinking Critically6. In the middle of the summer a florist receives a

large shipment of short-day plants. Infer what theflorist must do to induce flowering.

7. Get the Big Picture Explain why the structure of awind-pollinated flower is often different from thatof an insect pollinated flower. For more help, referto Get the Big Picture in the Skill Handbook.


24.2 FLOWERS AND FLOWERING 645

Spinach and lettuce (left) arelong-day plants that flowerin midsummer.

B

Many plants are day-neutral. Flowering in cucumbers (left),tomatoes, and corn is not influenced by a dark period.

C

Short-day plantsinclude pansies(right) andgoldenrod.

AFigure 24.11Photoperiodism refers to aplant’s response to thechanging length of night.

As long as the proper growing con-ditions exist, some plants will flowerover a range in the number of day-light hours. These plants are calledday-neutral plants. This categoryincludes many tropical plants, roses,cotton, dandelions and many otherweeds, as well as the cucumbersshown in Figure 24.11C.

An intermediate-day plant willnot flower if days are shorter or

longer than its critical period.Several grasses and sugarcane are inthis category.

Photoperiodism is a physiologicaladaptation of all flowering plants thatensures the production of flowers at a time when there is an abundantpopulation of pollinators. This isimportant because pollination is acritical event in the life cycles of allflowering plants.

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Ian West/O.S.F./Earth Scenes

24.3SECTION PREVIEWObjectivesSurvey and identify themethods of reproduction,growth, and developmentin flowering plants.Outline the processes inwhich cells differentiateduring the formation ofseeds and fruits and dur-ing seed germination.

Review Vocabularypollination: transfer of

pollen grains from malereproductive organs tofemale reproductiveorgans in plants; usuallywithin the same species(p. 254)

New Vocabularypolar nucleidouble fertilizationendospermdormancygerminationradiclehypocotyl


“Tall oaks from little acornsgrow.” —David Everett (1769–1813)Finding Main Ideas On a piece of paper,construct an outline about the life cycleof a flowering plant, such as the beeorchid shown to the right. Use the redand blue titles in this section as aguideline. As you read the paragraphsthat follow the titles, add importantinformation and vocabulary words toyour outline.

Example:I. The Life Cycle of an Anthophyte

A. Development of the female gametophyte1. Occurs in ovule2. Involves meiosis3. Polar nuclei formed

Use your outline to help you answer ques-tions in the Section Assessment on page 657.For more help, refer to Outline in the SkillHandbook.

The Life Cycle of an AnthophyteThe life cycle of flowering plants is similar to that of conifers in many

ways. In both coniferophytes and anthophytes, the gametophyte generationis contained within the sporophyte. Many of the reproductive structures arealso similar. However, anthophytes are the only plants that produce flow-ers and fruits. Figure 24.12 summarizes the life cycle of a flowering plant.

Development of the female gametophyteIn anthophytes, the female gametophyte is formed inside the ovule within

the ovary. In the ovule, a cell undergoes meiosis and produces haploidmegaspores. One of these megaspores can develop into the female gameto-phyte, but the other three spores usually die. In most flowering plants, themegaspore’s nucleus undergoes mitosis three times, producing eight haploidnuclei. These eight nuclei make up the female gametophyte. Cell walls formaround each of six nuclei, one of which is now called the egg cell.

Bee orchid, Ophrys speculum

The Life Cycle of a Flowering Plant

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nSPOROPHYTEGENERATION

2n Fertilization

Meiosis

Youngseedling

Germinatingseed

SeedFruitwith seeds

Adultsporophyteplant

Pollentube

Egg

Femalegametophyte

Tubenucleus

Sperm Malegametophyte

Microsporesin fours

Anther

Pollen sacwith microsporemother cells

Ovary

Fourmegaspores

Ovule withmegasporemother cell

Zygote

Pollen tube

Endospermnucleus

Doublefertilization

Pollengrain

24.3 THE LIFE CYCLE OF A FLOWERING PLANT 647

Figure 24.12In the life cycle of a flowering plant, the sporophyte generationnourishes and protects the developing gametophyte.

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(female gametophyte)

Ovule

Embryo sac

Central cell

Egg cell

Micropyle

Figure 24.13 The eight nuclei produced by themegaspore form the female gameto-phyte inside an ovule of an antho-phyte’s flower. Differentiate How dothese nuclei differ from those inthe leaf cells of this anthophyte?

MEIOSIS

Microsporemother cell (2n)

Tubenucleus

Generativenucleus

Pollen grain(male gametophyte)

Microspores(n)

ANTHER

Pollen sacs

The two remaining nuclei, which arecalled polar nuclei, are enclosed in onecell. This cell, the central cell, is locatedat the center of the female game-tophyte. The egg cell is near the micro-pyle, as shown in Figure 24.13. Theother five cells eventually disintegrate.

Development of the malegametophyte

The formation of the male ga-metophyte begins in the anther, as

seen in Figure 24.14. Haploid micro-spores are produced by meiosis withinthe pollen sac. The nucleus of eachmicrospore undergoes mitosis. Athick, protective wall surrounds thesetwo nuclei. This structure is theimmature male gametophyte, orpollen grain. The nuclei within thepollen grain are the tube nucleus andthe generative nucleus. When thepollen grains mature, the anther usu-ally splits open.

Figure 24.14 Meiotic division of each ofmany cells within the antherproduces four microspores.These microspores developinto male gametophytes orpollen grains.

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(tl)Raymond Mendez/Animals Animals, (tr)Bill Beatty/Earth Scenes, (bl)Anthony Mercieca/Photo Researchers, (br)Leonard Lessin/Photo Researchers

PollinationIn anthophytes, pollination is the

transfer of the pollen grain from theanther to the stigma. Depending onthe type of flower, the pollen can becarried to the stigma by wind, water,or animals. Plant reproduction ismost successful when the pollinationrate is high, which means that the pis-til of a flower receives enough pollenof its own species to fertilize the eggin each ovule. Although it may seemwasteful for wind-pollinated plants toproduce such large amounts of pollen,it does help ensure pollination. Many

anthophytes 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.

Most anthophytes that are polli-nated by animals produce nectar intheir flowers. Nectar is a highly con-centrated food sought by visitors tothese flowers. It is a liquid made up of proteins and sugars and usually collects in the cuplike area at the baseof petals. Animals, such as insects and birds, brush up against the antherswhile trying to get to the nectar.


Figure 24.15The shape, color, and size of aflower reflect its relationshipwith a pollinator.

Ragweed’s wind-pollinatedflowers are small and greenand lack structures thatwould block wind currents.

A

Flowers polli-nated by hum-mingbirds are oftentubular and coloredbright red or yellow butmay have little scent.

C

A butterfly can bea pollinator whenit visits a flower tosip nectar.

B

The ultravioletmarkings of someflowers guidepollinatinginsects to aflower’s nectar.

D

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The pollen that attaches to them canbe carried to another flower, resultingin pollination. Some insects also gatherpollen to use as food. By producingnectar and attracting animal pollina-tors, animal-pollinated plants are ableto promote pollination with lesseramounts of pollen.

Some nectar-feeding pollinators areattracted to a flower by its color orscent or both. Some of the bright, vividflowers attract pollinators, such as but-terflies and bees. Some of these flowershave markings that are invisible to thehuman eye but are easily seen byinsects, as shown in Figure 24.15D.Flowers that are pollinated by beetlesand flies have a strong scent but areoften dull in color.

Many flowers have structural adap-tations that favor cross-pollination—pollination between two plants of thesame species. This results in greatergenetic variation because a spermfrom one plant fertilizes an egg fromanother. For example, the flowers ofcertain species of orchids resemble

female wasps. A male wasp visits theflower and attempts to mate with itand becomes 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 events takeplace before fertilization occurs. Insideeach pollen grain are two haploidnuclei, the tube nucleus and the gener-ative nucleus. The tube nucleus directsthe growth of the pollen tube downthrough the pistil to the ovary, asshown in Figure 24.16. The genera-tive nucleus divides by mitosis, pro-ducing two sperm nuclei. The spermnuclei move through the pollen tubeto a tiny opening in the ovule calledthe micropyle.

Within the ovule is the femalegametophyte. One of the sperm uniteswith the egg forming a diploid zygote,which begins the new sporophytegeneration. The other sperm nu-cleus fuses with the central cell, which


Figure 24.16In flowering plants,part of the male game-tophyte grows throughthe pistil to reach thefemale gametophyte.Double fertilizationinvolves two spermnuclei. A zygote (2n)and endosperm (3n)are formed.

Pollen grain

Stigma

Style

Ovary

Central cell

Ovule

Egg cell

Two spermnuclei

Pollen tube

Tube nucleus

Double Fertilization

One spermfertilizes thecentral cell(3n)

One spermfertilizes theegg cell (2n)

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endosperm fromthe Greek wordsendon, meaning“within,” andsperma, meaning“seed”; The endo-sperm is storagetissue found in theseeds of manyanthophytes.


Ovules

Ovary

Sepals

Fusedpetals

Stamen

FruitFigure 24.17A fruit usually consists of seeds andthe surrounding mature ovary of aflowering plant.

When the eggs in the ovules of a blue-berry flower have been fertilized, thepetals, stamens, and stigma eventuallywither and fall away.

A

The wall of the blueberryflower’s ovary becomes fleshyas the seeds develop.

B

The remains of the sepalsand some dried stamensusually can be seen at thetop of a blueberry fruit.

C

contains the polar nuclei, to form acell with a triploid (3n) nucleus. Thisprocess, in which one sperm fertilizesthe egg and the other sperm joins with the central cell, is called doublefertilization. Double fertilization isunique to anthophytes and is illus-trated in Figure 24.16. The triploidnucleus will divide many times, even-tually forming the endosperm of theseed. The endosperm is food storagetissue that supports development ofthe embryo in anthophyte seeds.

Many flower ovaries contain morethan one ovule. For each ovule tobecome a seed, at least one pollen grainmust land on the stigma for each ovulecontained in the ovary. In a water-melon plant, for example, hundreds ofpollen grains are required to pollinate a

flower if each ovule is to be fertilized.You are probably familiar with the hun-dreds of seeds in a watermelon that arethe 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, as shown inFigure 24.17, help ensure the sur-vival of the next generation.

Seed formationAfter fertilization takes place, seed

development begins. Inside the ovule,the zygote divides and develops into theembryo plant. The triploid central celldevelops into the seed’s endosperm.

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(t)Mark E. Gibson, (bl br)Aaron Haupt


Fleshy fruits are juicy and full of waterand sugars.

B

Greens Keeper

Do you like to workoutside? Is golf your

favorite sport? Then youalready know the value of a well-manicured golffairway. Maintaining golf fairways is one of theresponsibilities of a greenskeeper.

Skills for the JobA greens keeper maintains both

the playing quality and the beauty of a golf course.Beginning greens keepers usually learn on the job and spendtheir days mowing the greens. Greens keepers who want tomanage large crews will need a two- or four-year degree inturf management and a certificate in grounds management.They must be thoroughly familiar with different types ofgrasses, the growing conditions that each require, and thepests, diseases, and environmental factors that can affectthem. Greens keepers also must know what corrective meas-ures to take when something affects the grasses so that fair-ways can be kept in excellent condition. Other careers in turfmanagement include maintaining the grounds of shoppingcenters, schools, sports playing fields, cemeteries, office build-ings, and other locations.

For more careers in related fields, visit

The wall of the ovule becomes theseed coat, which can aid in seed dis-persal and help protect the embryountil it begins growing.

Fruit formationAs the seeds develop, the surround-

ing ovary enlarges and becomes thefruit. Sometimes other flower organsbecome part of the fruit. A fruit is thestructure that contains the seeds of ananthophyte.

A fruit is as unique to an antho-phyte as is its flower, and manyanthophytes can be identified byexamining their fruit. You are familiarwith plants that develop fleshy fruits,such as apples, grapes, melons, toma-toes, and cucumbers. Other plantsdevelop dry fruits such as peanuts,sunflower “seeds,” and walnuts. Indry fruits, the ovary around the seedshardens as the fruit matures. Someplant foods that we call vegetables orgrains are actually fruits, as shown inFigure 24.18.

Figure 24.18 A fruit is usually the ripened ovary ofa flower that can contain one ormore seeds. The most familiar fruitsare those we consume as food.

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

A

ca.bdol.glencoe.com/careers

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http://ca.bdol.glencoe.com/careers

(l)Jerome Wexler/Photo Researchers, (tr)John Sohlden/Visuals Unlimited, (br)Alvin E. Staffan

Can you think of any vegetables thatare actually fruits? For example,green peppers are fleshy fruits thatare often referred to as vegetables.

Seed dispersalA fruit not only protects the seeds

inside it, but also may aid in dispersingthose seeds away from the parentplant and into new habitats. The dis-persal of seeds, as shown in Figure24.19, is important because it reducescompetition for sunlight, soil, andwater between the parent plant and itsoffspring. Animals such as raccoons,deer, bears, and birds help distributemany seeds by eating fruits. They cancarry the fruit some distance awayfrom the parent plant before consum-ing it and spitting out the seeds. Orthey may eat the fruit, seeds and all.Seeds that are eaten usually passthrough the digestive system undam-aged and are deposited in the animal’swastes. Squirrels, birds, and other nutgatherers may drop and lose seedsthey collect, or even bury them only

to forget where. These seeds can thengerminate far from the parent plant.

Plants, such as water lilies andcoconut palms that grow in or nearwater, produce fruits or seeds with airpockets in the walls that enable them tofloat and drift away from the parentplant. The ripened fruits of manyplants split open to release seeds withstructural adaptations for dispersal bywind or by clinging to animal fur.Orchid seeds are so tiny that they canbecome airborne. The fruit of thepoppy flower forms a seed-filled cap-sule that bobs about in the wind andsprinkles its tiny seeds like a salt shaker.Tumbleweed seeds are scattered as theplant rolls along the ground.

Seed germinationAt maturity, seeds are fully formed.

The seed coat dries and hardens,enabling the seed to survive environ-mental conditions that are unfavor-able to the parent plant. The seeds ofsome plant species must germinatewithin a short period of time or die.


Figure 24.19A wide variety of seed-dispersal mechanisms haveevolved among floweringplants.

The ripe pods of violets snap open witha pop, which sends a shower of smallseeds in all directions.

A

Clinging fruits, likethose of the cockleburand burdock, are cov-ered by hooks thatstick to the hair, fur, orfeathers of passinganimals or the clothesof passing humans.

B

Wind-dispersed seeds havestructural adaptations thatenable them to be heldaloft while they drift awayfrom their parent.

C

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(t)Matt Meadows, (b)Jim W. Grace/Photo Researchers

However, the seeds of some plantspecies can remain in the soil untilconditions are favorable for growthand development of the new plant.This period of inactivity in a matureseed is called dormancy. The lengthof time a seed remains dormant canvary from one species to another.Some seeds, such as willow, magnolia,and maple remain dormant for only afew weeks after they mature. These

seeds cannot survive harsh conditionsfor long periods of time. Other plantsproduce seeds, like those shown inFigure 24.20, that can remain dor-mant for remarkably long periods oftime. Even under harsh conditions,the seeds of desert wildflowers andsome conifers can survive dormantperiods of 15 to 20 years. Scientistsdiscovered ancient seeds of the EastIndian Lotus, Nelumbo nucifera, inChina, which they have radiocarbondated to be more than a thousandyears old. Imagine their amazementwhen these seeds 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 water and the presenceof oxygen and favorable temperaturesusually end dormancy, but there maybe 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 rate of respiration inthe plant embryo increases rapidly.

Many seeds germinate best at tem-peratures between 25°C and 30°C. Attemperatures below 0°C or above45°C, most seeds won’t germinate at all.

Some seeds have specific require-ments for germination. For example,some germinate more readily afterthey have passed through the acidenvironment of an animal’s digestivesystem. Others require a period offreezing temperatures, such as apple


Figure 24.20Seeds can remain dor-mant for long periodsof time. Lupine seedslike these can germi-nate after remainingdormant for decades.

Figure 24.21Many wildflower seeds require fire to germi-nate. This is especially true in prairie environ-ments where fires are periodically set to inducethe germination of prairie wildflower seeds.

0646-0663 C24S3 BDOL-829900 8/4/04 12:14 PM Page 654

seeds, extensive soaking in saltwater,such as coconut seeds, or certain daylengths. The seeds of some coniferswill not germinate unless they havebeen exposed to fire. The same is trueof certain wildflower species, includ-ing lupines and gentians, as shown inFigure 24.21.

The germination of a typical dicotembryo is shown in Figure 24.22.Once the seed coat has been softenedby water, the embryo starts to emergefrom the seed. The first part of theembryo to appear is the embryonicroot called the radicle (RA dih kul).The radicle grows down into the soil

and develops into a root. The portionof the stem nearest the seed is calledthe hypocotyl (HI poh kah tul). Insome plants, the first part of the stemto push above ground is an archedportion of the hypocotyl. As thehypocotyl continues growing, itstraightens, bringing with it thecotyledons and the plant’s first leaves.In monocots, the cotyledon remainsbelow the soil’s surface. As growthcontinues, the leaves turn green, andthe plant produces its own foodthrough photosynthesis. To learnmore about germinating seeds, try theMiniLab on page 657.


Figure 24.22Germination of a bean seed is stimulated bywarm temperatures and water, which softensthe seed coat.

The hypocotylis the firstpart of thestem toappear.

B

As new leavesmature, thecotyledonswither andfall away.

D

As thehypocotylstraightens,the plant’sfirst leaves are exposed to sunlight.

C

The radiclewill becomethe primaryroot.

A Radicle

Cotyledon

Seed coat

Primary root

Secondaryroots

Hypocotyl

Epicotyl

Cotyledon

Withered cotyledons

Hypocotyl

Forces exertedby seedlingsWhen a seedlingemerges from aseed, its stempushes upwardthrough the soil.This push or forceresults from waterpressure inside theseedling’s cells.Also, the numberof cells within theseedling increasesas it grows.Therefore, thetotal force exertedon the soil by theseedling increasesas the seedlinggrows.

PhysicalScience

Connection

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OrchidOrchidaceae

GrassPoaceae

LilyLiliaceae

PalmArecaceae

RaspberryRosaceae

ColeusLamiaceae

CarawayApiaceae

ChicoryAsteraceae

CocoaSterculiaceae

CactusCactaceae

MagnoliaMagnoliaceae

OakFagaceae

MilkweedAsclepiadaceae

Dicots

Protists

Monocots

Figure 24.23There are two classes of anthophytes—monocots and dicots. Within each classthere are many different families, whichshow great diversity in their structures.

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David M. Dennis

Vegetative reproductionThe roots, stems, and leaves

of plants are called vegetative struc-tures. When these structures producea new plant, it is called vegetativereproduction. Vegetative reproduc-tion is common among anthophytes.Some modified stems of anthophytes,such as potato tubers, can produce anew plant from each “eye” or bud.Farmers make use of this featurewhen they cut potato tubers intopieces and plant them.

Although cloning animals is a rela-tively new phenomenon, for yearsgardeners have relied on cloning toreproduce plants. Using vegetativereproduction to grow numerousplants from one plant is frequentlyreferred to as vegetative propagation.Some plants, such as begonias, can be propagated by planting cuttings,which are pieces of the stem or a leafthat has been cut off another begonia.Even smaller pieces of plants can be used to grow plants by tissue cul-ture. Minute pieces of plant tissue areplaced on nutrient agar in test tubesor petri dishes. The plants grownfrom cuttings and tissue cultures havethe same genetic makeup as the plantsfrom which they came and are botan-ical clones.

There is great diversity in the flow-ers, seeds, fruits, and vegetative struc-tures of anthophytes. Anthophytes

are divided into families based onthese differences. The relationshipsamong these families are shown inFigure 24.23 on the opposite page.Which of the families is part of yourenvironment?


Understanding Main Ideas1. Explain the relationship between the pollination

of a flower and the production of one or moreseeds.

2. Name the part(s) of an anthophyte flower thatbecomes the fruit.

3. Describe the process of double fertilization inanthophytes.

4. Infer how the production of nectar could enhancethe pollination of a flowering plant.

Thinking Critically5. Sequence the formation of the female gameto-

phyte in a flowering plant using illustrations.

6. Make and Use Tables Make a table that identi-fies whether each organ of a flower is involved inpollination, fruit formation, seed production, orseed dispersal. For more help, refer to Make andUse Tables in the Skill Handbook.


ObserveLooking at Germinating SeedsSeeds are made up of a plant embryo, aseed coat, and in some plants, a food-storage tissue. Monocot and dicot seedsdiffer in structure.

Procedure! Obtain from your teacher a soaked,

ungerminated corn kernel (monocot), a bean seed (dicot), and germinating corn and bean seeds.

@ Remove the seed coats from each of the ungerminatedseeds. Examine the seed’s structures under a microscopeusing low-power magnification. Locate and identify theembryo’s structures and other seed structures.

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

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

and label their structures. Draw the germinating seeds,and label their structures.

2. Distinguish List at least three major differences youobserved in the internal structures of the corn and beanseeds.

Corn seedgermination


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Before You BeginFlowers are the reproduc-tive structures of antho-phytes. They come inmany colors and shapes.Often their colors orshapes are related to themanner in which pollina-tion takes place. Themajor organs of a flowerinclude the petals, sepals,stamens, and pistils. Someflowers are incomplete,which means they do nothave all four kinds oforgans. You will study acomplete flower.

Examining the Organs of a Flower

ProblemWhat do the organs of a flower look like? How are theyarranged?

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

Materialsflower—any complete dicot flower microscope

that is available locally, single-edged such as phlox, carnation, or razor bladetobacco flower coverslips (2)

hand lens (or dissecting microscope) droppercolored pencils (red, green, blue) watermicroscope slides (2) forceps

(or tweezers)

Safety PrecautionsCAUTION: Handle the razor blade with extreme caution.Always cut away from you. Use caution when handling a microscope, slides, and coverslips.

Skill HandbookIf you need help with this lab, refer to the Skill Handbook.

1. Examine your flower. Locate the sepals and petals. Notetheir numbers, size, color, and arrangement. Record thisdata. Remove the sepals and petals from your flower bygently pulling them off the stem.

2. Using the hand lens, locate the stamens, each of whichconsists of a thin filament with an anther on the tip.Note and record the number of stamens.

3. Locate the pistil. The stigma at the top of the pistil isoften sticky. The style is a long, narrow structure thatleads from the stigma to the ovary.

PROCEDUREPROCEDURE

PREPARATIONPREPARATION

Matt Meadows

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4. Place an anther onto a microscope slide and add a drop of water. Cut the anther into several pieces with the razorblade. Place a coverslip over the cut-up anther. CAUTION: Always take care when using a razor blade.

5. Examine the anther under low and high power of your microscope. The small, dotlike structures are pollen grains. Remove the slide from the stage of the microscope.

6. Slice the pistil in half lengthwise with the razor blade.Mount one half, cut side facing up,on a microscope slide.

7. Identify and examine the pistil’sovary with a hand lens or dissectingmicroscope. The many dotlikestructures in the ovary are ovules. A tiny stalk connects each ovule tothe wall of the ovary.

8. Make a diagram of the flower, label-ing all its parts. Color the femalereproductive parts red. Color themale reproductive parts green.Color the remaining parts blue.

9. Clean allequipment as instructed by yourteacher, and return everything to itsproper place. Properly dispose ofcoverslips and flower organs. Washyour hands thoroughly.

CLEANUP AND DISPOSAL

Field Investigation Use a field guide toidentify common wildflowers in your area.Most field guide identifications are made onthe basis of color, shape, numbers, andarrangement of flower parts. If collecting ispermitted, pick a few common flowers topress and make into a display of local flora.

Web Links To find out more about flowers, visit

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Observe How many stamens are present inyour flower? How many pistils, ovaries,sepals, and petals?

2. Compare and Contrast Make a reasonableestimate for your flower of the number ofpollen grains in the anther and the number ofovules in the ovary. Calculate the class aver-age of the estimated number of pollen grainsand estimated number of ovules.

3. Interpret Data Which number is greater?Pollen grains in one anther or ovules in oneovary? Give a possible explanation for youranswer.

(t)Aaron Haupt, (b)Matt Meadows

ca.bdol.glencoe.com/flowers

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http://ca.bdol.glencoe.com/flowers

660 REPRODUCTION IN PLANTSEnvision/Michael Major

Hybrid Plants

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 diseases and pests.

The perfect ear of corn The first step in cre-ating a hybrid is the selection of parent plantswith desirable characteristics. A breeder mightselect a corn plant that ripens earlier in the sea-son, one that can be sown earlier in the springbecause its seeds germinate well in cool, moistsoil, or one with more kernels per ear.

The next step is to grow several self-pollinated generations of each plant to form atrue-breeding line—plants that produce off-spring that always show the desired characteris-tic. To do this, each plant must be preventedfrom cross-pollinating with other corn. Thefemale flowers, called silks, grow on cobs nearthe middle of the corn stalk. The breeder coversthe silks to prevent wind-borne pollen from fer-tilizing them. The pollen-producing male tasselsare removed, and the breeder uses selectedpollen to hand-pollinate each silk.

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.

Plant breeding today Cell culture andgenetic engineering technologies are new plantbreeding techniques. Protoplast fusion removesthe cell walls from the cells of leaves orseedlings, then uses electricity or chemicals tofuse cells of two different species. Some of these

fused cells have been successfully cultured in thelab and grown into adult plants, though nonehave produced seeds.

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

Technician performing hybridization studies

Form an Opinion Scientists have geneticallyengineered hybrid corn to produce an insecticidethat kills one of corn’s most damaging insects, theEuropean corn borer. The hybrid corn produces Btdelta endotoxin. Since its introduction in 1996, Bt-corn has been the subject of much controversy.Research this topic and form an opinion for oragainst the growing of Bt-corn. Create a file of sci-entific articles and information that supports yourposition. Discuss this topic with someone of theopposite opinion.

To find out more about hybrid seeds, visit ca.bdol.glencoe.com/biotechnology

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http://ca.bdol.glencoe.com/biotechnology

Section 24.1Vocabularymegaspore (p. 638)micropyle (p. 639)microspore (p. 638)protonema (p. 635)vegetative reproduction

(p. 634)

Vocabularyanther (p. 641)day-neutral plant

(p. 645)long-day plant (p. 644)ovary (p. 643)petals (p. 641)photoperiodism (p. 644)pistil (p. 643)sepals (p. 641)short-day plant (p. 644)stamen (p. 641)

Vocabularydormancy (p. 654)double fertilization

(p. 651)endosperm (p. 651)germination (p. 654)hypocotyl (p. 655)polar nuclei (p. 648)radicle (p. 655)

Flowers andFlowering

The Life Cycleof a FloweringPlant

STUDY GUIDESTUDY GUIDE

CHAPTER 24 ASSESSMENT 661

Life Cycles ofMosses, Ferns,and Conifers

Section 24.3

Section 24.2

To help you review the lifecycles of mosses, ferns, and conifers, use theOrganizational Study Fold on page 633.

Key Concepts■ The male gametophyte develops from a

microspore in the anther. The femalegametophyte develops from a megasporein the ovule.

■ Double fertilization occurs when a spermnucleus joins with an egg to form a zygote.The second sperm nucleus joins the centralcell to form endosperm.

■ Fruits and seeds are modified for dispersal.Seeds can stay dormant for a long timebefore they germinate.

Key Concepts■ The gametophyte generation is dominant

in mosses. Archegonia and antheridia formon separate or the same gametophyte. Fer-tilization requires a film of water on thegametophyte.

■ Archegonia and antheridia develop on aprothallus, the fern gametophyte. Fertili-zation requires a film of water on the gametophyte. The sporophyte generationis dominant in ferns.

■ Conifers have cones in which a male orfemale gametophyte forms. Sperm nucleiform in pollen grains and eggs form inovules. The embryo is protected in a seed.

Key Concepts■ Flowers are made up of four organs: sepals,

petals, stamens, and pistils. A flower lack-ing any organ is called incomplete. Com-plete flowers have all four organs.

■ Photoperiodism—responses of floweringplants to daylight-darkness conditions—affects flower production. Plants are calledshort-day, long-day, day-neutral, or inter-mediate depending upon their photoperi-odic response.

(t)Noble Proctor/Photo Researchers, (c)Aaron Haupt, (b)Runk-Shoenberger/Grant Heilman Photography

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662 CHAPTER 24 ASSESSMENT

Review the Chapter 24 vocabulary words listed inthe Study Guide on page 661. Match the wordswith the definitions below.

1. in seed plants, the opening in the ovulethrough which the pollen tube enters

2. male reproductive organ of a flower thatconsists of an anther and a filament

3. the first part of the stem to push aboveground from a germinating seed

4. in mosses, the small green filament of hap-loid cells that develops from a spore

5. plants that produce flowers only after expo-sure to short nights

6. Moss gametophytes are ________ and formgametes by ________.A. diploid—meiosis C. diploid—mitosisB. haploid—mitosis D. haploid—meiosis

7. Which of the following structures candevelop as part of an anthophyte?A. C.

B. D.

8. The endosperm of an anthophyte seed is________.A. haploid C. triploidB. monoploid D. diploid

9. Producing a new plant from part of anotherplant is ________.A. photoperiodismB. double fertilizationC. vegetative reproductionD. germination

10. A flower that naturally has eight petals andfour sepals and four pistils is called ________.A. asexual C. completeB. incomplete D. partial

11. The male gametophyte of a conifer is calleda ________.A. pollen grain C. coneB. needle D. cotyledon

12. Meiosis begins the ________ stage in aplant’s life cycle.A. bryophyte C. sporophyteB. pterophyte D. gametophyte

13. Ferns produce male gametes in ________.A. antheridia C. rhizomesB. protonemas D. archegonia

14. Open Ended Many animals, includinghumans, eat seeds such as peas, beans,peanuts, and almonds. Why are seeds a goodfood source?

15. Open Ended How does dormancy help thesurvival of a plant species in a desert biome?

16. Open Ended The root is usually the firstorgan to emerge from a germinating seed.Evaluate how this benefits the plant morethan if another organ emerged first.

17. Predict You are given three seeds. One isprickly, another has a winglike structure, andthe third is tiny. How might each of theseeds be dispersed?

18. The U.S. Fishand Wildlife Service has a list of endan-gered or threatened flowering plant species.Visit to investigatefive plants on this list. Determine why theyare considered endangered and what isbeing done to preserve them. What mightresult if one of your plants became extinct?Prepare a poster or give multimedia presen-tation of your results to your class.

REAL WORLD BIOCHALLENGE

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CHAPTER 24 ASSESSMENT 663

0

50

100

Perc

ent

germ

inat

ion

Exposure level

Control High Medium Low

Germination of Beans AfterExposure to Radiation

Nu

mb

er o

f ca

tcla

w f

ruit

s p

er p

lan

t

Severity of infection

0

30

60

90

120

150

Mistletoe Infection andCatclaw Fruit Production

Light Moderate Severe

Multiple ChoiceDesert mistletoe grows on a desert plant namedcatclaw, Acacia greggii. Predict a trend from thedata to answer question 19. Analyze the graphand answer questions 20 and 21.

19. The data was collected during a drought.When drought is not severe, mistletoe infes-tation does not reduce fruit production. Agraph of this situation would show ________.A. bars unequal in heightB. bars much lower than the bars in the

graph aboveC. two high bars and one low barD. all bars the same height

20. As infection of catclaw becomes moresevere, ________.A. fruit production increasesB. fruit production decreasesC. fruit production remains unchangedD. catclaw plants die

21. If the rate of infestation by mistletoe of cat-claw increases over the years, catclaw num-bers may ________.A. decrease C. remain the sameB. increase D. decrease, then increase

Analyze and evaluate the graph below to answerquestions 22–24.

22. Which group of beans had the highest per-centage of germination?A. control C. medium-exposure levelB. high-exposure D. low-exposure level level

23. As the radiation dose increases, germination ________.A. increases C. stopsB. decreases D. is not affected

24. When beans are given a low dose of radiation, about ________ germinate.A. 25 percent C. noneB. 50 percent D. 100 percent

Constructed Response/Grid InRecord your answer on your answer document.

25. Open Ended Plant geneticists have learned that about one-third of the genes in two differentplant genomes are not found in any sequenced fungus or animal genome. Infer why plants havemany genes that do not have counterparts in the fungus kingdom or the animal kingdom.

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Alternation of Generations

The leafy moss gametophyteis haploid. The spore stalksand capsules are diploid.

664

Plants Plants provide food and shelter for most of Earth’s

organisms. Through the process of photosynthesis, they transform the radiant energy in light into chemicalenergy in food and release oxygen. All plants are multi-cellular eukaryotes whose cells are surrounded by a cellwall made of cellulose.

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

A spore is a haploid (n) reproductive cell, producedby meiosis, which can withstand harsh environmen-tal conditions. When conditions become favorable,a spore can develop into the haploid, gameto-phyte generation of a plant.

Mosses, Liverworts, and Hornworts Bryophyta (mosses), Hepaticophyta (liverworts),

and Anthocerophyta (hornworts) are divisions ofnon-seed nonvascular plants that live in cool,moist habitats. These plant groups have no vascu-lar tissues to move water and nutrients from onepart of the plant to another. They usually grow nomore than several centimeters tall.

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

division Lycophyta. They are found primarily inmoist environments and are usually only a fewcentimeters high. Fossil lycophytes grew as high as30 m and formed a large part of the vegetation ofPaleozoic forests.

Mosses often grow in masses thatform thick carpets.

The life cycles of plants have two stages, called generations. Thediploid (2n) stage is called the sporophyte generation. It produces

haploid (n) spores that develop into the gametophyte generation. Thegametophyte generation produces male and female gametes that canunite and begin a new sporophyte generation.

In nonvascular plants, the sporophyte is smaller than the gameto-phyte, and usually remains attached to the gametophyte. The nonvascu-lar sporophyte is dependent upon the nonvascular gametophyte forwater and its nutrition. In vascular plants, the sporophyte is dominantand independent of the gametophyte. Vascular gametophytes areminute and usually are buried in soil or are enclosed within the vascularsporophyte.

(t)Jan & Peter Lahall/Peter Arnold, Inc., (b)Ed Reschke/Peter Arnold, Inc.

0664-0669 U7 BD/Rev BDOL-829900 8/4/04 2:17 PM Page 664

Plants

PLANTS 665

ConifersExamples: Pine, spruce, fir, larch, yew, red-wood, juniper.Plant giants: Giant sequoias of central Cali-fornia, to 99 m tall, the most massive organ-isms in the world; coast redwoods of California,to 117 m, the tallest trees in the world.

Fern fronds are sometimes divided into leaflets.

Horsetails Horsetails are non-seed vascular plants in the

division Arthrophyta. They are common in areaswith damp soil, such as stream banks and some-times along roadsides. Present-day horsetails aresmall, but their ancestors were treelike.

Ferns Ferns, division Pterophyta, are diverse non-seed

vascular plants. They have leaves called fronds thatgrow up from an underground stem called the rhi-zome. Ferns are found in many different habitats,including shady forests, stream banks, roadsides,and abandoned pastures.

Seed Plants A seed is a reproductive structure that contains

a sporophyte embryo and a food supply that areenclosed 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.

Cycads, Ginkgoes, and Joint FirsPlants in divisions Cycadophyta (cycads),

Ginkgophyta (ginkgoes), and Gnetophyta (jointfirs), along with those in Coniferophyta, are some-times called gymnosperms. Seeds of these plants

are not part of a fruit. Their male and femalereproductive organs are in separate structures.Seeds develop in the female reproductive struc-ture. These plants have different forms and growin diverse environments.

Conifers Conifers, division Coniferophyta, produce seeds,

usually in woody strobili called cones, and haveneedle-shaped or scalelike leaves. Most conifersare evergreen plants, which means they bearleaves all year round.

Conifers are common in cold or dry habitats.Their needles have a compact shape and a thick,waxy covering that helps reduce water loss.Conifer stems are covered with a thick layer ofbark that insulates the tissues inside. These struc-tural adaptations enable conifers to survive belowfreezing temperatures.

Seeds of conifersdevelop at thebase of eachwoody scale of female cones.

(l)Rod Planck/Tom Stack & Associates, (r)Pat & Tom Leeson/Photo Researchers


Flowering Plants The flowering plants, division Anthophyta, form

the largest and most diverse group of plants onEarth today. They provide much of the food eatenby humans. Anthophytes produce flowers anddevelop seeds that are part of a fruit.

Monocots and Dicots The anthophytes are in two classes: the mono-

cotyledons and the dicotyledons. Cotyledons, or“seed leaves,” are part of the seed along with theplant embryo. Monocots have one seed leaf anddicots have two seed leaves.

Flowers Flowers are the organs of reproduction in

anthophytes. The pistil is the female reproductiveorgan. At the base of the pistil is the ovary. Insidethe ovary are the ovules. Ovules contain thefemale gametophyte. A female gamete—an eggcell—forms in each ovule. The stamen is the malereproductive organ of a flower. Pollen grains that

form inside the anthereventually contain malegametes called sperm.

Moving from Water to Land

Stigma

Style

Ovary

Pistil

Petals

Sepal Peduncle

Stamen AntherFilament

Plants probably evolvedfrom filamentous green algae.

Plants

666

Monocots include grasses, orchids(left photo), and palms. Dicotsinclude many flowering trees(right photo) and wildflowers.

LM Magnification: 400�

A ll plants probably evolved from filamentous green algaethat lived in nutrient-rich waters of Earth’s ancient oceans.

Water and dissolved minerals can diffuse directly into the cellsof an ocean-dwelling alga. As land plants evolved, new struc-tures developed that allowed water and dissolved minerals to betaken in from the environment and to be transported to allparts of the plant.

Nonvascular plants In nonvascular plants, water and nutri-ents travel from one cell to another by the processes of osmosisand diffusion. As a result, nonvascular plants are limited tomoist environments.

(tl)Richard & Susan Day/Earth Scenes, (tr)Jerome Wyckoff/Earth Scenes, (b)Manfred Kage/Peter Arnold, Inc.


Plants

667

Water

Water lostthrough leaves

Sourceof sugars

Sink

Vascular plants The stems of most plantscontain vascular tissues made up of tubelike,elongated cells through which water, food, andother materials move from one part of the plantto another. One reason vascular plants can growlarger than nonvascular plants is that vasculartissue allows a more efficient method of internaltransport than osmosis and diffusion. In addi-tion, vascular tissues include thickened fibersthat can support upright growth.

An unbroken column of watertravels from the roots in xylemtissues. Sugars formed by pho-tosynthesis travel throughoutthe plant in phloem tissues.

Pollen In seed plants, the sperm develop inside of

the thick-coated pollen grains. Pollen is animportant structural adaptation that hasenabled seed plants to live in diverse land habi-tats. Pollination is the transfer of pollen grainsfrom the anther to the stigma of the pistil.

Pollinators Pollen can be transferred by wind, insects, birds,

and even bats. Some flowers have colorful or per-fumed petals that attract pollinators. Flowers alsocan contain sweet nectar, as well as pollen, whichprovides pollinators with food.

FruitFollowing fertilization, a fruit with seeds can

develop. Fruits help protect seeds until they aremature. Some flowering plants develop fleshyfruits, such as apples, melons, tomatoes, or squash.Other flowering plants develop dry fruits, such aspeanuts, walnuts, or sunflowers. Fruits also canhelp disperse seeds.

Plant Responses Plants respond to stimuli in their environment

such as light, temperature, and water availability.Chemicals called hormones control some of theseresponses by increasing cell division and growth.

Plants that dependon the wind to carrypollen from anther tostigma tend to havesmall, inconspicuousflowers. The pollen ofcorn is carried by thewind from these maleflowers to femaleflowers that make upan ear of corn.

The phototropic responseshown here is the resultof increased cell growthon the side of the stemaway from the light.

(l)Larry Lefever/Grant Heilman Photography, (r)Jerome Wexler/Photo Researchers


668 UNIT 7 STANDARDIZED TEST PRACTICE

Multiple Choice

Blueberry plants grow best and have optimal berryproduction in soils that have a pH range of 4.0–5.2.Recall that a substance with a pH less than 7 iscalled acidic. Scientists tested the soils on 104 com-mercial blueberry farms, then compiled their data.Analyze the graph of their data below to answerquestions 1 and 2.

1. Which of the following statements bestdescribes the soils at most blueberry farms?A. within the pH range for optimal berry

productionB. lower than the pH range for optimal berry

productionC. higher than the pH range for optimal berry

productionD. both lower and higher than the pH range

for optimal berry production

2. What percentage of the blueberry farms havesoils too acidic for optimal berry production?A. 57%B. 35%C. 100%D. 9%

Study the graph below about seed productionresponses of purple needlegrass in California toanswer questions 3–5.

3. The largest seed mass of purple needlegrassoccurs when the habitat is ________.A. grazed and unburnedB. ungrazed and unburnedC. grazed and burnedD. ungrazed and burned

4. The smallest seed mass of purple needle grassoccurs when the habitat is ________.A. grazed and unburnedB. ungrazed and unburnedC. grazed and burnedD. ungrazed and burned

5. If you wanted to grow purple needlegrass withseeds with larger mass, how would you manageyour land?A. graze onlyB. burn onlyC. graze and burnD. neither graze nor burn

See

d m

ass

(mg

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Burned

Seed Production of Purple Needlegrass

Grazed Ungrazed Grazed Ungrazed

Unburned

Nu

mb

er o

f fa

rms

Soil pH

Soil pH at Blueberry Farms

2

4

6

8

10

12

03.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4

Plan Your Work and Work Your Plan

Set up a study schedule for yourself well inadvance of your test. Plan your workload sothat you do a little each day rather than a lotall at once. The key to retaining information isto repeatedly review and practice it. There istruth in the saying, “Use it or lose it.”

Standards Practice


UNIT 7 STANDARDIZED TEST PRACTICE 669

The roots of some nonnative plants produce chemi-cals that interfere with the growth of nativegrasses. Analyze the graph below to answer ques-tions 6 and 7.

6. In this study, the effect of carbon on nativeplants ________.A. is somewhat beneficialB. is harmfulC. completely counteracts the effect of the

chemical given off by the roots of the non-native plants

D. cannot be determined by analyzing thegraph

7. When native grasses are grown with nonnativeplants nearby and carbon is not added to the soil,the biomass of the native grasses is ________.A. less than grasses grown without the

nonnative plants nearby regardless ofwhether carbon is added to the soil

B. greater than without nonnative plants nearby

C. not affected by the addition of carbon tothe soil

D. less than without nonnatives nearby, butgreater than when carbon is added to the soil

8. Mosses, ferns, and club mosses are alikebecause they ________.A. require water for fertilizationB. have adaptations for conserving waterC. need insects for pollinationD. grow best in warm, sunny, and dry habitats

9. Club mosses, horsetails, and conifers havespecialized leaves that form reproductivestructures known as ________.A. sori C. protonemaB. flowers D. strobili

Bio

mas

s (g

)

2

1

0

No nonnatives

With nonnatives, no carbon added to soilWith nonnatives, carbon added to soil

Native Grass Biomass

Constructed Response/Grid InRecord your answers or fill in the bubbles on your answer document using the correct place value.

10. Open Ended Snow buttercups in the alpine tundra have flowers that orient themselves toward andtrack the apparent movement of the sun across the sky. Biologists hypothesized that this response was stimulated by the blue wavelength component of light. It is known that red filters can block bluewavelengths. Describe an experiment that would test this hypothesis.

11. Open Ended Why do vascular plants have an adaptive advantage over nonvascular plants?

12. Open Ended Using the table to the right, choose twogeographic areas and infer why the number of seedplants in the two areas differ.

13. Grid In A change in pH of one represents a 10-timeschange in the hydrogen ion concentration of a sub-stance. A change in pH of two represents a 100-timeschange in the hydrogen ion concentration of a sub-stance. Using the graph about soil pH on page 668 forquestions 1 and 2, find to the nearest whole number the change in soil pH from the lowest to the highest recorded number. This number represents a ________-times change in hydrogen ionconcentration.

Estimated Number of Seed Plants in Geographical Areas

Geographic Area Estimated Number

Europe 13 638

Africa 74 232

Australia/ New Zealand 81 876

Pacific Islands 10 758

Northern America 34 455

Southern America 115 242

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Documents

Chapter 24: Reproduction in Plants - CoconinoHighSchool · on reproduction in plants ... vegetative reproduction protonema megaspore microspore ... gametophyte is the familiar form,