22-1 Seed Plants—The SpermopsidaGuide For Readingm In what ways are seed plants able to

survive on land?

ii What are the functions of roots, stems,and leaves?

11 In what ways are plants adapted toreproduce on land?

Compared to life in water, life on land offers several bene¬fits to plants. Life on land provides abundant sunlight for pho¬tosynthesis. On land there is continuous free movement ofgaseous carbon dioxide and oxygen, which plants use duringphotosynthesis and respiration.

But life on land also presents significant problems toplants. Water and nutrients are available to most land plantsonly from the soil. On land, dry air draws water from exposedplant tissues by the process of evaporation. On land, photosyn-thetic tissues must be held upright to capture sunlight. And un¬like the reproductive cycles of mosses and ferns, the reproduc¬tive cycles of most land plants must work without standingwater. &

Seed Plants—Designed for Life on LandSeed plants, members of the subphylum Spermopsida,

exhibit numerous adaptations that allow them to survive thedifficulties of life on land. Note that seed plants did not evolve

CHAPTER

GUIDE FOR READING

Journal Activity

Plantswith

Seeds

try to imagine what life would be like without plants. It's a rather

difficult image to conjure up, especially because without plants there

would be no animals. Almost every animal on the face of the Earth

ultimately depends on food produced by plants. And just as

importantly plants shape environments in which animals live.

Humans and other land animals are able to benefit from plants

only because members of one certain plant group have evolved in

ways that allow them to live in a variety of different places. Most

mosses and ferns cannot survive in many habitats because they

need an almost constant supply of water. But seed plants—which

include nearly all the plants you encounter—have, as a result of

many evolutionary changes, been freed from dependence on water.

It Is this evolutionary story that you will uncover In this chapter.

After you read the followingsections, you will be able to

22-1 Seed Plants—The Spermopsida

• Describe several adaptations ofseed plants to life on land.

• Identify the functions of roots,stems, and leaves.

• Explain why reproduction in seedplants is not dependent uponwater.

22-2 Evolution of Seed Plants• Describe the evolution of seed

plants.

• List several characteristics ofgymnosperms and angiosperms.

• Compare monocots and dicots.

22-3 Coevolution of FloweringPlants and Animals

• Describe the process ofpollination in seed plants.

• Explain plant-animal coevolution.• Discuss the importance of seed

dispersal to the success of theseed plants.

YOU AND YOUR WORLDPoets have long written about thebeauty of plants. Why don't you try,too? Write a short poem about aflower, tree, or other plant you seeon the way to school each day.

Figure 22-1 Fields of sunflowersfollow the daily movement of thesun. Here thousands of plantsgrow in conditions that are quitefavorable. But plants often growin less hospitable places, such asa tiny crack in the surface of aroad.

This bee is busy gathering nectar from flowers. Pollenproduced by the flowers sticks to the bee's body. Anoak tree produces more than enough acorns to satisfyhungry squirrels and more than enough to producenew oaks.

Figure 22-2 Roots, such as theseof a corn plant, anchor the plantin the soil (top). The stem of thewhite pine is strong enough tosupport the plant for many metersabove the ground (bottom, left).The leaves of most plants aregreen, the color of chlorophyll.However, leaves such as those ofthe brilliantly colored croton oftenshow other colors besides green(bottom, right).

these adaptations because they "wanted" to or because theprocesses of evolution somehow "knew" that such adaptationswould be useful on dry land. Rather, in every generation ofplants the types of genetic variations we discussed in earlierchapters produced individuals with different characteristics.Over time, those individuals with characteristics best suitedto their environments survived and produced offspring.

In this way, over hundreds of millions of years, the ances¬tors of seed plants evolved a variety of new adaptations thatenabled them to survive in many places in which mosses andferns could not. These ancient plants evolved well-developedvascular tissues that conduct water and nutrients betweenroots and leaves. They evolved roots, stems, leaves, and struc¬

tures that enable them to live everywhere—from frigid moun¬tains to scorching deserts. And, seed plants, as their nameimplies, evolved seeds—the key adaptation in a new form ofsexual reproduction that does not require standing water. Letus briefly examine these adaptations one at a time.

Roots, Stems, and LeavesJust like the cells in your body, the cells in a plant are or¬

ganized into different tissues and organs. The three mainorgans in a plant are roots, stems, and leaves. Each organshows adaptations that make the plant better able to survive.

ROOTS Roots perform several important functions. Theyabsorb water and dissolved nutrients from moist soil. They an¬chor plants in the ground. Roots also hold plants upright andprevent them from being knocked over by wind and rain. Rootsare able to do all these jobs because as they grow, they developcomplex branching networks that penetrate the soil and grow

between soil particles.

STEMS Stems hold a plant's leaves up to the sun. Al¬though plenty of sunlight reaches the Earth, plants compete

468

with one another for this solar energy. Many plants have tallstems and branches that reach above other plants aroundthem. To support such tall plants, stems must be very sturdy.

LEAVES Leaves are the organs in which plants capturethe sun's energy—a process vital to photosynthesis. Leavesevolved because plants that had broad, flat surfaces over whichto spread their chlorophyll were able to capture more solar en¬ergy than plants that did not have such surfaces. So over time,in most habitats, plants with leaves had higher fitness—andproduced more offspring—than plants without leaves. Butthose broad, flat leaves also exposed a great deal of tissue tothe dryness of the air. These tissues must be protected againstwater loss to dry air. That's why most leaves are covered with awaxy coating called the cuticle. Because water cannot passthrough the cuticle, this coating slows down the rate of evapo¬ration of water from leaf tissues. Adjustable openings in the cu¬ticle help conserve water while allowing oxygen and carbondioxide to enter and leave the leaf as needed.

Vascular TissueAs plants evolved longer and longer stems, the distance be¬

tween their leaves and roots increased. The leaves of a tall treemight be 100 meters above the ground. Thus tall plants face animportant challenge: Water must be lifted from roots to leaves,and compounds produced in leaves must be sent down toroots. Over time, the evolutionary forces of variation, chance,and natural selection produced a well-developed vascular sys¬tem. This remarkable two-way plumbing system consists of twokinds of specialized tissue: xylem and phloem.

XYLEM Xylem is the vascular tissue primarily responsi¬ble for carrying water and dissolved nutrients from the roots tostems and leaves. Because xylem cells often have thick cellwalls, they also provide strength to the woody parts of largeplants such as trees. Oddly enough, most xylem cells grow tomaturity and die before they function as water carriers.

PHLOEM Phloem tissue carries the products of photo¬synthesis and certain other substances from one part of theplant to another. Whereas xylem cells conduct water in onlyone direction (upward), phloem cells can carry their contentseither upward or downward. Unlike xylem cells, functioningphloem cells are alive and filled with cytoplasm.

Reproduction Free from WaterLike other plants, seed plants have alternation of genera¬

tions. However, the life cycles of seed plants are well adaptedto the rigors of life on land. All of the seed plants you see

Figure 22-3 Growing tall can bean advantage to a plant'ssurvival. Tall plants receive moreof the sun's light and are lesslikely to be shaded by otherplants. Vascular tissues transportwater from the roots to leaves atthe tallest part of a plant.

around you are members of the sporophyte generation. Bycomparison, the gametophytes of seed plants are tiny, consist¬ing of only a few cells. This size difference can be seen as thefinal result of an evolutionary trend in plants in which the ga-metophyte becomes smaller as the sporophyte becomes larger.

FLOWERS AND CONES The tiny gametophytes of seedplants grow and mature within the parts of the sporophyte wecall flowers and cones. Flowers and cones are special repro¬ductive structures of seed plants, which we shall discuss later.Because they develop within the sporophyte plant, neither thegametophytes nor the gametes need standing water to function.Thus the special reproductive structures of seed plants(flowers and cones) can be considered important adaptationsthat have contributed to the success of these plants.

POLLINATION The entire male gametophyte of seedplants is contained in a tiny structure called a pollen grain.Sperm produced by this gametophyte do not swim throughwater to fertilize the eggs. Instead, the entire pollen grain iscarried to the female gametophyte by wind, insects, birds,small animals, and sometimes even by bats. The carrying ofpollen to the female gametophyte is called pollination. Pollina¬tion is an important process that we shall discuss shortly.

SEEDS Seeds are structures that protect the zygotes ofseed plants. After fertilization, the zygote grows into a tinyplant called an embryo. The embryo, still within the seed,stops growing while it is still quite small. When the embryobegins to grow again later, it uses a supply of stored food insidethe seed. A seed coat surrounds the embryo and protects itand the food supply from drying out. Inside the seed coat, theembryo can remain dormant for weeks, months, or even years.Seeds can survive long periods of bitter cold, extreme heat, ordrought—beginning to grow only when conditions are onceagain right. Thus the formation of seeds allows seed plants tosurvive and increase their number in habitats where mossesand ferns cannot.

OO SECTIONLL" 1 REVIEW

1. What are three adaptations of seed plants that enablethem to live on land?

2. What are the functions of roots, stems, and leaves?

3. How are xylem and phloem tissues similar? How are theydifferent?

4. Connection—You and Your World What is a seed?What are two ways seeds provide food for people?

Figure 22-5 Seeds are a promiseand a plant's insurance. A seedcontains the promise of a plant tocome and the insurance that aspecies will have a chance tosurvive.

Figure 22-4 Texas bluebonnetsare a wildflower that grows inhuge numbers. Flowers are aplant's reproductive structures.

470

22-2 Evolution of Seed PlantsThe history of plant evolution is marked by several great

adaptive radiations. Each time a group of plants evolved auseful new adaptation (such as vascular tissue or seeds),that group of plants gave rise to many new species. Becauseof the new adaptation, some new species were able to survivein previously empty niches. For other new species, the new ad¬aptation made them better suited to their environments thanexisting species that did not possess the new adaptation. Overtime, the better adapted species survived and the older speciesbecame extinct.

It is important to remember that Earth's environments didnot remain constant through time. Over a period of millions ofyears *landmasses moved and mountain ranges rose. In some

cases, plant species produced by an adaptive radiation contin¬ued to evolve in ways that enabled them to survive as their en¬vironment changed. Such species survived for long periods. Inother cases, plant species could not survive changing environ¬ments. These species became extinct.

Mosses and ferns, for example, underwent major adaptiveradiations during the Devonian and Carboniferous periods, 300to 400 million years ago. During these periods, land environ¬ments were much wetter than they are today. Tree ferns, treelycopods, and other spore-bearers grew into lush forests thatcovered much of the Earth.

But over a period of millions of years, continents becamemuch drier, making it harder for spore-bearing plants to sur¬vive and reproduce. For that reason, many moss and fern spe¬cies became extinct. They were replaced by seed plants whoseadaptations equipped them to deal with drier conditions. Tohelp you understand how seed plants became successful, weshall now trace the evolution of these fascinating organisms.

Guide For Reading¦ How do useful adaptations give

rise to new plant species?¦ What are some characteristics

of gymnosperms?

¦ What are some characteristicsof angiosperms?

_ How do monocots differ fromdicots?

Figure 22-6 Seed ferns are partof the fossil record. Theyrepresent a link between fernsthat do not form seeds and seedplants that do. This ancient planthad leaves that resemble theleaves of modern ferns.

Seed FernsThe first seed-bearing plants, which appeared during the

Devonian Period, resembled ferns. But these plants were differ¬ent from ordinary ferns in one very important respect: They rp-Pj^uced4»yjsing s,eeds instead,of spores. Fossils of theseso-called seed ferns document several evolutionary stages inthe development of seed plants. Although seed ferns were quitesuccessful for a time, they were rapidly replaced by other plantspecks. Today, no seed ferns survive.

GymnospermsThe most ancient surviving seed plants belong to three

classes; the Cycadae, Ginkgoae, and Coniferae. In plants ofthese classes, a number of leaves have evolved into specializedmale and female reproductive structures called scales. Scales

Figure 22-7 Confusingly namedthe sago palm, this cycad is not apalm at all (left). Cycads growprimarily in warm and temperateareas. Cycads producereproductive structures that looklike giant pine cones (right).

Figure 22-8 The ginkgo is oftenplanted on city streets because itcan tolerate the air pollutionproduced by city traffic.

are grouped into larger structures called male and femalecones. Male cones produce male gametophytes called pollen.Female cones produce female gametophytes called eggs. Later,the female cones hold seeds that develop on their scales. Eachseed is protected by a seed coat, but the seed is not covered bythe cone. Because their seeds sit "naked" on the scales,cycads, ginkgoes. and conifers are called naked seed plants, orgymnosperms (gymno- means naked; -sperm means seed).

CYCADS Cycads are beautiful palmlike plants that firstappear in the fossil record during the Triassic Period, 225 mil¬lion years ago. Huge forests of cycads thrived when dinosaursroamed the Earth. Many biologists think that some species ofdinosaurs ate the young leaves and seeds of cycads. Today,only nine genera of cycads, including the confusingly namedsago palm, remain. Cycads can be found growing naturally intropical and subtropical places such as Mexico, the WestIndies, Florida, and parts of Asia, Africa, and Australia.

GINKGOES Ginkgoes were common when dinosaurs werealive, but today only a single species, Ginkgo biloba, remains.The living ginkgo species looks almost exactly like its fossil an¬cestors, so it is truly a living fossil. In fact, Ginkgo biloba maybe the oldest seed plant species alive today. This single speciesmay have survived only because the Chinese have grown it intheir gardens for thousands of years.

Conifers: Cone BearersConifers, commonly called evergreens, are the most abun¬

dant gymnosperms today. They are also the most familiar andimportant. Pines, spruce, fir, cedars, sequoias, redwoods, andyews are all conifers. Some conifers, such as the dawn red¬wood, date back 400 million years to the Devonian Period-well before the time of the cycads. But although other classesof gymnosperms are largely extinct, conifers still cover vast

-i

areas of North America, China, Europe, and Australia. Conifersgrow on mountains, in sandy soil, and in cool moist areas alongthe northeast and northwest coasts of North America. Someconifers live more than 4000 years and can grow more than100 meters tall.

ADAPTATIONS The leaves of conifers are long and thin,and are often called needles. Although the name evergreen iscommonly used for these plants, it is not really accurate be¬cause needles do not remain on conifers forever. A few speciesof conifers, like larches and bald cypresses, lose their needlesevery fall. The needles of other conifer species remain on theplant for between 2 and 14 years. These conifers seem as if theyare "evergreen" because older needles drop off gradually allyear long and the trees are never completely bare.

REPRODUCTION Like other gymnosperms, most conifersproduce two kinds of cones. The scales that form these conescarry structures called sporangia that produce male and femalegametophytes. Both male and female gametophytes are verysmall. Male cones, called pollen cooes, produce male gameto¬phytes in the form of pollen grains. Female cones, called seedcones, house the female gametophytes that produce ovules.Some species of conifers produce male and female cones onthe same plant, whereas other species have separate male andfemale plants.

Each spring, pollen cones release millions of dustlike pol¬len grains that are carried by the wind. Many of these pollengrains fall to the ground or land in water and are wasted. Butsome pollen grains drift onto seed cones (female cones), wherethey may be caught by a sticky secretion. When a pollen grainlands near a female gametophyte, it produces sperm cells bymitosis. These sperm cells burst out of the pollen grain and fer¬tilize ovules. After fertilization, zygotes grow into seeds on thesurfaces of the scales that make up the seed cones. It may takemonths or even years for seeds on the female cones to mature.In time, and if they land on good soil, the mature seeds may de¬velop into new conifers.

Angiosperms: Flowering PlantsAngiosperms are the flowering plants. All angiosperms re¬

produce sexually through their flowers in a process that in¬volves pollination. Unlike the seeds of gymnosperms, the seedsof angiosperms are not carried naked on the flower parts. In¬stead, angiosperm seeds are contained within a protective wallthat develops into a structure called a fruit. The scientific termfruit refers not only to the plant structures normally calledfruits but also to many structures often called vegetables. Thus,by definition, apples, oranges, beans, pea pods, pumpkins, to¬matoes, and eggplants are all fruits.

Figure 22-9 Pine cones may beeither male or female. Male cones(top) produce wind borne pollenthat is carried to female cones(bottom). Female cones nurtureand protect the developing seeds,which often take two years tomature.

Figure 22-10 These pear flowersare a form of floral advertisingthat attracts bees and otherinsects. The insects pollinate theflowers. Six weeks afterpollination has occurred, thedeveloping pears are still quitesmall. In time they will ripen.

Today, angiosperms are the most widespread of all landplants. More than a quarter of a million species of angiospermslive everywhere from frigid mountains to blazing deserts, fromhumid rain forests to temperate backyards near your home.Some angiosperms even live under water. Different species ofangiosperms have evolved specialized tissues that allow themto survive extreme heat and cold, as well as long periods ofdrought.

Angiosperms can be separated into two subclasses: theMonocotyledonae (mahn-oh-kaht-'l-EED-'n-ee), called mono-cots for short, and the Dicotyledonae (digh-kaht-'I-EED-'n-ee),called dicots for short. The monocots include corn, wheat,lilies, daffodils, orchids, and palms. The dicots include plantssuch as roses, clover, tomatoes, oaks, and daisies.

There are several differences between monocots anddicots. The simplest difference has to do with the number ofleaves the embryo plant has when it first begins to grow, orgerminate. The leaves of the embryo are called cotyledons, orseed leaves. Monocotyledons have one seed leaf (mono- meansone). Dicotyledons start off with two seed leaves (di- meanstwo). In some species, cotyledons are filled with food for thegerminating plant. In other species, the cotyledons are the firstleaves to carry on photosynthesis for the germinating plant.

Figure 22-12 shows several characteristics of monocotsand dicots. These differences are summarized below:

Figure 22-11 Flowers can varyin appearance. This orchid floweris colorful and has petals andsepals of different shapes.

Veins in monocot leaves usually lie parallel to one another.Veins in dicot leaves form a branching network.In monocot flowers, petals and other flower parts areusually found in threes or multiples of three (3, 6, 9, andso on). In dicot flowers, petals and other flower partsoccur in fours or fives or in multiples of four (4, 8, 12) orfive (5, 10, 15).In monocot stems, xylem and phloem tissues are gatheredinto vascular bundles that are scattered throughout thestem. In dicot stems, these vascular bundles are arrangedin a ring near the outside of the stem.

Leaves

Dicots

Flower

Vascularbundlesinstem

Veins in leavesof most monocotsare parallelto each other.

Flower parts inthrees or multiplesof three.

Vascular bundles arescattered in across section ofa stem.

Veins in leavesform a branchingnetwork.

Flower parts infours or fives ormultiples offour or five.

Vascular bundles arearranged in a ringin a cross sectionof a stem.

Vascularbundlesinroot

Stemthickness

Bundles of xylemand phloem alternatewith one anotherin a circle.

Stems of mostmonocots do notgrow thicker fromyear to year.

A single mass of xylemforms an "X" in thecenter of the root;phloem bundles arelocated between thearms of the "X."

Stems can growthicker fromyear to year.

Figure 22-12 Flowering plants are placed into two main sub¬classes, Monocotyledonae and Dicotyledonae. This chartidentifies the differences between these two classes. Which classcontains plants whose leaves have veins that are parallel to oneanother?

4. In monocot roots, bundles of xylem and phloem alternatewith each other in a circular arrangement, like the spokesof a bicycle wheel. In dicot roots, a single mass of xylemtissue forms an X in the center of the root, and bundles ofphloem tissue are positioned between the arms of the "X."

5. Most monocots have stems and roots that do not growthicker from year to year. For this reason there are veryfew treelike monocots. Palms are one of the few treelikemonocots. Some dicot stems and roots can grow thickerfrom year to year. Most of the flowering trees you see aredicots.

22 JA SECTIONREVIEW

1. How do useful adaptations give rise to new plant species?

2. Compare gymnosperms and angiosperms.

3. Which generation is more obvious in seed plants? Howdo the relative sizes of these generations follow a trendin the evolution of plant reproduction?

4. Critical Thinking—Applying Concepts Suppose youfound a plant whose leaves have parallel veins and whoseflowers have six petals. Is this plant a monocot or a dicot?What is your reasoning?

Figure 22-13 This tiny beanseed has pushed its stem abovethe soil surface and into the light.Just below the leaves at the topof the plant, the two bean-shapedcotyledons remain attached to thestem. Later, when the plant islarge enough to make its ownfood, the cotyledons will shriveland fall off.

475

Figure 22-14 Many differentanimals pollinate plants. Bees,such as this honeybee coveredwith pollen, are perhaps the mostcommon (right). Bees areresponsible for the pollination ofmany of the plant varieties thatproduce the fruits we eat.Bananas, like this one growing inSoutheast Asia, are oftenpollinated by bats, not by bees(left).

22-3 Coevolution of FloweringPlants and Animals

Watching bees travel from flower to flower is such a com¬mon experience that most of us probably do not think about it.We take for granted the fact that flowers are brightly coloredand beautifully perfumed. Rarely do we wonder why fruits aretasty and nutritious as well as colorful. But how and why didinsects begin exhibiting flower-visiting behavior? When did an¬imals begin to eat fruits and seeds? And why have plant flowersand fruits evolved into their present forms?

The process by which two organisms evolve structures andbehaviors in response to changes in each other over time iscalled coevolution. Some of the most fascinating examples ofcoevolution involve relationships between angiosperm flowersand fruits and a wide variety of animal species.

To understand plant-animal coevolution, we must lookonce again at the evolutionary history of plants. The first flow¬ering plants probably evolved during the early Cretaceous Pe¬riod, about 125 million years ago. At that time, gymnospermsand other plants formed huge forests. Dinosaurs were the dom¬inant land animals. During the Cretaceous Period, the firstbirds and mammals began to appear in the fossil record. Flyinginsects, particularly beetles of several types, became common.Thus the first flowering plants evolved at about the same timeas the earliest mammals, a short time after the earliest birds,and a good while after the earliest insects.

Then, toward the end of the Cretaceous Period, the Earth'sclimate changed dramatically. Dinosaurs and many gymno-

Gulde For Reading¦ What is the importance of

pollination?How do plants and animals affecteach other's evolution?

How does seed dispersal contributeto the success of seed plants?

sperms became extinct. This mass extinction opened up manyniches for other organisms. New adaptive radiations of bothanimals and plants occurred.)New species of birds and mam¬mals evolved and filled niches vacated by the dinosaurs. Newspecies of angiosperms replaced disappearing gymnosperms.And many new species of insects—including moths, bees, andbutterflies—evolved.

The coincidence of angiosperm evolution with the evolu¬tion of modern insects, birds, and mammals is very important.Flowers and fruits are specialized reproductive structures thatcould evolve only in the presence of insects, birds, and mam¬mals. Let us now see how and why this is so.

Flower PollinationPollination is essential to the reproduction of flowering

plants. Over millions of years, a variety of ways to ensure thatpollination will occur has evolved. For example, some plantsare pollinated by the wind. Wind-pollinated plants include wil¬low trees, ragweed, and grasses such as corn and wheat. Thetiny pollen grains of these plants fall off their flowers withoutdifficulty, making it easy for them to be carried by the wind toother flowers. Wind-pollinated plants usually have small, plainsimple flowers with little or no fragrance.

But most angiosperms are not pollinated by the wind.Most flowering plants are pollinated by insects, birds, or mam¬mals that carry pollen from one flower to another. In return,the plants provide the pollinators with food. The food may takethe form of pollen or a liquid called nectar, which may contain upto 25 percent glucose, or a combination of pollen and nectar.

Figure 22-15 Hummingbirds areable to flap their wings so fastthat they hover in place. Thishummingbird is drinking nectarfrom a flower. Becausehummingbirds are able to see redand orange quite well, they areattracted to these flower colors.

477

Figure 22-16 This flower looksdifferent under natural sunlight (top)than it does under ultraviolet light(bottom). Insects can perceiveultraviolet light whereas humanscannot. The pattern that shows upunder ultraviolet light may attractinsects to the center of the flower,where the flower's reproductivestructures are found. This makes itmore likely that the insect willpollinate the plant.

It is easy to imagine how pollinators such as bees firstlearned to visit certain flowers. When a bee finds food on a par¬ticular flower, it remembers clearly the color, shape, and odorof that flower. So if a bee finds edible pollen on a flower of aparticular type, it will search for more flowers of that sametype. While feeding on different flowers, a bee may accidentallypick up extra pollen that it then carries to the next flower itvisits. Because the bees remember the color and odor offlowers so well, it is probable that pollen picked up from oneflower will be deposited on another flower of the same species.

This kind of interaction between animals and -plants in¬creases the evolutionary fitness of both organisms. Insectsbenefit by learning to identify dependable sources of food.Plants benefit because this kind of vector pollination, or pol¬lination by the actions of animals, is a very efficient way of get¬ting the male gametophyte to the female gametophyte. Vectorpollination is much more efficient than wind pollination, whichwastes enormous amounts of pollen.

Of course, flowers that depend upon specific animals topollinate them could only have evolved after those animalsevolved. When angiosperms first appeared, this sort of rela¬tionship began accidentally. But over time the coevolutionaryrelationship strengthened because it proved beneficial to thesurvival of both plants and animals. Coevolutionary relation¬ships can be very specific. The following examples of flower-pollinator pairs illustrate this fact.

One common pollinator is the honeybee. To attract bees totheir flowers, many plants have brightly colored flower petalsthat bees can see well. Because bees can see ultraviolet, blue,and yellow light the best, these are the colors of most bee-pollinated flowers. We cannot see ultraviolet light underordinary circumstances. But special film can make this colorvisible to our eyes. In Figure 22-16 you can see a picture of aflower taken in ultraviolet light. The petals of some flowerseven have markings that point to the center of the flower.These markings are like a secret sign for bees alone to see!The markings direct the bee to the center of the flower—thesource of nectar. On its way to the food, the bee might pollinatethe flower, thus ensuring the survival of the plant species.Flowers that are pollinated by bees usually have some kind oflanding platform because bees gather nectar only when theyare standing, not when they are flying.

Flowers that have coevolved with animals other than beesshow different methods of attracting pollinators. For example,some flowers are pollinated by night-flying moths that cannotsee color but have an excellent sense of smell. The petals ofthese flowers are often plain and white, but the flowers them¬selves are very fragrant—especially at night. (We use many ofthese floral fragrances—jasmine, for example—in perfumes.)Moth-pollinated flowers usually do not have landing platformsbecause unlike bees, moths feed while hovering in midair. The

478

nectar of moth-pollinated flowers is usually contained deepwithin the flower, where only the long tongue of a moth canreach it.

Several species of flowers are pollinated by flies that laytheir eggs in the bodies of dead and decaying animals. You cer¬tainly would not want to grow these flowers in your house be¬cause they smell like rotting meat! The smell produced by theflowers attracts the flies that are looking for a place to lay theireggs. The flowers of these plants even heat up when they areready to be pollinated, thus intensifying the smell they produceto lure additional flies that may act as vector pollinators.

Some flowers are pollinated by birds. Birds have a verypoor sense of smell but a good sense of sight. Birds can easilysee the colors orange and red. Not surprisingly, bird-pollinatedflowers, such as the beautiful bird-of-paradise flower, are a red¬dish-orange color. These flowers usually have no fragrance.

Seed DispersalJust as flowers have different methods that ensure pol

lination, angiosperm fruits have adaptations that help scatter seeds away from the parent plant. The process ofdistributing seeds away from parent plants is seed dispersal,Seed dispersal is very important to plants. Why? ff the seeds "ofa plant are not dispersed but instead fall to the ground beneaththe parent plant, the seedlings will compete with one anotherand with the parent plant for sunlight, water, and nutrients.This competition will reduce the chances of survival for thegrowing seeds. Seed dispersal also enables plants to colonizenew environments. Although adult plants cannot move around,their seeds can be carried to new environments.

Figure 22-18 'The seeds of themilkweed (left) and the dandelion(right) are carried by the wind.

Figure 22-17 The stapeliaflower, also called the carrionflower, smells like a piece ofrotting meat. Although notattractive to us, the smell provesalluring to a fly looking for aplace to lay her eggs.

Several different methods of seed dispersal have been ob¬served in angiosperms. The seeds and fruits of some angio-sperms, like those of dandelions, are carried by the wind. Inother angiosperms, pressure builds inside the fruit and finallyforces seeds out of the ripe fruit like bullets from a gun. Thecommon garden plant Impatiens has fruits that spi ing openwhen touched, scattering the seeds over substantial distances.

480

Designer Genes—Problem or Promise?

SCIENCE, ||TECHNOLOGY. JAN D SOCIETY j

makes it possible to design and produce plantsthat have traits that people could once onlydream about. People who support this new fieldassure us that a new agricultural revolution hasbegun. However, other researchers warn thatwe must be careful about the ways in which ge¬netic engineering is used. What sorts of prob¬lems could occur? Some researchers worry thataccidental cross-pollination could produce "su¬per weeds" immune to insects or herbicides.

Some ecologists wor¬ry that if herbicide-resistant varieties ofplants (such as cot¬ton) are made avail¬able, farmers will beencouraged to spraymore or stronger poi¬sons on their fields.

So far, geneticengineers point outthat no problems withgenetically alteredorganisms have oc¬

curred. Should geneticengineering be re¬stricted in organismsthat are moved out¬side of the laboratory?What do you think?

At one agricultural laboratory, a singletomato plant in a cage full of hungry caterpillarsremains untouched while its neighbor isstripped bare of leaves. The cells of the un¬touched plant are able to manufacture an in¬secticide because it has genes transplantedfrom a bacterium.

At another greenhouse, two rows of cottonplants grow side by side. T he benches theygrow in have been treated with an herbicide, achemical used to killweeds. In one benchthe cotton plants arestunted and dying-much like the weedsthe powerful herbicidekills. In the other bench,the cotton plants thrive.The thriving plants car¬ry a gene that confersresistance to that par¬ticular herbicide, agene that was graftedonto the plants' genomeby genetic engineers.

These are just twonew and improvedplants produced by theapplication of geneticengineering, which

Many fruits have coevolved with animal species that helpdisperse the fruits' seeds. For example, some fruits have sharpbarbs that catch in fur or feathers, allowing the seeds inside tobitch rides on mammals or birds. As they move from place toplace, such animals may enter a new environment. If the seedsfall off the animals and land on a spot that provides good grow¬ing conditions, they will develop into new plants. In this way,plants are carried to new environments.

Some fruits have attractive colors, pleasant tastes, and con¬tain a variety of nutritious compounds. These fruits and theseeds inside them are eaten by mammals and birds. The fleshy,nourishing, and tasty pulp of the fruit is digested by the animal,but the seeds, which are protected by tough seed coats, arenot. These seeds pass through the digestive tract of the animalwithout being damaged. While inside the animal, seeds may becarried over great distances. Eventually the seeds are depos¬ited, along with a convenient dose of natural fertilizer, in a newlocation where they can grow.

Have you ever wondered why so many unripe fruits aregreen and have a bitter taste? Think about the function of fruitsin relation to the evolutionary fitness of plants. Inside the unri-pened fruits the seeds are still maturing. If the fruits are eatentoo soon, the immature seeds will not be able to grow. Theplant's fitness for survival would decrease. But plants manufac¬ture bitter-tasting compounds that they pump into fruits as thefruits develop. These bitter-tasting compounds discourage ani¬mals from eating fruits that are not ripe. The green color ofunripe fruits makes it more likely that animals will not noticethe fruits hidden among the green leaves of plants. When theseeds are mature, plants either remove the bitter-tasting com¬pounds from the fruits or chemically break down the com¬pounds completely. Plants then pump sugars into the fruits. Atthe same time, the fruits change color. The brightly coloredfruits are more easily noticed by birds and other animals. Thedistribution of seeds in fruits is yet another example of plant-animal coevolution.

C f f SECTION I ^ REVIEW _ _

1. Why is pollination important?

2. Explain how plant-animal coevolution has led to thedevelopment of relationships between vector pollinatorsand flowers.

3. What is seed dispersal? Why is it important?

4. Critical Thinking—Relating Concepts Explain howfruits are dispersed by animals. How does fruit dispersalcontribute to seed dispersal?

Figure 22-19 The tiny seeds ofthe cocklebur have many hooks(top). The hooks catch onto thefur of animals and are carried tonew environments. When theseeds are ripe, raspberries turn abright red and can easily be seenby birds and other animals(bottom). After the fruits areeaten, the indigestible seeds passthrough the animal and aredeposited, along with other solidwastes, in a new environment.

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PROBLEMWhy do fruits get ripe?

MATERIALS (per group)

unripe banana hot plateripe banana rulerbalance scalpel400-mL beaker 4 test tubesBenedict's solution test tube holdersugar (dextrose) test tube racksolution hand lens

2 100-mL graduated glass-marking pencilcylinders

PROCEDURE

1.

A o o> H l|Fill a 400-mL beaker halfway with water.Place the beaker on a hot plate. Turn the hotplate on high.

2. Use a glass-marking pencil to label four testtubes. Label the first test tube C, for control;the second 5, for sugar; the third, R, for ripebanana; and the fourth, U, for unripe banana.

3. Use a graduated cylinder to put 5 mL of waterinto the test tubes labeled C, R, and U. Place 5mL of a sugar (dextrose) solution into thetest tube labeled S.

4. With a clean graduated cylinder, add 5 mL ofBenedict's solution to each of the test tubes.

5. Observe the color and appearance of theunripe banana. Peel it. Use a scalpel to cut aslice, or cross section, 5 mm thick. CAUTION:Always cut away from yourself and others.

6. Cut the slice of banana in half along its diam¬eter. Then make a cut parallel to the diame¬ter, about 5 mm from the cut edge, as shownin the accompanying illustration.

7. Measure the mass of the cut piece. It shouldhave a mass of about 1 g. Put this piece of ba¬nana into the test tube marked U.

8. Repeat steps 5 to 7 with the ripe banana.Make sure the mass of the piece of ripe ba¬nana is the same as the mass of the unripebanana. Place this piece in the test tubemarked R.

9. Place the test tubes in the beaker of boilingwater on the hot plate. CAUTION: Use the testtube holder. Place the tubes carefully.

10. Observe the four test tubes. When the testtube that contains the sugar solution changescolor, observe the color of the other testtubes.

11. Use the test tube holder to remove the testtubes from the beaker. Place the test tubes inthe test tube rack. Turn off the hot plate andallow the beaker to cool.

12. Make several more slices of the ripe banana.Use a hand lens to examine the region nearthe center of each slice.

OBSERVATIONS

1. What did the peel of the unripe banana looklike? The ripe banana?

2. In which test tubes did the greatest changeoccur?

3. Describe the structures you observed in thecenter of the banana slices.

ANALYSIS AND CONCLUSIONS

1. What do the results of the tests with Bene¬dict's solution show?

2. What are the structures in the center of abanana?

3. How do animals help disperse banana seeds?4. What changes occur when a banana ripens?5. Why would an animal be more likely to find

and eat ripe bananas than unripe bananas?

m

SUMMARIZING THE CONCEPTS

The key concepts in each section of this chapter are listed below to help youreview the chapter content. Make sure you understand each concept and itsrelationship to other concepts and to the theme of this chapter.

22-1 Seed Plants—The Spermopsida• Seed plants have roots, stems, and leaves

that show adaptations that enable them toperform different functions.

• Seed plants are able to reproduce withoutthe need for standing water. Seed plants pro¬duce seeds that are able to survive periodsof time that are unfavorable for growth.

22-2 Evolution of Seed Plants

• The gymnosperms are the most ancientgroup of surviving seed plants. The namegymnosperm means naked seed.

• The most common gymnosperms are theconifers. Conifer means cone-bearing. Most

conifers produce cones, which are specialreproductive organs.

• All flowering plants belong to the angio-sperms. Flowers are the angiosperms' repro¬ductive organs.

• There are two main subclasses of angio¬

sperms: monocots and dicots. Monocots

have one seed leaf; dicots have two. Theveins in monocot leaves are parallel to oneanother. The veins of dicots form a branch¬ing network in the leaves. The flower partsof monocots occur in threes or multiples ofthree. The flower parts of dicots occur infours or fives or multiples of four or five. Thevascular bundles in dicots form a ringaround the stem. The vascular bundles ofmonocots are scattered around the stem.

22-3 Coevolution of Flowering Plantsand Animals

• Some flowering plants are pollinated by thewind. These plants shed vast amounts of pol¬len into the air.

• The process by which two organisms evolvestructures and behaviors in relation to orcomplementary to one another is calledcoevolution.

• Many animals are pollinators of flowers, oragents that transfer pollen from one flowerto another.

REVIEWING KEY TERMS

Vocabulary terms are important to your understanding of biology. The key termslisted below are those you should be especially familiar with. Review these termsand their meanings. Then use each term in a complete sentence. If you are notsure of a term's meaning, return to the appropriate section and review its definition.

22-1 Seed Plants—The Spermopsida

pollen grainpollination

. embryoseed coat

22-2 Evolution ofSeed Plantsscalegymnospermpollen coneangiosperm

flowerfruitmonocot

dicotcotyledonvascular bundle

22-3 Coevolutionof Flowering Plantsand Animalscoevolutionvector pollinationseed dispersal

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B. In each of the following sets of terms, three of the terms are related. One termdoes not belong. Determine the characteristic common to three of the terms andthen identify the term that does not belong.

5. net veins, parallel veins, one cotyledon, nine petals6. bee, bird, bat^wind )7. strawberry, blueberry, apple, potato

CONCEPT MASTERY

Use your understanding of the concepts developedof the following in a brief paragraph.

1. What is seed dispersal? How does itcontribute to the survival of a plant species?

2. What is a cotyledon?3. How do seed plants help humans survive?4. Why do botanists consider a tomato and a

squash fruits?5. How do roots and vascular tissues

contribute to a redwood tree's great size?

in the chapter to answer each

6. How are seed plants better able to survivedrier conditions than mosses and ferns?

7. What is a conifer? How does a conifer differfrom an angiosperm?

8. What is wind pollination? How does windpollination differ from vector pollination?

9. Why is it important that seeds provide foodfor the embryo plant?

CRITICAL AND CREATIVE THINKING

Discuss each of the following in a brief paragraph.

1. Applying concepts In nature, flowershave a limited range of colors. In a garden,however, flowers can have many more

colors. Apply your knowledge of pollinationand artificial selection to explain why.

'1. Making predictions In the future, aterrible, fatal disease is found to affect allmonocots. Predict the effect of this diseaseon the human population. ^ ^

3. Relating cause and effect Scientists iSTinvent a new insecticide that can kill all fffteinsects in the world. What importanTharmful effect would this have on plants?

4. Interpreting diagramsExamine the plant inthis photograph. Howmany cotyledons wouldthe seeds of this planthave? Explain yourreasoning.

5. Applying concepts A farmer decides notto plant her fields one year. Later in theyear heavier than normal rains fall on thefield. Now the farmer wishes she hadplanted her crops. Why do you think shechanged her mind?

6. Applying concepts Making a cut throughthe bark of a tree in a complete circlearound the trunk often results in the death ofthe tree. Using your knowledge of vasculartissue, explain why this might happen.

7. Relating facts The seeds of agymnosperm are probably not likely to bedispersed by animals, whereas the seeds ofangiosperms are likely to be dispersed byanimals. Explain why this is so.

8. Using the writing process Suppose all^¦gymnosperms died out tomorrow. Write astory that details ways in which your lifewould be changed.

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