576 What You’ll Learn ■ You will identify the charac- teristics of the major plant groups. ■ You will identify and compare the distinguishing features of vascular and nonvascular plants. ■ You will analyze the advan- tages of seed production. Why It’s Important You can classify plants according to their diverse characteristics. Knowing about these character- istics of plants will help you ana- lyze the relationships among plant divisions. Visit to • study the entire chapter online • access Web Links for more information and activities on diversity of plants • review content with the Inter- active Tutor and self-check quizzes The Diversity of Plants The Diversity of Plants Plants can be categorized as either non-seed plants or seed plants. The ferns and mosses covering the ground in this for- est are non-seed plants. The trees and woody shrubs are seed plants. Understanding the Photo bdol.glencoe.com Adam Jones
Transcript
576
What You’ll Learn■ You will identify the charac-
teristics of the major plantgroups.
■ You will identify and comparethe distinguishing features ofvascular and nonvascularplants.
■ You will analyze the advan-tages of seed production.
Why It’s ImportantYou can classify plants accordingto their diverse characteristics.Knowing about these character-istics of plants will help you ana-lyze the relationships amongplant divisions.
Visit to• study the entire chapter online• access Web Links for more
information and activities ondiversity of plants
• review content with the Inter-active Tutor and self-checkquizzes
The Diversity of Plants
The Diversity of Plants
Plants can be categorized aseither non-seed plants or seedplants. The ferns and mossescovering the ground in this for-est are non-seed plants. Thetrees and woody shrubs are seed plants.
22.1SECTION PREVIEWObjectivesIdentify the structures ofnonvascular plants.Compare and contrastcharacteristics of the different groups of non-vascular plants.
Review Vocabularyfertilization: fusion of
male and femalegametes (p. 253)
New Vocabularyantheridiumarchegonium
Nonvascular Plants
Plants Divisions AdaptationsOrigins
Nonvascular
Non-seedvascular
Seed vascular
Make a Table As you read Chapter 22, complete the table about the diversityof plants.
Diversity Make the following Foldable to help you organize information about the diversity of plants.
Fold one piece of paper in half lengthwise twice.
Fold the paper widthwise into fourths.
Unfold, lay the paper length-wise, and draw lines along the folds.
Label your table as shown.
STEP 1
STEP 3
STEP 2
STEP 4
22.1 NONVASCULAR PLANTS 577
Figure 22.1 Bryum is a type of mossfrequently found inmoist forest habitats.
What is a nonvascular plant?Nonvascular plants are not as common or as widespread in their distri-
bution as vascular plants because life functions, including photosynthesisand reproduction, require a close association with water. Because a steadysupply of water is not available everywhere, nonvascular plants are limitedto moist habitats by streams and rivers or in temperate and tropical rainforests. Recall that a lack of vascular tissue also limits the size of a plant. Indrier soils, there is not enough water to meet the needs of most nonvascu-lar plants. Their long-term survival in dry environments is limited by thisresource—water. However, nonvascular plants, such as the moss inFigure 22.1, are successful in habitats with adequate water.
Alternation of generations As in all plants, the life cycle of nonvascular plants includes an alternation
of generations between a diploid sporophyte and a haploid gametophyte.However, nonvascular plant divisions include the only plants that have adominant gametophyte generation. Sporophytes grow attached to anddepend on gametophytes to take in water and other substances.
William J. Weber/Visuals Unlimited
Non-photosynthetic sporophytes, likethose shown in Figure 22.2A, dependon their gametophytes for food.
Gametophytes of nonvascular plantsproduce two kinds of sexual reproduc-tive structures. The antheridium (anthuh RIH dee um) is the male repro-ductive structure in which sperm areproduced. The archegonium (ar kihGOH nee um) is the female reproduc-tive structure in which eggs are pro-duced. Fertilization, which begins thesporophyte generation, occurs in thearchegonium.
Adaptations inBryophyta
There are several divisions of non-vascular plants. The first division you’llstudy are the mosses, or bryophytes.
Figure 22.2Mosses usually grow as carpets of small green plants that cover damp soil or other damp surfaces.
Brown stalks with sporecapsules, thesporophytegeneration, grow from thegreen, leafygametophyte.
A
578 THE DIVERSITY OF PLANTS
Moss species sometimes are referred to as pioneer speciesbecause they often are some of the first species toinhabit newly formed or disturbed environments. Thesemosses are growing on the damp surfaces of fallen trees.Analyze Describe the flow of matter between thesemosses, trees, and the environment.
B
Interpret DataDo bryophytes grow in South Texas? The presence anddistribution of moss species in South Texas are displayed in the graph below.
Solve the ProblemBryophytes need a moist environment and South Texas can bevery hot and dry. Interpret the data and infer why the distribu-tion of the numbers of moss species in South Texas counties isso varied.
Thinking Critically1. Estimate Which is greater, counties in South Texas that
have many moss species or counties in South Texas thathave few moss species?
2. Calculate What percentage of counties in South Texashave five or fewer moss species?
3. Calculate What percentage of counties in South Texashave more than 20 moss species?
4. Infer How might the environment or geography in coun-ties that have more than 20 moss species be different fromthe environment of counties that have no or very few mossspecies?
Nu
mb
er o
f co
un
ties
Number of species
26242220181614121086420
0–5 6–11 11–15 16–20 21–25 26–30
Moss Species in South Texas Counties
(l)Jan-Peter Lahall/Peter Arnold, Inc. (r)David M. Dennis/Tom Stack & Associates
Bryophytes are the most familiar of thenonvascular plant divisions. Mosses aresmall plants with leafy stems. Theleaves of mosses are usually one cellthick. Mosses have rhizoids, colorlessmulticellular structures, which helpanchor the stem to the soil. Althoughmosses do not contain true vascular tis-sue, some species do have a few, longwater-conducting cells in their stems.
Mosses usually grow in dense car-pets of hundreds of plants, as shownin Figure 22.2B. Some have uprightstems; others have creeping stemsthat hang from steep banks or treebranches. Some mosses form exten-sive mats that help retard erosion onexposed rocky slopes.
Mosses grow in a wide variety ofhabitats, as stated in the Problem-Solving Lab on the opposite page.They even grow in the arctic duringthe brief growing season where suffi-cient moisture is present.
A well-known moss is Sphagnum,also known as peat moss. This plantthrives in acidic bogs in northernregions of the world. It is harvestedfor use as fuel and is a commonly usedsoil additive. Dried peat moss absorbslarge amounts of water, so florists andgardeners use it to increase the water-holding ability of some soils.
Adaptations inHepaticophyta
Another division of nonvascularplants is the liverworts, or hepatico-phytes. Like mosses, liverworts aresmall plants that usually grow inclumps or masses in moist habitats.The name of the division is derivedfrom the word hepar, which refers tothe liver. The flattened body of a liv-erwort gametophyte is thought toresemble the shape of the lobes of ananimal’s liver. Liverworts occur inmany environments worldwide.
A liverwort can be categorized aseither thallose or leafy, as shown inFigure 22.3. The body of a thalloseliverwort is called a thallus. It isbroad and ribbonlike and resemblesa fleshy, lobed leaf. Thallose liver-worts like Marchantia, shown inFigure 22.3A, are usually foundgrowing on damp soil. Leafy liver-worts grow close to the ground andusually are common in tropical jun-gles and areas with persistent fog.Their stems have flat, thin leavesarranged in three rows—a row alongeach side of the stem and a row ofsmaller leaves on the stem’s lowersurface. Liverworts have rhizoidsthat are composed of only one elon-gated cell.
Examine a hepatico-phyte’s method of growth. Evaluateits significance as an adaptation toa hepaticophyte’s environment.
22.1 NONVASCULAR PLANTS 579
antheridium fromthe Medieval Latinword anthera,meaning “pollen”;Sperm are pro-duced in theantheridium.archegonium fromthe Greek wordarchegonos, mean-ing “originator”;Eggs are producedin the archegonium.
AA
BB
Figure 22.3 Liverworts may have aflattened plant bodycalled a thallus (A) orleafy stems (B).
(t)Ed Reschke/Peter Arnold, Inc. (b)Les Saucier/Photo/Nats
Understanding Main Ideas1. Compare and contrast a leafy liverwort and a thal-
lose liverwort.2. Explain how the bryophyte sporophyte generation
is dependent on the bryophyte gametophyte generation.
3. Identify characteristics shared by all nonvascularplants.
4. Evaluate the significance of nonvascular plantadaptations to their moist environments.
Thinking Critically5. Identify the growth and development methods of
mosses. Explain why these are advantageous intheir environments.
6. Compare and Contrast Describe the variationsin gametophyte and sporophyte generations ofnonvascular plants. For more help, refer toCompare and Contrast in the Skill Handbook.
SKILL REVIEWSKILL REVIEW
580 THE DIVERSITY OF PLANTS
Figure 22.4 The upright sporophyte of the hornwort resemblesan animal horn and givesthe plant its commonname.
Sporophyte with sporangium (2n)
Gametophyte (n)
bdol.glencoe.com/self_check_quiz
Adaptations inAnthocerophyta
Anthocerophytes are the smallestdivision of nonvascular plants, cur-rently consisting of only about100 species. Also known as horn-worts, these nonvascular plants aresimilar to liverworts in severalrespects. Like some liverworts, horn-worts have a thallose body. Thesporophyte of a hornwort resemblesthe horn of an animal, as shown inFigure 22.4, which is why membersof this division are commonly called“hornworts.” Another feature uniqueto hornworts is the presence of one toseveral chloroplasts in each cell of thesporophyte depending upon thespecies. Unlike other nonvascularplants, the hornwort sporophyte, not
the gametophyte, produces most ofthe food used by both generations.
Origins of NonvascularPlants
Fossil and genetic evidence suggeststhat liverworts were the first landplants. Fossils that have been positivelyidentified as nonvascular plants firstappear in rocks from the earlyPaleozoic Era, more than 440 millionyears ago. However, paleobotanistssuspect that nonvascular plants werepresent earlier than current fossil evi-dence suggests. Both nonvascular andvascular plants probably share a com-mon ancestor that had alternatingsporophyte and gametophyte genera-tions, cellulose in their cell walls, andchlorophyll for photosynthesis.
22.2SECTION PREVIEWObjectivesEvaluate the significanceof plant vascular tissue tolife on land.Identify and analyze the characteristics of thenon-seed vascular plantdivisions.
Vocabulary Reviewalternation of generations:
type of life cycle foundin some algae, fungi,and all plants where anorganism alternatesbetween a haploid (n)gametophyte genera-tion and a diploid (2n)sporophyte generation(p. 516)
New Vocabularystrobilusprothallusrhizomesorus
22.2 NON-SEED VASCULAR PLANTS 581
What is a non-seed vascular plant?The obvious difference between a vascular and a nonvascular plant is
the presence of vascular tissue. As you may remember, vascular tissue ismade up of tubelike, elongated cells through which water and sugars are
transported. Vascular plants are able toadapt to changes in the availability ofwater, and thus are found in a variety ofhabitats. You will learn about three divi-sions of non-seed vascular plants:Lycophyta, Arthrophyta, and Pterophyta.
Alternation of generations Vascular plants, like all plants, exhibit
an alternation of generations. Unlikenonvascular plants, the spore-producingvascular sporophyte is dominant andlarger in size than the gametophyte, asshown in Figure 22.5. The maturesporophyte does not depend on thegametophyte for water or nutrients.
Plants with PipesUsing an Analogy You take a drink froma water fountain then watch the unusedwater flow down the drain. In the lab,another student turns a tap to get50 mL of water for an investigation.These activities are possible becauseyour school has plumbing that deliverswater and carries away wastes. Plants,such the tree fern and others shown tothe right, have plumbing too. Thisplant plumbing—called vascular tis-sues—distributes water and other dis-solved substances throughout a plant.Concept Map After you read about non-seedvascular plants, make a concept map that identifiesand analyzes the relationships among these organisms.
Non-Seed Vascular Plants
Gametophyte (n)
Sporophyte (2n)Figure 22.5In non-seed vascularplants, the sporo-phyte generation isdominant.
Tree fern
Patti Murray/Earth Scenes
A major advance in this group ofvascular plants was the adaptation ofleaves to form structures that protectthe developing reproductive cells. Insome non-seed vascular plants, spore-bearing leaves form a compact clustercalled a strobilus (stroh BIH lus). Thespores are released from the strobilusand can grow to form gametophytes. Afern gametophyte is called a prothallus(proh THA lus). Gametophytes are rel-atively small and live in or on the soil.Antheridia and archegonia develop onthe gametophyte, as illustrated inFigure 22.6. Sperm are released fromantheridia and require a continuousfilm of water to reach eggs in thearchegonia. If fertilization occurs, azygote can grow into a large, domi-nant sporophyte.
Adaptations in Lycophyta
From fossil evidence it is knownthat tree-sized lycophytes weremembers of the early forest commu-nity. Modern lycophytes, like theone in Figure 22.7, are muchsmaller than their early ancestors.Lycophytes are commonly called
club mosses and spikemosses. Their leafy stemsresemble moss gameto-phytes, and their repro-ductive structures are clubor spike shaped. However,unlike mosses, the sporo-phyte generation of thelycophytes is dominant. Ithas roots, stems, and smallleaflike structures. A sin-gle vein of vascular tissueruns through each leaflikestructure. The stems oflycophytes may be uprightor creeping and have rootsgrowing from the base ofthe stem.
582 THE DIVERSITY OF PLANTS
Archegonium
Prothallus
Antheridium
Egg
RhizoidsSperm
Figure 22.6In the proper environment, a non-seedplant spore can grow into a gametophytelike this prothallus. Each gametophyte canproduce eggs and sperm.
Figure 22.7Selaginella species inhabit a variety ofenvironments, from damp greenhousefloors to tropical forests. The resurrec-tion plant, Selaginella lepidophylla, isadapted to survive extreme drought(A). When moisture returns, the plantresumes normal functions (B).
AA
BB
(l)Richard F. Trump/ Photo Researchers (r)Virginia P. Weinland/Photo Researchers
The club moss, Lycopodium, is com-monly called ground pine because it isevergreen and resembles a miniaturepine tree. Some species of groundpine have been collected for decora-tive uses in such numbers that theplants have become endangered.
Explain why waterlimits the long-term survival of non-seed vascular plants.
Adaptations inArthrophyta
Arthrophytes, or horsetails, repre-sent a second group of ancient vascu-lar plants. Like the lycophytes, earlyhorsetails were tree-sized members of the forest community. Today’sarthrophytes are much smaller thantheir ancestors, usually growing toabout 1 m tall. There are only about15 species in existence, all of thegenus Equisetum.
The name horse-tail refers to the bushyappearance of somespecies. These plants alsoare called scouring rushesbecause they contain silica, an abra-sive substance, and were once used toscour cooking utensils. If you runyour finger along a horsetail stem,you can feel how rough it is.
Most horsetails, like the ones shownin Figure 22.8, are found in marshes,in shallow ponds, on stream banks, andother areas with damp soil. Somespecies are common in the drier soil offields and roadsides. The stem struc-ture of horsetails is unlike most othervascular plants; it is ribbed and hollow,and appears jointed. At each joint,there is a whorl of tiny, scalelike leaves.
Like lycophytes, arthrophyte sporesare produced in strobili that form at the tips of non-photosyntheticstems. After the spores are released,they can grow into gametophyteswith antheridia and archegonia.
22.2 NON-SEED VASCULAR PLANTS 583
strobilus from the Greek wordstrobos, meaning“whirling”; Spore-bearing leavesform a compactcluster called astrobilus.
Figure 22.8 The sporophyte generation of ahorsetail, Equisetum, has thin,narrow leaflike structures thatcircle each joint of the slender,hollow stem. Spore-producingstems are not photosynthetic.
According to fossil records, ferns—division Pterophyta—first appearednearly 375 million years ago duringthe time when club mosses and horse-tails were the predominant membersof Earth’s plant population. Ancientferns grew tall and treelike andformed vast forests. Over time, fernsevolved into many species, adapted todifferent environments, and today aremore abundant than club mosses orhorsetails.
Ferns range in size from a fewmeters tall, like tree ferns, to small,floating plants that are only a few cen-timeters in diameter, such as those inFigure 22.9. You may have seenshrub-sized ferns on damp forestfloors or along stream banks. Someferns inhabit dry areas, becomingdormant when moisture is scarce andresuming growth and reproductiononly when water is available again.Explore the relationship betweenwater and non-seed vascular plants inthe Problem-Solving Lab on this page.
584 THE DIVERSITY OF PLANTS
Figure 22.9 Today there are about 12 000 speciesof living ferns. Ferns occupy widelydiverse habitats and have a varietyof different forms and sizes.
The clover fern is an aquaticplant. Its roots grow under-ground at the muddy bottomof a body of water.
B
The center of this fern, where the frondsform, resembles a bird’s nest. Bird’s nestferns often are grown as houseplants.
A
Many species of tall treeferns exist in the tropics.
C
Art to update
Apply ConceptsIs water needed for fertilization? Non-seed vascular plants have a number of shared characteristics. One of thesecharacteristics is related to certain requirements needed forreproduction.
Solve the ProblemExamine the following data table. Notice that some of theinformation is incomplete.
Thinking Critically1. Explain How would you complete the column titled
“Sperm Must Swim to Egg?”2. Explain How is the column titled “Water Needed for
Fertilization?” related to answers in the previous column?3. Describe What environment is required for the growth of
these three plant divisions?4. Predict What other means might be possible for plant
sperm delivery to eggs without the use of water?
Data Table
Sperm Must Water NeededDivision Example Swim to Egg? for Fertilization?
Fern structures As with most vascular plants, it is
the sporophyte generation of the fernthat has roots, stems, and leaves. Thepart of the fern plant that we mostcommonly recognize is the sporo-phyte generation. The gametophytein most ferns is a thin, flat structurethat is independent of the sporophyte.In most ferns, the main stem is under-ground. This thick, undergroundstem is called a rhizome. It containsmany starch-filled cells for storage.The leaves of a fern are called frondsand grow upward from the rhizome,as shown in Figure 22.10. The frondsare often divided into leaflets calledpinnae, which are attached to a cen-tral rachis. Ferns are the first of thevascular plants to have evolved leaveswith branching veins of vascular tis-sue. The branched veins in fernstransport water and food to and fromall the cells.
Figure 22.10Fern sporophyte structures include a rhizome, fronds, androots. Compare and Contrast In general, how do the sizesof fern sporophytes and gametophytes compare?
The form of fern frondsvaries from species tospecies. Some fronds arebroad, flat structures andothers are finely dividedinto leaflets. Fronds aresupported by a stemlikestructure called a rachis.
B
New fern fronds are calledfiddleheads because their shapeis similar to the neck of a violin.
C
22.2 NON-SEED VASCULAR PLANTS 585
The fern rhizomeis an undergroundstem. Roots andfronds grow fromnodes along therhizome.
The fern life cycle is representativeof other non-seed vascular plants.Fern spores are produced in struc-tures called sporangia. Clusters ofsporangia form a structure called asorus (plural, sori). Sori are usuallyfound on the undersides of fronds, asshown in Figure 22.11, but in someferns, spores are borne on modifiedfronds. Practice your lab skills andlearn more about fern spores and spo-rangia in the MiniLab on this page.
Origins of Non-SeedVascular Plants
The earliest evidence of non-seedvascular plants is found in fossils fromearly in the Devonian Period, around375 million years ago. Large tree-sized lycophytes, arthrophytes, andpterophytes were extremely abundantin the warm, moist forests that domi-nated Earth during the CarboniferousPeriod. Ancient lycophyte speciesgrew as tall as 30 m. Many of thesespecies of non-seed vascular plantsdied out about 280 million yearsago—a time when Earth’s climate wascooler and drier. Today’s non-seed
586
Figure 22.11Sori found on the underside offern fronds look like brown orrust-colored dust.
Most sori are found as roundclusters. The shape, color, andarrangement of clusters on afrond vary with fern species.
A
Some species of ferns have sorion the edges of fronds.
B
ExperimentIdentifying Fern Sporangia Whenyou admire a fern growing in a garden or forest, you are admiringthe plant’s sporophyte generation.Upon further examination, you shouldbe able to see evidence of sporesbeing formed. Typically, the evidenceyou are looking for can be found on theunderside of the fern’s fronds.
ProcedureCAUTION: Use caution when handling a microscope, glass slides, and coverslips.! Place a drop of water and a drop of glycerin at opposite
ends of a glass slide.@ Use forceps to gently pick off one sorus from a frond.
Place it in the drop of water and add a coverslip.# Add a second sorus to the glycerin and add a coverslip.$ Observe both preparations under low-power magnifica-
tion and note any similarities and differences. Look forlarge sporangia (resembling heads on a stalk) and spores(tiny round bodies released from a sporangium).
Analysis1. Compare and Contrast How does the appearance of
spores in water and in glycerin differ?2. Explain How did the glycerin affect the sporangium?3. Form a Hypothesis Explain how sporangia naturally
burst.4. Form a Hypothesis Explain how sporangia were affected
by glycerin.
Fern sporangium
Stained LM Magnification: 100�
(t)Ed Reschke/Peter Arnold, Inc.(bl)Doug Martin (br)Glenn Foss/Photo Researchers
Understanding Main Ideas1. Explain why most non-seed vascular plants live in
moist habitats.2. Identify and analyze the characteristics of lyco-
phyte and pterophyte sporophytes.3. Compare and contrast non-seed vascular plants
that exist today and those that lived in ancientforest communities.
4. What are the similarities and differences betweenthe sporophyte of a non-seed vascular plant andthe sporophyte of a nonvascular plant?
5. List the three structures common to all fern sporo-phytes and describe the function of each.
Thinking Critically6. Hypothesize why there are fewer non-seed vascular
plants today than there were 300 million years ago.Analyze, critique, and review your hypothesis as toits strengths and weaknesses based on scientificinformation.
7. Observe and Infer How do you think the pres-ence of silica in the stems of arthrophytes mightprotect these plants from being eaten by animals?For more help, refer to Observe and Infer in theSkill Handbook.
SKILL REVIEWSKILL REVIEW
22.2 NON-SEED VASCULAR PLANTS 587
PRESENT CENOZOIC PALEOZOIC PRECAMBRIANMESOZOIC
Arthrophytes15 species
Lycophytes1150 species
Psilophytes6 species
Pterophytes11 000 species
Protists
Bryophytes20 000 species
Anthocerophytes100 species
Hepaticophytes6500 species
Lepidodendron
Calamites
Species numbers are approximate and subject to change pending discoveries or extinctions.
Figure 22.12The divisions of non-seed vascular plantsand nonvascular plants are closelyrelated.
nonvascular plants are much smallerand less widespread in their distribu-tion than their prehistoric ancestors.
The evolution of vascular tissueenabled these plants to live on land
and to maintain larger body sizes incomparison with nonvascular plants.As you can infer from Figure 22.12,non-seed vascular plants are closelyrelated to nonvascular plants.
22.3SECTION PREVIEWObjectivesIdentify and analyze thecharacteristics of seedplants.Analyze the advantagesof seed and fruit production.
Review Vocabularyreproduction: production
of offspring by anorganism; a characteristicof all living things (p. 7)
New Vocabularypollen grainovuleembryocotyledonfruit deciduous plantmonocotyledondicotyledonannualbiennialperennial
588 THE DIVERSITY OF PLANTS
. . . they’re everywhere, they’re everywhere . . .Using Prior Knowledge Items derived fromseed plants are a significant part of everyone’slife. Cotton fabrics are woven from processedcottonseed fibers. Most paper begins as woodpulp, a product of seed plants. You eat seedplants and drink their products. Other organ-isms that eat seed plants are part of most diets.Products of seed plants are used in construc-tion and manufacturing. Without seed plants,life on Earth would be impossible for mostorganisms.Creative Writing As a class, brainstorm and recorditems you use that are either seed plants or are derivedfrom seed plants. Identify the seed plant(s) for eachitem. After reviewing your list, write a story aboutwhat your life would be like without seed plants.
What is a seed plant?Some vascular plants produce seeds in which reduced sporophyte
plants are enclosed within a protective coat. The seeds may be sur-rounded by a fruit or carried on the scales of a cone.
Seed plants produce sporesIn seed plants, as in all other plants, spores are produced by the sporo-
phyte generation. These spores develop into the male and female game-tophytes. The male gametophyte develops inside a structure called apollen grain that includes sperm cells, nutrients, and a protective outercovering. The female gametophyte, which produces the egg cell, is con-tained within a sporophyte structure called an ovule.
Fertilization and reproductionThe union of the sperm and egg, called fertilization, forms the sporo-
phyte zygote. In most seed plants, this process does not require a contin-uous film of water as required by nonvascular and non-seed vascularplants. Remember that in non-seed plants, the sperm must swim througha continuous film of water in order to reach eggs in the archegonia of agametophyte. Because they do not require a continuous film of water forfertilization, seed plants are able to grow and reproduce in a wide varietyof habitats that have limited water availability.
Seed Plants
These girls are surroundedby items that are derivedfrom seed plants.
Doug Martin
After fertilization, the zygote devel-ops into an embryo. An embryo is anearly stage of development of anorganism. In plants, an embryo is theyoung diploid sporophyte stage of theplant. Embryos of seed plants includeone or more cotyledons. Cotyledons(kah tuh LEE dunz) usually store orabsorb food for the developingembryo. In conifers and many flower-ing plants, cotyledons are the leaflikestructures on the plant’s stem whenthe plant emerges from the soil.
Advantages of seedsA seed consists of an embryo and its
food supply enclosed in a tough, pro-tective coat, as shown in Figure 22.13.Seed plants have several importantadvantages over non-seed plants. Theseed contains a supply of food tonourish the young plant during theearly stages of growth. This food isused by the plant until its leaves aredeveloped enough to carry out photo-synthesis. In conifers and some flow-ering plants, the embryo’s food supplyis stored in the cotyledons. Theembryo is protected during harshconditions by a tough seed coat. Theseeds of many species are also adaptedfor easy dispersal to new areas. Thenthe new plants do not have to com-pete with their parent plant for sun-light, water, soil nutrients, and livingspace. You can learn more about seedstructure in MiniLab 22.2.
22.3 SEED PLANTS 589
Figure 22.13Seeds exhibit a variety ofstructural adaptations.
A tough seed coatprotects some pineseeds until favorableconditions exist forgermination. As growthbegins, the seed coatbreaks down and eventuallydrops off.
A
The feathery tuftattached tomilkweed seedsaids in theirdispersal.
B
Compare and ContrastComparing Seed Types Anthophytes are classified into twoclasses, the monocotyledons (monocots) and dicotyledons(dicots) based on the number of cotyledons.
Procedure! Copy the data table shown below.@ Examine the variety of seeds given to you. Use forceps to
gently remove the seed coat or covering from each seed ifone is present.
# Determine the number of cotyledons present. If two cotyle-dons are present, the seed will easily separate into twoequal halves. If one cotyledon is present, it will not separateinto halves. Record your observations in the data table.
$ Add a drop of iodine to each seed. Note the color change.Record your observations in the data table. CAUTION:Wash your hands with soap and water after handlingchemicals.
Analysis1. Observe Starch turns purple when iodine is added to
it. Describe the color change when iodine was added toeach seed.
2. Form a Hypothesis Why do seeds contain stored starch?
Data Table
Number of Monocot Color withSeed Name Cotyledons or Dicot Iodine
Diversity of seed plantsIn some plants, seeds develop on
the scales of woody strobili calledcones. This group of plants is some-times referred to as gymnosperms.The term gymnosperm means “nakedseed” and is used with these plantsbecause their seeds are not protectedby a fruit. The gymnosperm plantdivisions you will learn about areCycadophyta, Ginkgophyta, Gneto-phyta, and Coniferophyta.
Flowering plants, also called angio-sperms, produce seeds enclosed withina fruit. A fruit includes the ripenedovary of a flower. The fruit providesprotection for seeds and aids in seeddispersal. The Anthophyta divisioncontains all species of flowering plants.
Adaptations inCycadophyta
About 100 species of cycads existtoday, exclusively in the tropics andsubtropics. The only present-dayspecies that grows wild in the UnitedStates is found in Florida, althoughyou may see cycads cultivated ingreenhouses or botanical gardens.
Cycads have male and femalereproductive systems on separateplants, as shown in Figure 22.14.The male system includes cones thatproduce pollen grains, which producemotile sperm. Cycads are one of thefew seed plants that produce motilesperm. The female system includescones that produce ovules. Thetrunks and leaves of many cycadsresemble those of palm trees, butcycads and palms are not closely relatedbecause palms are anthophytes.
Adaptations inGinkgophyta
Today, this division is representedby only one living species, Ginkgobiloba. All ginkgoes are cultivatedtrees, and they are not known to existin the wild. Like cycads, ginkgo maleand female reproductive systems areon separate plants. The male ginkgoproduces pollen grains in strobiluslikecones that grow from the bases of leafclusters. Also like cycads, ginkgopollen grains produce motile sperm.The female ginkgo produces ovuleswhich, when fertilized, develop fleshy,apricot-colored seed coats, as shownin Figure 22.15. These soft seedcoats give off a foul odor when broken or crushed. Ginkgoes oftenare planted in urban areas becausethey tolerate smog and pollution.Gardeners and landscapers usuallyonly plant male gingkoes becausethey do not produce seeds with soft
Figure 22.15Unlike cycads, the seedsof the ginkgo develop afleshy outer covering(A). The ginkgo is some-times called the maid-enhair tree because itslobed leaves (B) resem-ble the fronds of amaidenhair fern.
Figure 22.14Cycads have a termi-nal rosette of leaves.Male cones (A) pro-duce pollen grainsthat are released ingreat masses into the air. Female cones (B) containovules with eggs.
seed coats. Do the BioLab at the endof this chapter to explore what othertrees are planted in urban areas.
Infer what structuraladaptations give ginkgoes a toler-ance of urban environments.
Adaptations inGnetophyta
Most living gnetophytes can befound in the deserts or mountains ofAsia, Africa, North America, andCentral and South America. The divi-sion Gnetophyta contains only threegenera, which have different structuraladaptations to their environments.The genus Gnetum is composed oftropical climbing plants. The genusEphedra contains shrublike plants andis the only gnetophyte genus found inthe United States. The third genus,Welwitschia, is a bizarre-looking plantfound only in South Africa. It growsclose to the ground, has a large tuber-ous root, and may live 1000 years.Ephedra and Welwitschia are pictured inFigure 22.16.
Adaptations inConiferophyta
The sugar pine is one of manyfamiliar forest trees that belong to thedivision Coniferophyta. The conifersare trees and shrubs with needlelike
or scalelike leaves. They are abundantin forests throughout the world, andinclude pine, fir, spruce, juniper,cedar, redwood, yew, and larch.
The reproductive structures of mostconifers are produced in cones. Mostconifers have male and female coneson different branches of the same tree.The male cones produce pollen. Theyare small and easy to overlook. Femalecones are much larger. They stay onthe tree until the seeds have matured.Examples of both types of cones areshown in Figure 22.17.
591
Figure 22.16Most gnetophytes haveseparate male and femaleplants.
Welwitschiamay live 1000years. The planthas only twoleaves, whichcontinue tolengthen as theplant growsolder.
B
Figure 22.17Male and female conesof conifers differ instructure and function.
In many conifers,including spruce,two seeds developat the base of eachof the woodyscales that makeup a female cone.
BMale cones are made upof thin papery scalesthat disintegrate soonafter the cones openand shed clouds ofpollen grains.
A
Members of the genusEphedra are a sourceof ephedrine, amedicine used to treatasthma, emphysema,and hay fever.
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Evergreen conifers Most conifers, like those pictured in
Figure 22.18, are evergreen plants—plants that retain some of their leavesfor more than one year. Althoughindividual leaves drop off as they ageor are damaged, the plant never losesall of its leaves at one time.
Plants that retain some of theirleaves year-round can photosynthesize
whenever favorable environmentalconditions exist. This is an advan-tage in environments where thegrowing season is short. Anotheradvantage of leaf retention is that a plant’s food reserves are notdepleted each spring to produce awhole set of new leaves.
Evergreen leaves usually have a heavy coating of cutin, a water-insoluble, waxy material that helpsreduce water loss. For conifers, leafshape—needlelike or scalelike—alsohelps reduce water loss. To learnmore about conifer needles, seeFigure 22.20 on the next page.
Deciduous trees lose their leavesA few conifers, including larches
and bald cypress trees, are deciduous,Figure 22.19. Deciduous plantsdrop all their leaves each fall or whenwater is scarce or unavailable as in thetundra or in deserts. Plants lose mostof their water through the leaves; verylittle is lost through bark or roots.Dropping all leaves is an adaptationfor reducing water loss. However, atree with no leaves cannot photosyn-thesize and must remain dormantduring this time.
deciduous fromthe Latin worddeciduus, meaningto “fall off”;Deciduous treesdrop all of theirleaves at the endof the growingseason.
The Douglas fir isone of the mostimportant lumbertrees in NorthAmerica. It growsstraight and tall, to a height of about 100 m.
A
Figure 22.19Some trees, includ-ing these baldcypress trees, losetheir leaves in thefall as an adaptationfor reducing waterloss.
592 THE DIVERSITY OF PLANTS
Spruce trees are popularornamental trees becauseof their graceful shapeand color variations.
Modified leaf cells There are leaf-cell modifica-tions in pine needles that help reduce water loss. Theoutermost layer of cells, called the epidermis, has athick, waxy, waterproof coating called a cuticle. Some-times the cuticle is so thick that it gives needles agrayish color. The layer of cells just below the epidermishas thick cell walls that also help to reduce water loss.
AA
22.3 SEED PLANTS 593
Bundles of needles The needles ofpines usually grow in bundles of two, three,or five needles. The number of needles perbundle and the shape of needles are gen-etically determined. A papery sheathsurrounds the base of each bundle.
BB
Recessed stomata A pine-needlestoma is located in a recessed cavity ofthe epidermis. This position providesmore protection from harsh environ-mental conditions than on the surface of the epidermis. The result is reducedwater loss from leaf tissues.
CC
Stoma
Epidermis
Papery sheath
Needle
Cross section ofneedle bundle
Pine NeedlesFigure 22.20 When you look at a snow-covered pine forest, you may be surprised to learn that wintercan be considered a dry time for plants. The cold temperature means that the soil moistureis unavailable because it is frozen. The needles of pines have several adaptations thatenable the plants to conserve water during the cold dry winter and the dry heat of thesummer. Critical Thinking How does the structure of pine needles enhance the sur-vival of conifers during hot and dry summers as well as cold and snowy winters?
Kenneth J. Stein, Ph.D./ Phototake
Adaptations inAnthophyta
Flowering plants are classified inthe division Anthophyta. They arethe most well-known plants on Earthwith more than 250 000 identifiedspecies. See if you are familiar withsome of the plants in Figure 22.21.Like other seed plants, anthophyteshave roots, stems, and leaves. Butunlike the other seed plants, antho-phytes produce flowers and formseeds enclosed in a fruit. Many differ-ent species of flowering plantsinhabit tropical forests. As you willdiscover in Biology and Society at theend of this chapter, different groups
of people have different viewpointson preserving this rich habitat.
Fruit productionAnthophyta is unique among plant
divisions. It is the only division inwhich plants have flowers and pro-duce fruits. A fruit develops from aflower’s female reproductive struc-ture(s). Sometimes, other flower partsbecome part of the fruit and, as inpineapples, the fruit develops frommore than one flower. A fruit usuallycontains one or more seeds. One ofthe advantages of fruit-enclosed seedsis the added protection the fruit pro-vides for the young embryo.
Fruits often aid in the dispersal ofseeds. Animals may eat them or carrythem off to store for food. Seeds ofsome species that are eaten passthrough the animal’s digestive tractunharmed and are distributed as theanimal wanders. In fact, some seedsmust pass through a digestive tractbefore they can begin to grow a newplant. Some fruits have structuraladaptations that help disperse the seedby wind or water. Some examples offruits are illustrated in Figure 22.22.
594 THE DIVERSITY OF PLANTS
Figure 22.22Fruits exhibit a widevariety of structuraladaptations that aid inseed protection anddispersal.
The fruit of a magnolia contains manyseeds each with a bright red coveringthat attracts birds and small animals.
B
The tough fibrousfruit of a coconutprovides protectionas well as a flotationdevice.
C
Figure 22.21A florist’s display is agood place to see anassortment of flower-ing plants.
The maple fruithas a winglikestructure thathelps keep itairborne.
divided into two classes: mono-cotyledons and dicotyledons. Thetwo classes are named for the num-ber of cotyledons in the seed.Monocotyledons (mah nuh kah tuhLEE dunz) have one seed leaf;dicotyledons (di kah tuh LEE dunz)have two seed leaves. These twoclasses often are called monocots anddicots. Table 22.1 compares thecharacteristics of monocots anddicots. About 65 000 species ofmonocots have been identified andinclude grasses, orchids, lilies, andpalms. Identified dicot species num-ber about 185 000. They includenearly all of the familiar shrubs andtrees (except conifers), cacti, wild-flowers, garden flowers, vegetables,and herbs.
Life spans of anthophytesWhy do some plants live longer
than people, and others live only afew weeks? The life span of a plant isgenetically determined and reflects
strategies for surviving periods ofharsh conditions.
Annual plants live for only a yearor less. They sprout from seeds, grow,reproduce, and die in a single grow-ing season. Most annuals are herba-ceous, which means their stems aregreen and do not contain woody tis-sue. Many food plants such as corn,wheat, peas, beans, and squash areannuals, as are many weeds of thetemperate garden. Annuals formdrought-resistant seeds that can sur-vive the winter.
Biennial plants have life spans thatlast two years. Many biennials developlarge storage roots, such as carrots,beets, and turnips. During the firstyear, biennials grow many leaves anddevelop a strong root system. Over thewinter, the aboveground portion of theplant dies back, but the roots remainalive. Under ground roots are able tosurvive conditions that leaves andstems cannot endure. During the sec-ond spring, food stored in the root isused to produce new shoots that pro-duce flowers and seeds.
22.3 SEED PLANTS 595
Table 22.1 Distinguishing Characteristics of Monocots and Dicots
Monocots One cotyledon Usually parallel Scattered Multiples ofthree
Dicots Two Usually netlike Arranged in ring Multiples ofcotyledons four and five
Perennials live for several years,producing flowers and seeds periodi-cally—usually once each year. Somesurvive harsh conditions by droppingtheir leaves or dying back to soil level,while their woody stems or under-ground storage organs remain intactand dormant. Examples of plants withdifferent lifespans are shown inFigure 22.23.
Origins of Seed PlantsSeed plants first appeared about
360 million years ago during thePaleozoic Era. Some seed plants, suchas ancient relatives of cycads andginkgoes, shared Earth’s forest with
I f you like to spend time in the forest, consider a career as a
lumberjack or a logging industryworker. Besides being outdoors,you will get lots of exercise.
Skills for the JobThe logging industry now
includes many different workers.Cruisers choose which trees to cut. Fallersuse chainsaws and axes to cut, or “fell,” the chosen trees.Buckers saw off the limbs and cut the trunk into pieces.Logging supervisors oversee these tasks. Other workers turnthe tree into logs or wood chips that are used to makepaper. After finishing high school, most loggers learn onthe job. However, with a two-year degree, you can becomea forest technician. A four-year degree qualifies you as aprofessional forester who manages the forest resources.Most logging jobs are in the Northwest, Northeast, South,and Great Lakes regions.
For more careers in related fields, visit
Some woody perennials, likebrambles, drop their leavesand become dormant duringthe winter.
B
These tomatoes areannual plants.
C
Figure 22.23Anthophytes may be annuals,biennials, or perennials.
596 THE DIVERSITY OF PLANTS
Vegetable gardenersgrow biennial parsleyfor its leaves.
Understanding Main Ideas1. Identify two adaptations that help seed plants
reproduce on land.2. Explain why needlelike leaves are an adaptation
in climates where water may be a limitedresource.
3. What adaptations help make flowering plants sosuccessful?
4. Compare and contrast characteristics of anth-ophytes and coniferophytes.
Thinking Critically5. Infer why the development of the seed might
have affected the lives of herbivorous animals living in Earth’s ancient forests.
6. Get the Big Picture Compare the formation of a spore in ferns and a seed in conifers. For morehelp, refer to Get the Big Picture in the SkillHandbook.
SKILL REVIEWSKILL REVIEW
22.3 SEED PLANTS 597
PRESENT CENOZOIC PALEOZOIC PRECAMBRIANMESOZOIC
Cycads200 species
Conifers600 species
Anthophytes250 000 species
Protists
Ginkgoes1 species
Gnetums65 species
Species numbers are approximate and subject to change pending discoveries or extinctions.
Figure 22.24The seed plant divisions are closelyrelated to each other.
bdol.glencoe.com/self_check_quiz
the dinosaurs during the MesozoicEra. However, about 65 million yearsago, most members of the Gink-gophyta died out along with manyorganisms during a mass extinction.
According to fossil evidence, thefirst conifers emerged around 250 mil-lion years ago. During the Jurassic
Period, conifers became predominantforest inhabitants and remain so today.
Anthophytes first appeared about140 million years ago late in theJurassic Period of the Mesozoic Era.
The evolutionary relationshipsamong the divisions of vascular seedplants are illustrated in Figure 22.24.
Imagine that you areemployed as a forester ina city’s department ofUrban Planning. You havejust been handed anassignment by the citymanager. The assignment?Research the types of treesthat would be most suit-able for planting alongthe streets of your community.
NorthernCatalpa,Catalpaspeciosa
California redbud,Cercis occidentalis
598 THE DIVERSITY OF PLANTS
Researching Trees on the Internet
ProblemUse the Internet to find different tree species that would besuitable for planting along the streets in your community.
ObjectivesIn this BioLab, you will:■ Research the characteristics of five different tree species.■ Use the Internet to collect and compare data from other
students.■ Conclude which tree species would be most suitable for
planting along the streets in your community.
MaterialsInternet access
Skill HandbookIf you need help with this lab, refer to the Skill Handbook.
1. Make a copy of the data table.2. Pick five tree species that you wish to research. (Note:
Your teacher may provide you with suggestions if necessary.)
3. Visit to find linksto information needed for this BioLab.
1. Define Operationally Explain the difference between trees classified aseither Coniferophyta or Anthophyta.
2. Analyze Was the information provided on the Internet helpful in com-pleting your data table? Explain your answer.
3. Think Critically What do you consider to be the most important charac-teristic when deciding on the most suitable tree for your community?Explain your answer.
4. Use the Internet Using the information yougathered from the Internet, which treespecies would most likely be the:a. most suitable along a street in your com-
munity? Explain your answer.b. least suitable along a street in your com-
munity? Explain your answer.5. Apply Explain why tree selections would dif-
fer if your community were located in: a. a desert biomeb. a taiga biomec. a tropical rain forest biome
bdol.glencoe.com/internet_lab
Classify Find this BioLab using the linkbelow, and post your findings in the tableprovided for this activity. Using the addi-tional data from other students on theInternet, prepare a dichotomous key thatallows you to identify your five trees.
(tl)Jerome Wyckoff/Earth Scenes (tr)Doug Wechsler/Earth Scenes (bl)Dick Keen/Visuals Unlimited (br)Michael A. Dirr, University of Georgia, Athens
Analyze Brainstorm in groups for reasons whysome trees may be suited for agroforestry while oth-ers are not. Discuss the link between agriculture andthe loss of tropical forests. How might more sustain-able agricultural methods decrease rain forestdestruction? Analyze, critique, and review your expla-nations as to their strengths and weaknesses usingscientific information and evidence.
To find out more about rain forest destruction and sustainable forestry, visit
Tropical rain forests are Earth’s most biologi-cally diverse ecosystems. Many people live
within areas of tropical rain forests and mostdepend directly or indirectly on the extraction ofresources from them through mining, growingcrops, ranching, and timber harvesting. This hasresulted in the loss of large areas of these forestseach year.
Agriculture in rain forests Traditional meth-ods of growing crops, ranching, and tree harvest-ing involved clearing land of tropical rain forestvegetation in order to provide space to growcrops, allow plants to grow as food for cattle, or toremove the most economically valuable trees.Often referred to as slash-and-burn agriculture,the land was cleared then used for agriculture untilthe soil’s nutrients were depleted, after which itwas abandoned to allow the regrowth of trees andother plants. This system worked as long as theremoval rate of the forest vegetation did notexceed the recovery rate of the forest vegetation.
A sustainable harvest Thousands of plantspecies grow in tropical rain forests, yet only afew species are considered valuable. Many ecolo-gists and conservation groups are working topromote more sustainable alternatives to obtain-ing resources from tropical rain forests.Agroforestry—an ecological approach to land usethat integrates the use of trees on individualfarms and in entire regions—has been used forcenturies to manage land for trees, crops, andanimals together. The philosophy behind agro-forestry is to maintain more of the tropical rainforest’s structure and function while providingfood and other resources for the farmer, such aswood that can be used or sold. In many areaswhere trees have been removed, new trees havebeen planted that do not compete with crops.This creates a more diverse and sustainable sys-tem. While agroforestry does not prevent theloss of species, it is hoped that by using moresustainable agricultural methods less tropical rainforest will be slashed and burned in the future.
Perspectives Only a small fraction of theworld’s tropical rain forests are managed sustain-ably. Efforts are being made to promote agro-forestry and other agricultural practices.However, not all trees and crops are suited to anagroforestry system, and agroforestry still resultsin a reduction of biodiversity because manyplants are removed from the understory to pre-vent competition with crops.
To help you review thediversity of plants, use the Organiza-tional Study Fold on page 577.
Section 22.2
bdol.glencoe.com/vocabulary_puzzlemaker(t)Ed Reschke/Peter Arnold, Inc. (c )Runk-Schoenberger/Grant Heilman Photography (b)Tom and Pat Leeson/DRK Photo
Review the Chapter 22 vocabulary words listed inthe Study Guide on page 601. Match the wordswith the definitions below.
1. the thick underground stem of a fern2. gametophyte’s female reproductive structure
in which eggs develop3. gametophyte of a fern4. seed structure that stores food for the
embryo5. anthophyte that has a life span of two years
6. Bryophytes, hepaticophytes, and anthocero-phytes are the three divisions of ________plants.A. vascular C. nonvascular B. seed D. evergreen
7. Lycophytes include ________.A. ferns C. mossesB. conifers D. club mosses
8. Anthophytes and coniferophytes are divi-sions that are BOTH ________.A. vascular and seed-producingB. vascular and non-seedC. nonvascular and non-seedD. nonvascular and seed-producing
9. Vascular tissue is important to a plantbecause it ________.A. anchors it in the soilB. reproducesC. transports water and nutrientsD. photosynthesizes
10. The plant in the photograph is a(n) ________.A. AnthophyteB. PterophyteC. ArthrophyteD. Gnetophyte
11. The gametophyte generation is dominant inwhich of the following plants?A. pine trees C. apple treesB. ferns D. mosses
12. Which of the following is NOT a part of aseed?A. gametophyte C. food supplyB. protective coat D. embryo
13. An orange tree would be classified in thesame division as which of the following?A. pine tree C. cycadB. moss D. sunflower
14. Open Ended Evaluate the significance ofthe adaptation—fertilization that does notrequire a film of water for sperm to reachthe egg—for land plants.
15. Open Ended Cycads and ginkgoes do nothave needlelike leaves like pines and sprucesdo. For these coniferophytes, explain thesignificance of leaf shape as an adaptation tothe biomes in which they grow.
16. Open Ended What might be the advantageof having the sporophyte dependent on the gametophyte?
17. Observe and Infer Examine the needlecross section in Figure 22.20. Infer how theposition of stomata helps reduce water loss.
18. Forests are essential to our economy and for the preservation of biodiversity. Visit
to investigate what is being done to preserve biodiversity offorest resources near you. Make a map ofone area and indicate where projects areplanned or are in place that help preservebiodiversity.
19. Concept Map Construct a concept map that shows the relationships among the fol-lowing terms: eggs, prothallus, archegonia,antheridia, sperm, fern, gametophyte.
Constructed Response/Grid InRecord your answers on your answer document.
25. Open Ended Which two plant divisions do you think arethe most important? Why?
26. Open Ended On gametophytes of certain mosses, theouter surfaces of cells are curved, as shown to the right.Describe the environment of mosses with such an adaptation.
Chloroplasts
Light
Outersurfaceof cell
Multiple ChoiceRussian olive is an introduced, nonnative seedplant that is crowding out native plants in certainparts of the United States. Study the graph belowand answer questions 20–22.
20. How many years does it take for there to be50 mature Russian olive plants in an area? A. 45 C. 40B. 100 D. none of these
21. After an area has 50 mature plants, howmany years must pass before there are100 mature plants?A. 45 C. 15B. 5 D. 25
22. The general pattern of development andestablishment of Russian olive plants in anarea can be described as ________.A. fast for many years then slow thereafterB. slow for many years then a rapid increaseC. steady throughout its time in an areaD. very rapid from the start
Use the graph below to answer questions 23 and 24.
23. What would be the minimum time youwould keep the experimental seeds underrefrigeration before planting?A. 1 month C. 6 monthsB. 3 months D. 80 months
24. How long does it take to get 50 percent germination?A. 21⁄2 months C. 1 monthB. 6 months D. 4 months
