chapter 23: plant structure and...

604

What You’ll Learn■ You will describe and compare

the major types of plant cellsand tissues.

■ You will identify and analyzethe structure and functions ofroots, stems, and leaves.

■ You will identify plant hor-mones and determine thenature of plant responses.

Why It’s ImportantHumans and the organismsaround them, including plants,share an environment. By know-ing about plant structure andhow plants function, you canbetter understand how humansand plants interact.

Plant Structure and Function

Plant Structure and Function

David Sieren/Visuals Unlimited

Visit to• study the entire chapter online• access Web Links for more

information and activities onplant structure and function

• review content with theInteractive Tutor and self-check quizzes

These pitcher plants look differ-ent from the plants that sur-round them. However, they andthe other plants have similarplant systems and subsystems.

Understandingthe Photo

bdol.glencoe.com

http://bdol.glencoe.com

23.1 PLANT CELLS AND TISSUES 605

Types of Plant Cells Like all organisms, plants are composed of cells. Plant cells are different

from animal cells because they have a cell wall, a central vacuole, and cancontain chloroplasts. Figure 23.1 shows a typical plant cell. Plants, just likeother organisms, are composed of different cell types.

ParenchymaParenchyma (puh RENG kuh

muh) cells are the most abundantkind of plant cell. They are foundthroughout the tissues of a plant.These spherical cells have thin,flexible cell walls. Most parenchymacells usually have a large centralvacuole, which sometimes containsa fluid called sap.

Chloroplast

Central vacuole

Cell wall

Plant Cells and Tissues

Figure 23.1 Plant cells have several distinguishingfeatures, such as a cell wall, chloro-plasts, and a large central vacuole.

SECTION PREVIEWObjectivesIdentify the major typesof plant cells.Distinguish among thefunctions of the differenttypes of plant tissues.

Review Vocabularyvacuole: membrane-bound,

fluid-filled space in thecytoplasm of cells usedfor the temporary stor-age of materials (p. 183)

New Vocabularyparenchymacollenchymasclerenchymaepidermisstomataguard celltrichomexylemtracheidvessel elementphloemsieve tube membercompanion cellmeristemapical meristemvascular cambiumcork cambium

23.1

Plants Cells and Tissues Make the following Foldable to help you compare the types of plant cells and tissues.

Plantcells

Plant tissues

Identify and Describe As you read Chapter 23, identify and describe the types of plant cells and plant tissues under each tab.

Fold a vertical sheet of paper in half from top to bottom.

Fold in half from side to side with the fold at the top.

Unfold the paper once. Cut only the fold of the top flap to make two tabs.

Turn the paper vertically and label the front tabs as shown.

STEP 1

STEP 3

STEP 2

STEP 4

Parenchyma cells, as shown inFigure 23.2A, have two main func-tions: storage and food production.The large vacuole found in these cellscan be filled with water, starch grains,or oils. The edible portions of manyfruits and vegetables are composedmostly of parenchyma cells. Paren-chyma cells also can contain numer-ous chloroplasts that produce glucoseduring photosynthesis.

Collenchyma

Collenchyma (coh LENG kuh muh)cells are long cells with unevenlythickened cell walls, as illustrated inFigure 23.2B. The structure of thecell wall is important because it allowsthe cells to grow. The walls of col-lenchyma cells can stretch as the cellsgrow while providing strength andsupport. These cells are arranged intubelike strands or cylinders that pro-vide support for surrounding tissue.The long tough strands you may havenoticed in celery are composed of collenchyma.

SclerenchymaThe walls of sclerenchyma (skle

RENG kuh muh) cells are very thickand rigid. At maturity, these cellsoften die. Although their cytoplasmdisintegrates, their strong, thick cellwalls remain and provide support forthe plant. Sclerenchyma cells can beseen in Figure 23.2C. Two types ofsclerenchyma cells commonly foundin plants are fibers and sclerids (SKLER

idz). Fibers are long, thin cells thatform strands. They provide supportand strength for the plant and are thesource of fibers used for making linenand rope. A type of fiber is associatedwith vascular tissue, which you willlearn about later in this section.Sclerids are irregularly shaped andusually found in clusters. They arethe gritty texture of pears and a majorcomponent of the pits found inpeaches and other fruits.

Compare and contrast the structures and func-tions of parenchyma, collenchyma,and sclerenchyma.

606 PLANT STRUCTURE AND FUNCTION

par- from theGreek word para,meaning “beside”coll- from theGreek word kolla,meaning “glue”scler- from theGreek wordskleros, meaning“hard” Parenchyma, col-lenchyma, and sclerenchyma areall types of planttissues.

Figure 23.2Plants are composed ofthree basic types ofcells, which are shownhere stained with dyes.

Parenchyma cells are foundthroughout a plant. Because theircell walls are flexible, parenchymacells can have different shapes.

A Collenchyma cells often are foundin parts of the plant that are stillgrowing. Notice the unevenlythickened cell walls.

B The walls of sclerenchyma cellsare very thick. These dead cellsare able to provide support forthe plant.

C

LM Magnification: 120�LM Magnification: 54� LM Magnification: 80�

(bl)Jack M. Bostrack/Visuals Unlimited (bc)Ken Wagner/Phototake, NYC (br)Biophoto Associates/Science Photo Library/Photo Researchers

607

Plant TissuesRecall that a tissue is a group of cells

that function together to perform anactivity. Tissues can be referred to asplant subsystems. There are several dif-ferent tissue types in plants.

Dermal tissuesThe dermal tissue, or epidermis, is

composed of flattened cells that coverall parts of the plant. It functionsmuch like the skin of an animal, cov-ering and protecting the body of aplant. As shown in Figure 23.3, thecells that make up the epidermis aretightly packed and often fit togetherlike a jigsaw puzzle. The epidermalcells produce the waxy cuticle thathelps prevent water loss.

Another structure that helps con-trol water loss from the plant, a stoma,is part of the epidermal layer. Stomata(STOH mah tuh) (singular, stoma) areopenings in leaf tissue that control theexchange of gases. Stomata are foundon green stems and on the surfaces ofleaves. In many plants, fewer stomataare located on the upper surface of theleaf as a means of conserving water.Cells called guard cells control theopening and closing of stomata. The

opening and closing of stomata regu-lates the flow of water vapor from leaftissues. You can learn more aboutstomata in the BioLab at the end of thischapter.

The dermal tissue of roots mayhave root hairs. Root hairs are exten-sions of individual cells that help theroot absorb water and dissolved min-erals. On the stems and leaves ofsome plants, there are structurescalled trichomes. Trichomes (TRI

kohmz) are hairlike projections thatgive a stem or a leaf a “fuzzy” appear-ance. They help reduce the evaporationof water from the plant. In somecases, trichomes are glandular andsecrete toxic substances that help pro-tect the plant from predators.Stomata, root hairs, and trichomesare shown in Figure 23.4.

Figure 23.3 The cells of the epider-mis fit together tightly,which helps protect theplant and preventswater loss.

Figure 23.4 A root hair (A) is an extension of a root epidermal cell. A trichome (B) isunicellular or multicellular growth from an epidermal cell. Stomata (C)are openings in the leaf epidermis. Each stoma is surrounded by twoguard cells.

Color-enhanced SEM Magnification: 200�

Stained LM Magnification: 100�

AA

CC

BB

Color-enhanced SEMMagnification: 63�


Mic

rofil

ed S

cien

tific

LT

D/S

cien

ce P

hoto

Lib

rary

/Pho

to R

esea

rche

rs (

bl)(

t)B

arry

L.R

unk

/Gra

nt H

eilm

an P

hoto

grap

hy

(bc)Andrew Syred/Science Photo Library/Photo Researchers (br)Andrew Syred/Science Photo Library/Photo Researchers

Vascular tissuesFood, dissolved minerals, and

water are transported throughout theplant by vascular tissue. Xylem andphloem are the two types of vasculartissues. Xylem is plant tissue com-posed of tubular cells that transportswater and dissolved minerals from theroots to the rest of the plant. In seedplants, xylem is composed of fourtypes of cells—tracheids, vessel ele-ments, fibers, and parenchyma.

Tracheids (TRA kee uhdz) aretubular cells tapered at each end. Thecell walls between adjoining tracheidshave pits through which water anddissolved minerals flow.

Vessel elements are tubular cellsthat transport water throughout theplant. They are wider and shorter thantracheids and have openings in theirend walls, as shown in Figure 23.5. Insome plants, mature vessel elementslose their end walls and water and dis-solved minerals flow freely from onecell to another.

Although almost all vascular plantshave tracheids, vessel elements aremost commonly found in anthophytes.Conifers have tracheids but no vesselelements in their vascular tissues.This difference in vascular tissuescould be one reason why anthophytesare the most successful plants on Earth. Anthophyte vessel elements arethought to transport water more effi-ciently than tracheids because watercan flow freely from vessel element tovessel element through the openings intheir end walls.

You can learn more about vasculartissues in the MiniLab on this page.What other types of tissues are foundin vascular plants? To answer thisquestion, look at Figure 23.6 on thenext page.

Sugars and other organic com-pounds are transported throughout avascular plant within the phloem.


Figure 23.5Tracheids and vessel elements are the conductingcells of the xylem. These cells die when theymature but their cell walls remain.

Tracheid

Vessel element

ObserveExamining Plant Tissues Pipes arehollow. Their shape or structure allowsthem to be used efficiently in trans-porting water. Plant vascular tissueshave this same efficiency in structure.

Procedure! Snap a celery stalk in half and remove

a small section of “stringy tissue” from itsinside.

@ Place the material on a glass slide. Add several drops ofwater. Place a second glass slide on top. CAUTION: Usecaution when handling a microscope and glass slides.

# Press down evenly on the top glass slide with your thumbdirectly over the plant material.

$ Remove the top glass slide. Add more water if needed.Add a coverslip.

% Examine the celery material under low-and high-powermagnification. Diagram what you see.

^ Repeat steps 2–5 using some of the soft tissue inside thecelery stalk.

Analysis1. Describe Write a description of the stringy tissue under

low- and high-power magnification.2. Describe Write a description of the soft tissue under

low- and high-power magnification.3. Explain Does the structure of these tissues suggest their

functions?

Envision/George Mattei

A Plant’s Body PlanFigure 23.6There seems to be an almost endless variety ofvascular plants. Regardless of their diversity andnumerous adaptations, all vascular plants havethe same basic body plan. They are composed ofcells, tissues, and organs. Critical Thinking Whatare the different types of meristems, andhow do they help produce new plant systems and subsystems?

23.1 PLANT CELLS AND TISSUES 609

Apical meristem

Leaves

Vascular tissues

Stem

Apical meristems

Cells Most new plant cells are produced by cell divisions in regions of a plant calledmeristems. Meristematic cells continuallydivide. After each cell division, one of the two new cells remains meristematic and the other begins to differentiate. Two types of meristems—apical and lateral—producedifferent cell types. Apical meristems producecells that add length to stems and roots.Lateral meristems produce cells that increasestem and root diameters.

AA

BB

CC

Vascular plants

Tissues Plants have four types of tissues:dermal, vascular, ground, and meristematic.Dermal tissues cover the plant body. Vasculartissues transport water, food, and dissolvedsubstances throughout the plant. Photosynthesis,storage, and secretion are functions of groundtissue. Meristematic tissues produce most of aplant’s new cells.

Organs The major plant organs are stems, leaves, androots. They differ in structure among plant divisions butshare common functions. A stem is a plant organ thatprovides structural support and contains vascular tissues.Leaves and reproductive structures grow from stems.Usually, leaves are the organs in which photosynthesisoccurs. Leaf form differs among plants. Roots anchor aplant in soil or on another plant or structure. Most rootsabsorb water and dissolved substances that then aretransported in vascular tissues throughout the plant.

Hans Reinhard/OKAPIA/Photo Researchers

Phloem is made up of tubular cellsjoined end to end, as shown in Figure 23.7. It is similar to xylembecause phloem also has long cylin-drical cells. However these cells,called sieve tube members, are aliveat maturity. Sieve tube members areunusual because they contain cyto-plasm but do not have a nucleus orribosomes. Next to each sieve tubemember is a companion cell.Companion cells are nucleated cellsthat help with the transport of sugarsand other organic compoundsthrough the sieve tubes of thephloem. In anthophytes, the end wallsbetween two sieve tube members arecalled sieve plates. The sieve plateshave large pores that allow sugar andorganic compounds to move fromsieve tube member to sieve tubemember. Phloem can transport mate-rials from the roots to the leaves also.You can learn more about vascular tis-sues in Problem-Solving Lab 23.1.

The vascular phloem tissue ofmany plants contains fibers. Althoughthe fibers are not used for transportingmaterials, they are important becausethey provide support for the plant.

Ground tissueGround tissue is composed mostly

of parenchyma cells but it may also include collenchyma and scle-renchyma cells. It is found through-out a plant and often is associatedwith other tissues. The functions ofground tissue include photosynthesis,storage, and support. The cells ofground tissue in leaves and somestems contain numerous chloroplaststhat carry on photosynthesis. Groundtissue cells in some stems and rootscontain large vacuoles that storestarch grains and water. Cells, such asthose shown in Figure 23.8, are oftenseen in ground tissue.

Companion cell

Sieve plate

Sieve tube member

Figure 23.7 Phloem tissue carries sugars and other organic compounds throughoutthe plant.

Figure 23.8 The numerous chloroplasts in this ground tissue produce food for the plant.


Chloroplasts Color-enhanced LMMagnification: 400�

Michael Eichelberger/Visuals Unlimited

Meristematic tissuesA growing plant produces new cells

in areas called meristems. Meristemsare regions of actively dividing cells.Meristematic cells are differentlyshaped parenchyma cells with largenuclei. There are several types ofmeristems; two types are shown inFigure 23.6 on page 609.

Apical meristems are found at ornear the tips of roots and stems. Theyproduce cells that allow the roots andstems to increase in length. Lateralmeristems are cylinders of dividingcells located in roots and stems. Theproduction of cells by the lateralmeristems results in an increase inroot and stem diameters. Most woodyplants have two kinds of lateral meris-tems—vascular cambium and corkcambium. The vascular cambiumproduces new xylem and phloem cellsin the stems and roots. The corkcambium produces cells with toughcell walls. These cells cover the sur-face of stems and roots. The outerbark of a tree is produced by the corkcambium.

A third type of lateral meristem isfound in grasses, corn, and othermonocots. This meristem adds cellsthat lengthen the part of the stembetween the leaves. These plants donot have a vascular or a cork cambium.

Understanding Main Ideas1. Describe the distinguishing characteristics of the

three types of plant cells.2. Identify and analyze the function of vascular tissue.

Name the two different types of vascular tissue.3. Explain the function of stomata.4. Draw a plant and identify and indicate where the

apical meristems would be located. How do theyfunction differently from lateral meristems in thedevelopment of a plant?

Thinking Critically5. Explain what type of plant cell you would expect

to find in the photosynthetic tissue of a leaf. Whatis another name for the photosynthetic tissue?

6. Compare and Contrast Compare and contrastthe cells that make up the xylem and the phloem.For more help, refer to Compare and Contrast inthe Skill Handbook.

SKILL REVIEWSKILL REVIEW

23.1 PLANT CELLS AND TISSUES 611bdol.glencoe.com/self_check_quiz

Apply ConceptsWhat happens if vascular tissue is interrupted? Antho-phytes have tissues within their organs that transport materialsfrom roots to leaves and from leaves to roots. What happens ifthis pathway is experimentally interrupted?

Solve the ProblemA thin sheet of metal was insertedhalfway through the stem of a liv-ing tree as shown in the diagram.One day later, the following analy-sis was made:• Concentration of water and dis-

solved minerals directly belowthe metal sheet was higher thanabove the metal sheet.

• Concentration of sugar directlyabove the metal sheet washigher than directly below themetal sheet.

Thinking Critically1. Explain What is the function of phloem? Why was the

concentration of sugars different on either side of themetal sheet?

2. Explain What is the function of xylem? Why was the con-centration of dissolved minerals and water different oneither side of the metal sheet?

3. Analyze How would the experimental findings differ if themetal sheet were inserted only into the bark of the tree?

Higherwater anddissolved minerallevels

Highersugarlevels

Metalplate

http://bdol.glencoe.com/self_check_quiz


RootsRoots are plant organs that anchor a plant, usually absorb water and

dissolved minerals, and contain vascular tissues that transport materialsto and from the stem. As shown in Figure 23.9, roots may be short orlong, and thick and massive or thin and threadlike. The surface area of aplant’s roots can be as much as 50 times greater than the surface area of

its leaves. Most roots grow in soilbut some do not.

The type of root system is geneti-cally determined but can varybecause of environmental factorssuch as soil type, moisture, and tem-perature. There are two main typesof root systems—taproots andfibrous roots. Carrots and beets aretaproots, which are single, thickstructures with smaller branchingroots. Taproots accumulate and storefood. Fibrous roots systems havemany, small branching roots thatgrow from a central point.

Some plants, such as the corn inFigure 23.10, have a type of root

Roots, Stems, and Leaves

(t)KS Studios (bl)Mark Douet/Tony Stone Images (br)John D. Cunningham/Visuals Unlimited

Figure 23.9 The taproot of a carrot plantcan store large quantities offood and water (A). The fibrousroots of grasses absorb waterand anchor the plant (B).

AA BB

23.2SECTION PREVIEWObjectivesIdentify and comparethe structures of roots,stems, and leaves.Describe and comparethe functions of roots,stems, and leaves.

Review Vocabularyorgan: group of two or

more tissues organizedto perform complexactivities within anorganism (p. 210)

New Vocabularycortexendodermispericycleroot capsink translocationpetiolemesophyll transpiration

Do you like to eat plant organs?Using Prior Knowledge The next timeyou eat a salad, look closely at its con-tents. Did you know that most of theitems you are eating are plant organs?Lettuce and spinach are leaves. Carrotsand radishes are roots. Asparagus is astem. Bean and alfalfa sprouts includeimmature leaves, stems, and roots. Thereare more than one-quarter million kinds ofplants on Earth, and their organs exhibit anamazing variety.Experiment After reading the first two sections of this chapter, design an investigation to demonstrate how vascular tissue is common to roots, stems, and leaves. Show your plan to yourteacher and get permission to perform the investigation. Be sure to follow all labora-tory safety rules. Share your findings with your class.

Salad of roots,stems, and leaves

called prop roots, which originateabove ground and help support a plant.Many climbing plants have aerial rootsthat cling to objects such as walls andprovide support for climbing stems.When bald cypress trees grow inswampy soils, they produce modifiedroots called pneumatophores, whichare referred to as “knees.” The kneesgrow upward from the mud, and even-tually, out of the water. Knees helpsupply oxygen to the roots.

The structure of rootsIf you look at the diagram of a root

in Figure 23.11, you can see that aroot hair is a tiny extension of an epi-dermal cell. Root hairs increase thesurface area of a root that contacts thesoil. They absorb water, oxygen, anddissolved minerals. The next layer is apart of the ground tissue called thecortex, which is involved in the trans-port of water and dissolved mineralsinto the vascular tissues. The cortex is

made up of parenchyma cells thatsometimes store food and water.

At the inner limit of the cortex liesthe endodermis, a layer of cells withwaterproof cell walls that form a sealaround the root’s vascular tissues.

Zea Mays/Visuals Unlimited

Cortex

Epidermis

Root hair

EndodermisPericycle

Endodermalcells Waterproof

seal

Figure 23.11Water and dissolved minerals moveinto the root along two pathways.

Dissolved minerals and waterenter root hairs and travelthrough and between thecells of the cortex.

Minerals dissolved in water can flowbetween the parenchyma cells, directlyinto the root cortex, then through thecells of the endodermis.

Figure 23.10As a corn plant grows,prop roots grow from thestem and help keep thetall and top-heavy plantupright.

23.2 ROOTS, STEMS, AND LEAVES 613

AA

BB

The waterproof seal of the endodermisforces water and dissolved minerals thatenter the root to pass through the cellsof the endodermis. Thus, the endoder-mis controls the flow of water and dis-solved minerals into the root. Next tothe endodermis is the pericycle. It isthe tissue from which lateral roots ariseas offshoots of older roots.

Xylem and phloem are located inthe center of the root. The arrange-ment of xylem and phloem tissues, asshown in Figure 23.12, accounts forone of the major differences betweenmonocots and dicots. In dicot roots,the xylem forms a central star-shapedmass with phloem cells between the rays of the star. Monocot rootsusually have strands of xylem thatalternate with strands of phloem.There is sometimes a central core ofparenchyma cells in the monocot rootcalled a pith.

Root growthThere are two areas of rapidly divid-

ing cells in roots where the productionof new cells initiates growth. The rootapical meristem produces cells that


pericycle from theGreek words peri,meaning “around,”and kykos, meaning“circle”; In vascularplants, the pericy-cle can producelateral roots.endodermis fromthe Greek wordsendon, meaning“within,” and dermis, meaning“skin”; In vascularplants, the endo-dermis is the inner-most layer of cellsof the root cortex.

Runk-Schoenberger/Grant Heilman Photography

Figure 23.13Roots develop by both cell division and elon-gation. As the number and size of cellsincreases, the root grows in length and width.

Phloem

Pericycle

Endodermis

Root cap

Apical meristem

Root hairs

Xylem

Figure 23.12The root structures of dicots andmonocots differ inthe arrangementof xylem andphloem.

The xylem in this dicot root is arranged in acentral star-shaped fashion. The phloem isfound between the points of the star.

A In this monocot, there are alternating strandsof xylem and phloem that surround a core ofparenchyma cells.

B

Xylem

Phloem

Parenchyma

Stained LM Magnification: 20� Stained LM Magnification: 15�

cause a root to increase in length. Asthese cells begin to mature, they dif-ferentiate into different types of cells.In dicots, the vascular cambium devel-ops between the xylem and phloemand contributes to a root’s growth byadding cells that increase its diameter.

Each layer of new cells producedby the root apical meristem is left far-ther behind as new cells are addedand the root grows forward throughthe soil. The tip of each root is covered by a protective layer ofparenchyma cells called the root cap.As the root grows through the soil,the cells of the root cap wear away.Replacement cells are produced bythe root apical meristem so the roottip is never without its protective cov-ering. Examine Figure 23.13 on theprevious page to see if you can locateall the structures of a root.

StemsStems usually are the aboveground

parts of plants that support leaves andflowers. They have vascular tissuesthat transport water, dissolved miner-als, and sugars to and from roots andleaves. Their form ranges from thethin, herbaceous stems of basil plantsto the massive, woody trunks of trees.Green, herbaceous stems are soft andflexible and usually carry out somephotosynthesis. Petunias, impatiens,and carnations are other examples ofplants with herbaceous stems. Trees,shrubs, and some other perennialshave woody stems. Woody stems arehard and rigid and have cork and vas-cular cambriums.

Some stems are adapted to storingfood. This can enable the plant tosurvive drought or cold, or growfrom year to year. Stems that act asfood-storage organs include corms,tubers, and rhizomes. A corm is ashort, thickened, underground stem

surrounded by leaf scales. A tuber is aswollen, underground stem that hasbuds from which new plants cangrow. Rhizomes also are undergroundstems that store food. Some examplesof these food-storing stems are shownin Figure 23.14.

23.2 ROOTS, STEMS, AND LEAVES 615(t)J.R. Waalandi/University of Washington/BPS (c)Image reprinted from The Visual Dictionary of Plants with permission from Dorling Kindersley Publishing (b)Robert & Linda Mitchell

A white potato is a tuber.A

This gladiolus cormis a thickened,underground stemfrom which roots,leaves, and flowerbuds arise.

B

The rhizome ofan iris growshorizontallyunderground.

C

Figure 23.14 Plants can use food stored in stems to survivewhen conditions are less than ideal.

Internal structure Both stems and roots have vascular

tissues. However, the vascular tissues instems are arranged differently from thatof roots. Stems have a bundled arrange-ment or circular arrangement of vascu-lar tissues within a surrounding mass ofparenchyma tissue. As you can see inFigure 23.15A and B, monocots anddicots differ in the arrangement of vas-cular tissues in their stems. In mostdicots, xylem and phloem are in a circleof vascular bundles that form a ring inthe cortex. The vascular bundles ofmost monocots are scattered through-out the stem.

Woody stemsMany conifers and perennial dicots

produce thick, sturdy stems, as shownin Figure 23.15C, that may last severalyears, or even decades. As the stems ofwoody plants grow in height, they alsogrow in thickness. This added thick-ness, called secondary growth, resultsfrom cell divisions in the vascular cam-bium of the stem. The xylem tissueproduced by secondary growth is alsocalled wood. In temperate regions, atree’s annual growth rings are the lay-ers of vascular tissue produced eachyear by secondary growth. Theseannual growth rings can be used toestimate the age of the plant. The vas-cular tissues often contain scle-renchyma fibers that provide supportfor the growing plant.

As secondary growth continues, theouter portion of a woody stem devel-ops bark. Bark is composed of phloemcells and the cork cambium. Bark is atough, corky tissue that protects thestem from damage by burrowinginsects and browsing herbivores.

Stems transport materialsWater, sugars, and other com-

pounds are transported within thestem. Xylem transports water and

616 PLANT STRUCTURE AND FUNCTIONCarolina Biological/Visuals Unlimited

Figure 23.15 Stems have vascularbundles.

Cork

Phloem

Annual growth rings

Vascularcambium

Xylem

Woody stems are composedprimarily of dead xylem cells.

C

The vascular bundlesin a monocot arescattered through-out the stem as seenin this cross section.

A

As seen in this crosssection, youngherbaceous dicotstems have separatebundles of xylem andphloem that form aring. In older stems,the vascular tissuesform a continuouscylinder.

B

Color-enhanced LM Magnification: 4�

Color-enhanced LM Magnification: 2�

dissolved minerals from the roots tothe leaves. Water that is lost throughthe leaves is continually replaced bywater moving in the xylem. Waterforms an unbroken column within thexylem. As water moves up throughthe xylem, it also carries dissolvedminerals to all living plant cells.

The contents of phloem are pri-marily dissolved sugars but phloemalso can transport hormones, viruses,and other substances. The sugarsoriginate in photosynthetic tissuesthat are usually in leaves. Any portionof the plant that stores these sugars iscalled a sink, such as the parenchymacells that make up the cortex in theroot. The movement of sugars in thephloem is called translocation (transloh KAY shun). Figure 23.16 showsthe movement of materials in the vas-cular tissues of a plant.

Growth of the stemPrimary growth in a stem is similar

to primary growth in a root. Thisincrease in length is due to the produc-tion of cells by the apical meristem,

which lies at the tip of a stem. As men-tioned earlier, secondary growth or anincrease in diameter is the result of celldivisions in the vascular cambium orlateral meristem. Meristems located atintervals along the stem, called nodes,give rise to leaves and branches.

LeavesThe primary function of the leaves

is photosynthesis. Most leaves have arelatively large surface area thatreceives sunlight. Sunlight passesthrough the transparent cuticle andepidermis into the photosynthetic tis-sues just beneath the leaf surface.

Leaf variationWhen you think of a leaf, you

probably think only of a flat, broad,green structure. This part of the leafis called the leaf blade. Sizes, shapes,and types of leaves vary enormously.The giant Victoria water lily thatgrows in some of the rivers of Guyanahas floating, circular leaves that canbe more than two meters in diameter.


Water

Water lostthrough leaves

Xylem

Phloem

SugarSourceof sugars

Sieve plateCompanion cell

Sink

Figure 23.16Xylem carries waterup from roots toleaves. Phloem trans-ports sugars fromphotosynthetic tissuesto sinks locatedthroughout the plant.

The open ends of xylem vessel cellsform complete pipelike tubes.

A Sugars in the phloem of this carrot plant are moving to sinks.

B

The leaves of duckweed, a commonfloating plant of ponds and lakes, aremeasured in millimeters. Some plantspecies commonly produce differentforms of leaves on one plant.

Some leaves, such as grass blades,are joined directly to the stem. Inother leaves, a stalk joins the leafblade to the stem. This stalk, which ispart of the leaf, is called the petiole(PE tee ohl). The petiole contains vas-cular tissues that extend from thestem into the leaf and form veins. Ifyou look closely, you will notice theseveins as lines or ridges running alongthe leaf blade.

Leaves vary in their shape andarrangement on the stem. A simpleleaf is one with a blade that is notdivided. When the blade is dividedinto leaflets, it is called a compoundleaf. Figure 23.17 gives some exam-ples of the variety of leaf shapes.

The arrangement of leaves on astem can vary. Leaves can grow fromopposite sides of the stem in an alter-nating arrangement. If two leavesgrow opposite each other on a stem,the arrangement is called opposite.Three or more leaves growing arounda stem at the same position is called awhorled arrangement.

Leaf structureThe internal structure of a typical

leaf is shown in Figure 23.18. Thevascular tissues are located in themidrib and veins of the leaf. Justbeneath the epidermal layer are twolayers of mesophyll. Mesophyll (MEH

zuh fihl) is the photosynthetic tissueof a leaf. It is usually made up of twotypes of parenchyma cells—palisademesophyll and spongy mesophyll.The palisade mesophyll is made up ofcolumn-shaped cells containingmany chloroplasts. These cells arefound just under the upper epidermis

618 PLANT STRUCTURE AND FUNCTION(l)Runk-Schoenberger/Grant Heilman Photography (c)Aaron Haupt (r)Runk-Schoenberger/Grant Heilman Photography

StomataGuard cell

PhloemXylem

Vascularbundle

Palisademesophyll

Cuticle

Spongy mesophyll

Lower epidermis

Upperepidermis

Figure 23.18 The tissues of a leaf areadapted for photosynthe-sis, gas exchange, limitingwater loss, and transport-ing water and sugars.

Figure 23.17Leaf shapes vary, butmost are adapted toreceive sunlight.

The needlelike leaves of theevergreen yew can receivesunlight year round.

B

The tulip poplar is a deciduoustree with broad, distinctive,simple leaves.

C

The leaves of thewalnut are compoundwith many leaflets.

A

and receive maximum exposure tosunlight. Most photosynthesis takesplace in the palisade mesophyll.Below the palisade mesophyll is thespongy mesophyll, which is com-posed of loosely packed, irregularlyshaped cells. These cells usually aresurrounded by many air spaces thatallow carbon dioxide, oxygen, andwater vapor to freely flow around thecells. Gases can also move in and outof a leaf through the stomata, whichare located in the upper and/or lowerepidermis.

TranspirationYou read previously that leaves

have an epidermis with a waxy cuticleand stomata that help reduce waterloss. Guard cells are cells that sur-round and control the size of a stoma,as shown in Figure 23.19. The loss ofwater through the stomata is calledtranspiration. Learn more abouthow a plant’s surroundings may influ-ence rate of transpiration in Problem-Solving Lab 23.2 on this page.

Stoma

Chloroplast

Epidermalcells

Guardcell

Thickenedwalls

Water

The guard cells have flexiblecell walls.

A When water enters the guardcells, the pressure causes themto bow out, opening the stoma.

B As water leaves the guard cells, thepressure is released and the cellscome together, closing the stoma.

C

Figure 23.19 Guard cells regulate the size of the open-ing of the stomata according to theamount of water in the plant.

Draw ConclusionsWhat factors influence the rate of transpiration? Plantslose large amounts of water during transpiration. This processaids in pulling water up from roots to stem to leaves where itcan be used in photosynthesis.

Solve the ProblemA student was interestedin seeing if a plant’s surroundings mightaffect its rate of waterloss. A geranium plantwas set up as a control. A second geranium wassealed within a plasticbag and a third gera-nium was placed infront of a fan. All threeplants were placed under lights. The student’s experimental data are shown in the graph.

Thinking Critically1. Infer Which line, A, B, or C, might best represent the stu-

dent’s control data? Explain.2. a. Infer Which line might best represent the data with

the plant sealed within a bag? Explain.b. Identify What abiotic environmental factor was being

tested? 3. Infer Which line might best represent the data with the

plant in front of a fan? Explain.4. Conclude Write a conclusion for the student’s experiment.

Rate of Water Loss

Wat

er lo

ss

Time

A

B

C

(bl)G

.R.R

ober

ts (

bc)D

r.W

m.M

.Har

low

/Pho

to R

esea

rche

rs (

br)A

lfred

Pas

ieka

/Sci

ence

Pho

to L

ibra

ry/P

hoto

Res

earc

hers

AA

BB

CC

The opening and closing of guardcells regulate transpiration. As you readabout how guard cells work, look againat the diagrams in Figure 23.19. Guardcells are cells scattered among the cellsof the epidermis. The walls of thesecells contain fiberlike structures. Whenthere is more water available in sur-rounding cells than in guard cells, waterenters guard cells by osmosis. Thesefiberlike structures in the cell walls ofguard cells prevent expansion in width,not in length. Because the two guardcells are attached at either end, thisexpansion in length forces them to bowout and the stoma opens. When there isless water in surrounding tissues, waterleaves the guard cells. The cells returnto their previous shape, which reducesthe size of the stoma. The proper func-tioning of guard cells is importantbecause plants lose up to 90 percent ofall the water they transport from theroots by transpiration.

Venation patternsOne way to distinguish among dif-

ferent groups of plants is to examine thepattern of veins in their leaves. Theveins of vascular tissue run through themesophyll of the leaf. As shown inFigure 23.20, leaf venation patternsmay be parallel, netlike, or dichoto-mous. You can learn more about leafvenation in the MiniLab shown here.

Figure 23.20Leaf venation patterns help distinguish between monocots and dicots.Leaves of corn plants have parallel veins (A), a characteristic of manymonocots. Leaves of lettuce plants are netlike (B), a characteristic ofmany dicots. Leaves of ginkgoes are dichotomously veined (C).

Compare and Contrast Observing Leaves Identifying leaf characteristics can helpyou identify plants. Use these leaf images to complete thisfield investigation.

ProcedureCAUTION: Keep your hands away from your mouth whiledoing this investigation. Wash your hands thoroughly afteryou complete your work.! With your teacher’s permission, examine leaves on five dif-

ferent plants on your school campus, or observe preservedleaves. Do not use conifers.

@ Sketch a leaf from each plant. Beside each sketch, labelthe leaf as simple or compound, list its venation, and writethe word that describes its arrangement on the stem.

Analysis1. Collect and Organize Data As a class, place leaves hav-

ing the same three characteristics into groups. List thecharacteristics and count the number of leaves in eachgroup. Display class results in a bar graph.

2. Infer Why would a botanist compare and contrast leafstructure?

Leaf type Leaf venation Leaf arrangement

Simple Palmate Opposite

CompoundPinnate Alternate

Parallel Whorled

Leaf modificationsMany plants have leaves with struc-

tural adaptations for functions besidesphotosynthesis. Some plant leaveshave epidermal growths, as shown inFigure 23.21A, that release irritantswhen broken or crushed. Animals,including humans, learn to avoidplants with such leaves. Cactus spinesare modified leaves that help reducewater loss from the plant and provideprotection from predators.

Carnivorous plants, like the pitcherplant in Figure 23.21B, have leaveswith adaptations that can trap insectsor other small animals. Other leafmodifications include tendrils, thecurly structures on sweet peas, the

overlapping scales that enclose andprotect buds, and the colorful bractsof poinsettias.

Leaves often function as water orfood storage sites. The leaves of Aloevera, shown in Figure 23.21C, storewater. This adaptation ensures thelong-term survival of the plant whenwater resources are scarce. A bulbconsists of a shortened stem, a flowerbud, and thickened, immature leaves.Food is stored in the bases of theleaves. Onions, tulips, narcissus, andlilies all grow from bulbs.

Evaluate the signifi-cance of leaf structural adaptationsto their environments.

Understanding Main Ideas1. Compare and contrast the arrangement of xylem

and phloem in dicot roots and stems.2. Infer where you would expect to find stomata in a

plant with leaves that float on water, such as awater lily. Explain.

3. Describe the primary function of most leaves. List some other functions of leaves.

4. Explain how guard cells function and regulate thesize of a stoma.

Thinking Critically5. Compare and contrast the function and structure

of the epidermis and the endodermis in a vascularplant.

6. Get the Big Picture Construct a table that sum-marizes the structure and functions of roots, stems,and leaves. For more help, refer to Get the BigPicture in the Skill Handbook.



(l)A

ndre

w S

yred

/Sci

ence

Pho

to L

ibra

ry/P

hoto

Res

earc

hers

(c)

Imag

e re

prin

ted

from

The

Vis

ual D

ictio

nary

of

Pla

nts

with

per

mis

sion

fro

m D

orlin

g K

inde

rsle

y P

ublis

hing

(r)Alan & Linda Detrick/Photo Researchers

Figure 23.21Modified leaves servemany functions in addi-tion to photosynthesis.

The surface of a tomato leaf hasglandular hairs that help repelinsects and other predators.

A

The leaves of the pitcherplant are modified fortrapping insects.

B

The leaves of Aloe vera are adapted tostore water in a dry desert environment.

C

LM Magnification: 350�

bdol.glencoe.com/self_check_quiz


23.3SECTION PREVIEWObjectivesIdentify the major typesof plant hormones.Identify and analyze thedifferent types of plantresponses.

Review Vocabularystimulus: anything in an

organism’s internal orexternal environmentthat causes the organ-ism to react (p. 9)

New Vocabularyhormoneauxingibberellincytokininethylenetropismnastic movement

Plant HormonesPlants, like animals, have hormones that regulate growth and develop-

ment. A hormone is a chemical that is produced in one part of an organ-ism and transported to another part, where it causes a physiological change.Only a small amount of the hormone is needed to make this change.

Auxins cause stem elongationThe group of plant hormones called

auxins (AWK sunz) promote cell elongation.Indoleacetic (in doh luh SEE tihk) acid(IAA)—a naturally occurring auxin—is pro-duced in apical meristems of plant stems. IAAweakens the connections between the cellu-lose fibers in the cell wall, which allows a cellto stretch and grow longer. The combinationof new cells from the apical meristem andincreasing cell lengths leads to stem growth.Auxin is not transported in the vascular sys-tem. It moves from one parenchyma cell tothe next by active transport.

Plant Responses

Envision/Priscilla Connell

Shoot tip

Budsinhibited

Buds grow into side branches

Shoot tip removed

AA BB


Humans and Plants Respond to SunlightUsing an Analogy You step outside into the bright sunlight and immedi-ately raise your hand to shade your eyes. You react quickly to the bright sunlight. Plants react to sunlight, too. Often, however, plant responses to things in their environment are so slow that they can only be cap-tured by time-lapse photography. When filmed in this way, the flower heads of sunflowers can be seen mov-ing with the sun’s apparent movement across the sky. In this section, you will read about other plant stimuli and responses.Make and Use Tables As you read this section, make a table of plant stimuli and responses. Include the source of the stimulus and describe how the plant responds. When studying this chapter, use the table to review this section.

Sunflowers

Figure 23.22Auxin produced in the tip ofthe main shoot inhibits thegrowth of side branches (A).Once the tip is removed, theside branches start to grow (B).

Auxins have other effects on plantgrowth and development. Auxin pro-duced in the apical meristem inhibits thegrowth of side branches. Removing thestem tip reduces the amount of auxinpresent and allows the development ofbranches, as shown in Figure 23.22.

Auxin also delays fruit formation andinhibits the dropping of fruit from theplant. When auxin concentrationsdecrease, the ripened fruits of sometrees fall to the ground and deciduoustrees begin to shed their leaves.

Infer how a fruitgrower might use auxins.

Gibberellins promote growthThe group of plant growth hor-

mones called gibberellins (jih buhREH lunz) cause plants to grow tallerbecause, like auxins, they stimulate cellelongation. Unlike auxins, gibberellinsare transported in vascular tissue.Many dwarf plants, such as those inFigure 23.23, are short because theplant does not produce gibberellins orits cells are not receptive to the hor-mone. If gibberellins are applied to thetip of a dwarf plant, it will grow taller.Gibberellins also increase the rate ofseed germination and bud develop-ment. Farmers have learned to use gib-berellins to enhance fruit formation.Florists often use gibberellins toinduce flower buds to open.

Cytokinins stimulate cell division

The hormones called cytokinins(si tuh KI nihnz) stimulate mitosis andcell division. Cytokinins stimulate theproduction of proteins needed formitosis and cell division. Most cyto-kinins are produced in root meristems.This hormone travels up the xylem toother parts of the plant. The effects ofcytokinins are often enhanced by thepresence of other hormones.

Ethylene gas promotes ripeningThe plant hormone ethylene (EH

thuh leen) is a simple, gaseous com-pound composed of carbon andhydrogen. It is produced primarily byfruits, but also by leaves and stems.Ethylene is released during a specificstage of fruit ripening. It causes cellwalls to weaken and become soft.Ethylene speeds the ripening of fruitsand promotes the breakdown of com-plex carbohydrates to simple sugars.If you have ever enjoyed a ripe redapple you know that it tastes sweeterthan an immature fruit.

Many farmers use ethylene toripen green fruits or vegetables afterthey have been picked, as shown inFigure 23.24.

23.3 PLANT RESPONSES 623

auxin from theGreek word auxein,meaning “toincrease”; Auxincauses stem elon-gation by increasingcell length.

(t)R

unk-

Sch

oenb

erge

r/G

rant

Hei

lman

Pho

togr

aphy

(bl)Envision/Osentoski & Zoda (br)A. Louis Goldman/Photo Researchers

Figure 23.23 The bean plants in thispicture are geneticdwarfs. The two plantson the right weretreated with gibberellinand have grown to anormal height.

Figure 23.24 Tomatoes are usually pickedwhen they are green then theyare treated with ethylene.Most of the tomatoes you seein grocery stores have beenripened in this manner.

Lightsource

Elongated cells

Plant ResponsesWhy do roots grow down and most

stems grow up? Although a plantlacks a nervous system and usuallycannot make quick responses to stim-uli, it does have mechanisms thatenable it to respond to its environ-ment. Plants grow, reproduce, andreposition their roots, stems, andleaves in response to environmentalconditions, such as gravity, light, tem-perature, and amount of darkness.

Tropic responses in plantsAt the beginning of this section,

you read that the flower heads of sun-flowers slowly respond to the sun’sapparent movement across the sky.Tropism is a plant’s response to anexternal stimulus. The tropism iscalled positive if the plant growstoward the stimulus. The tropism iscalled negative if the plant growsaway from the stimulus.

The growth of a plant toward light iscalled phototropism. It is caused by anunequal distribution of auxin in theplant’s stem. There is more auxin onthe side of the stem away from thelight. This results in cell elongation,but only on that side. As these cellslengthen, the stem bends toward thelight, as shown in Figure 23.25A. Youcan learn more about phototropism inthe Problem-Solving Lab on this page.

There is another tropism associatedwith the upward growth of stems andthe downward growth of roots. Gravi-tropism is plant growth in response togravity. Gravitropic responses are ben-eficial to plants. Roots that grow downinto the soil are able to anchor theplant and can take in water and dis-solved minerals. Stems usually exhibit anegative gravitropism.

Some plants exhibit another tropismcalled thigmotropism, which is agrowth response to touch. The tendrils

Jim W. Grace/Photo Researchers

Figure 23.25Phototropism is the growthresponse of a plant towardlight (A). Thigmotropism isa growth response totouch (B).

624

AA

BB

Draw a ConclusionHow do plant stems respond to light? While working with young oat plants, Charles Darwin made discoveries about the response of young plant stems to light that helped explainwhy plants undergo phototropism. Scientists now know that this response is the result of an auxin that causes rapid cell elongation to occur along one side of a young plant stem. However, auxins were unknown during Darwin’s time.

Solve the ProblemStudy the before and after diagrams. The three plants areyoung oat stems. Note that the light source is directed at theplants from one side.

Thinking Critically1. Interpret Scientific Illustrations Which diagram (or

diagrams) supports the conclusion that light is the stimulusfor phototropism? Explain.

2. Interpret Scientific Illustrations Which diagram (or diagrams) supports the conclusion that the stem tip is thestimulus for phototropism? Explain.

3. Conclude Where might the auxin responsible for phototropism be produced? Explain.

BeforeA B C A B C

After

Opaquecover

Understanding Main Ideas1. Define a hormone.

2. Compare and contrast tropic responses and nasticmovements.

3. Explain how a plant can bend toward sunlight.What term describes this response?

4. Name one plant hormone and describe how itfunctions.

5. Explain why gardeners often remove stem tips ofchrysanthemum plants during early summer.

Thinking Critically6. One technique that has been used for years to

ripen fruit is to put a ripened banana in a paperbag with the unripe fruit. Infer what happensinside the bag.

7. Experiment Explain how you would design anexperiment to investigate the effects of differentcolors of light on the phototropism of a plant. For more help, refer to Experiment in the SkillHandbook.


23.3 PLANT RESPONSES 625(tl)Christi Carter/Grant Heilman Photography (bl)Christi Carter/Grant Heilman Photography (tr)Runk-Schoenberger/Grant Heilman Photography

of the vine in Figure 23.25B havecoiled around a fence after makingcontact during early growth.

Because tropisms involve growth,they are not reversible. The positionof a stem that has grown severalinches in a particular direction cannotbe changed. But, if the direction ofthe stimulus is changed, the stem willbegin growing in another direction.

Nastic responses in plantsA responsive movement of a plant

that is not dependent on the directionof the stimulus is called a nasticmovement. An example of a nasticmovement is the movement ofMimosa pudica leaflets when they are

touched, as shown in Figure 23.26A.This is caused by a change in waterpressure in the cells at the base ofeach leaflet. A dramatic drop in pres-sure causes the cells to become limpand the leaflets to change orientation.

Another example of a nasticresponse is the sudden closing of thehinged leaf of a Venus’s-flytrap, Figure 23.26B. If an insect triggerssensitive hairs on the inside of theleaf, the leaf snaps shut. Nasticresponses that are due to changes incellular water pressure are reversiblebecause they do not involve growth.The Mimosa pudica and Venus’s-flytrap leaves return to their originalpositions once the stimulus ends.

AA BB

Figure 23.26When leaflets ofMimosa pudica aretouched, they moveinward (A). Trigger hairsmust be touched toclose the hinged leaf ofa Venus’s-flytrap (B).Infer How do theseadaptations helpensure the long-termsurvival of eachspecies?

bdol.glencoe.com/self_check_quiz



Before YouBegin

If asked to count the totalnumber of stomata on aleaf, you might answer bysaying “that’s an impossi-ble task.” It may not benecessary for you to counteach and every stoma.Sampling is a techniquethat is used to arrive at ananswer that is close to theactual number. You willuse a sampling techniquein this BioLab.

1

2

3

4

5

Total

Average

Number of StomataTrial

Data Table 1

Determining the Number of Stomata on a Leaf

Problem How can you count the total number of stomata on a leaf?

Objectives In this BioLab, you will:■ Measure the area of a leaf.■ Observe the number of stomata seen under a high-power

field of view.■ Calculate the total number of stomata on a leaf.■ Use the Internet to collect and compare data from other

students.

Materials microscope rulerglass slide coverslipwater and dropper green leaf from an onion single-edged razor blade plant

Safety PrecautionsCAUTION: Wear latex gloves when handling an onion.

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

1. Copy Data Table 1 and Data Table 2.2. To calculate the area of the high-power field of view

for your microscope, go to Math Skills in the SkillHandbook. Enter the area in Data Table 2.

3. Obtain an onion leaf and carefully cut it open lengthwiseusing a single-edged razor blade. CAUTION: Be carefulwhen cutting with a razor blade.

4. Measure the length and width of your onion leaf inmillimeters. Record these values in Data Table 2.

5. Remove a small section of leaf and place it on a glassslide with the dark green side facing DOWN.

6. Add several drops of water and gently scrape away all greenleaf tissue using the razor blade. An almost transparentlayer of leaf epidermis will be left on the slide.

PROCEDUREPROCEDURE

PREPARATIONPREPARATION

LM Magnification: 200�

7. Add water and a coverslip to the epidermis. Observe under low-power magnification and locate an area where guard cells and stomata can be seen clearly. CAUTION: Use caution when handling a microscope, microscope slides, and coverslips.

8. Switch to high-power magnification.9. Count the number of stomata in your field of view.

This is Trial 1. Record your count in Data Table 1.10. Move the slide to a different area. Count the number

of stomata in this field of view. This is Trial 2. Record your count in Data Table 1.

11. Repeat step 10 for Trials 3, 4, and 5. Calculate the average number of stomata observed.

12. Calculate the total number of stomataon the entire onion leaf by followingthe directions in Data Table 2.

13. Clean allequipment as instructed by yourteacher, and return everything to itsproper place. Dispose of leaf tissueand coverslips properly. Wash yourhands thoroughly.

CLEANUP AND DISPOSAL

Area of high-power field of view

Length of leaf portion in mm

Width of leaf portion in mm

Calculate area of leaf (length � width)

Calculate number of high-power fields of view on leaf (area of leaf � the area of one high-power field of view)

Calculate total number of stomata (numberof high-power fields of view � average number of stomata from Data Table 1)

� _____ mm2

� _____ mm

� _____ mm2

� _____

� _____

Data Table 2

� _____ mm

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Communicate Compare your data with those of your classmates. Offerseveral reasons why your total number of stomata for the leaf may not beidentical to your classmates.

2. Predict Would you expect all plants to have the same number of stomataper high-power field of view? Explain your answer.

3. Compare and Contrast What are theadvantages to using sampling techniques?What are some limitations?

4. Analyze the following pro-cedures from this experiment and explainhow you can change them to improve theaccuracy of your data.a. five trials in Data Table 1b. calculating the area of your high-power

field of view

ERROR ANALYSISInterpret Data Find this BioLab using thelink below, and post your data in the datatables provided for this activity. Using theadditional data from other students on theInternet, analyze the combined data andcomplete your graph.

bdol.glencoe.com/internet_lab

23.3 PLANT RESPONSES 627Andrew Syred/Science Photo Library/Photo Researchers

http://bdol.glencoe.com/internet_lab


Red Poppyby Georgia O’Keeffe (1887–1986)

“When you take a flower in your hand andreally look at it,” O’Keeffe said, cupping herhand and holding it close to her face, “it’syour world for the moment. I want to givethat world to someone else. Most people inthe city rush around so, they have no time tolook at a flower. I want them to see itwhether they want to or not.”

American artist GeorgiaO’Keeffe attracted

much attention when thefirst of her many floralscenes was exhibited inNew York in 1924.Everything about thesepaintings—their color,size, point of view, andstyle—overwhelmed theviewer’s senses, just as theircreator had intended.

In describing her huge paintingsof solitary flowers, Georgia O’Keeffe said: “I decided that I wasn’t going to spend my lifedoing what had already been done.” Indeed, shedid do what had not been done by paintingenormous poppies, lilies, and irises on giant can-vases. Her use of colors and emphasis on shapessuggests nature rather than copying it with pho-tographic realism. Her work can be described asabstract. “I found that I could say things withcolor and shapes that I couldn’t say in any otherway—things that I had no words for,” she said.

The viewer’s eye is drawn into theflower’s heart In this early representation ofone of her familiar poppies, O’Keeffe directedthe viewer’s eye down into the poppy’s center bycontrasting the light tints of the outer ring ofpetals with the darkness of the poppy’s center.

The viewer’s eye is drawn to the center of theflower, much as the flower naturally attracts aninsect for reproduction purposes. The over-whelming size and detailed interiors ofO’Keeffe’s flowers give an effect similar to aphotographer’s close-up camera angle.

During her long life, O’Keeffe created hun-dreds of paintings. Her subjects included the flow-ers for which she is perhaps most famous, as wellas other botanical themes. Her paintings of NewMexico deserts are characterized by sweepingforms that portray sunsets, rocks, and cliffs.

Georgia O’Keeffe died in New Mexico in1986. She is remembered for her bold, vividpaintings that are, indeed, larger than life.

(l)Paul Strand/National Portrait Gallery, Smithsonian Institution/Art Resource, NY (r)Art Resource, NY/The Georgia O’Keeffe Foundation/Artists Rights Society, New York

Critique It’s easy to identify the flowers inO’Keeffe’s paintings, but can they be consideredscientific models? Look at the poppy flower photo-graph on page 957 of this book. Write a critiquethat evaluates each of these models—O’Keeffe’spoppy and the photograph—according to its ade-quacy in representing a poppy flower.

To find out more about Georgia O’Keeffe,visit bdol.glencoe.com/art

http://bdol.glencoe.com/art

Section 23.1

Section 23.2

Key Concepts■ Most plant tissues are composed of

parenchyma cells, collenchyma cells, and sclerenchyma cells.

■ Dermal tissue is a plant’s protective covering.■ Xylem moves water and dissolved minerals

up from roots and throughout the plant.Phloem transports sugars and organiccompounds throughout the plant.

■ Ground tissue often functions in food production and storage.

■ Meristematic tissues undergo cell divisions.Most plant growth results from new cellsproduced in the meristems.

Vocabularyapical meristem (p. 611)collenchyma (p. 606)companion cell (p. 610)cork cambium (p. 611)epidermis (p. 607)guard cell (p. 607)meristem (p. 611)parenchyma (p. 605)phloem (p. 610)sclerenchyma (p. 606)sieve tube member

(p. 610)stomata (p. 607)tracheid (p. 608)trichome (p. 607)vascular cambium

(p. 611)vessel element (p. 608)xylem (p. 608)

Plant Cells andTissues

Vocabularycortex (p. 613)endodermis (p. 613)mesophyll (p. 618)pericycle (p. 614)petiole (p. 618)root cap (p. 615)sink (p. 617)translocation (p. 617)transpiration (p. 619)

CHAPTER 23 ASSESSMENT 629(t)Microfiled Scientific LTD/Science Photo Library/Photo Researchers (c)Runk-Schoenberger/Grant Heilman Photography (b)Runk-Schoenberger/Grant Heilman Photography

To help you reviewplant structure and function, use theOrganizational Study Fold on page 605.

STUDY GUIDESTUDY GUIDE

Vocabularyauxin (p. 622)cytokinin (p. 623)ethylene (p. 623)gibberellin (p. 623)hormone (p. 622)nastic movement

(p. 625)tropism (p. 624)

PlantResponses

Section 23.3

Roots, Stems,and Leaves


Key Concepts■ Roots anchor plants and contain vascular

tissues. Root hairs absorb water, oxygen,and dissolved minerals. A root cap coversand protects each root tip.

■ Stems provide support, contain vasculartissues, and produce leaves. Some stemsare underground.

■ Leaves undergo photosynthesis. A stoma is an opening in the leaf epidermis, is sur-rounded by two guard cells, and takes inand releases gases. Veins in leaves are bundles of vascular tissues.

Key Concepts■ Plant hormones affect plant growth and

functions.■ Tropisms are growth responses to external

stimuli.■ Some nastic responses are caused by

changes in cell pressure.

bdol.glencoe.com/vocabulary_puzzlemaker

http://bdol.glencoe.com/vocabulary_puzzlemaker

630 CHAPTER 23 ASSESSMENT

Review the Chapter 23 vocabulary words listed inthe Study Guide on page 629. For each set ofvocabulary words, choose the one that does notbelong. Explain why it does not belong.

1. parenchyma—sclerenchyma—apical meristem2. vessel element—sieve tube member—

companion cell3. stomata—vascular cambium—epidermis4. root cap—translocation—sink5. cytokinin—hormone—tropism

6. The tissue that makes up the protective covering of a plant is ________ tissue.A. vascular C. groundB. meristematic D. dermal

7. This root cross sectionwith a core of vasculartissue is typical of________ plants.A. horsetailB. monocotC. dicotD. moss

8. A cambium and a meristem are examples of________ tissues.A. support C. growthB. protective D. transport

9. One of the primary structural differencesbetween dicot roots and stems is the________.A. arrangement of vascular tissues in roots

and stemsB. presence of stomata in roots C. lack of an epidermis in stemsD. presence of an apical meristem in stems

only10. The ripening of fruit is stimulated by the

presence of ________.A. gibberellinB. ethyleneC. auxinD. cytokinin

11. Which diagram correctly shows the func-tioning of guard cells?

A.

B.

12. Which terms complete this concept map?A. tracheids and

vessel elementsB. companion cells

and fibersC. tracheids and

sieve tubesD. companion cells

and sieve tubes

13. Open Ended In late winter, some sugarmaple trees have holes drilled in their trunksin order to collect their sap, a sugary fluid.This sap is processed to make maple syrup.Explain the source of the sap, and identify theplant system and subsystem that contains it.

14. Open Ended How does the endodermiscontrol the flow of water and ions into rootvascular tissues?

15. Compare and Contrast Identify and ana-lyze characteristics of plant systems and subsystems.

16. More than5000 products are made from the vasculartissues of about 1000 tree species in theUnited States. Investigate the production oflumber, paper, fuel, charcoal and its prod-ucts, fabrics, maple syrup, spices, dyes, anddrugs that come from vascular tissues. Visit

to research these topics. Prepare and present a poster or multimedia presentation of your findings.

REAL WORLD BIOCHALLENGE

Water

Water

bdol.glencoe.com

Water istransported in

is composed of

6.xylem

bdol.glencoe.com/chapter_test

http://bdol.glencoe.com/chapter_test

http://bdol.glencoe.com

bdol.glencoe.com/standardized_test

Multiple ChoiceUse data in the graph below to answer questions17 and 18.

17. Copper is an important soil micronutrientfor plants. According to the graph, the cop-per concentration that resulted in the wood-iest stem is ________.A. 3 ppmB. 0.5 ppmC. 1.5 ppmD. 4 ppm

18. Without enough copper, branches of someconifers twist as they grow. If you were atree grower and some of your conifer trees’branches were twisted and bent, what is thecorrect course of action to take first?A. Water the trees more.B. Apply fertilizer.C. Test the soil to determine

nutrient levels.D. Apply a pesticide.

Leaf samples from the same plant species werecollected from four different locations. Stomatawere counted and averaged, and the data weregraphed as shown below. Use the graph toanswer questions 19 and 20.

19. In which location might there be the mostrainfall?A. State AB. State BC. State CD. State D

20. How might the number of stomata correlatewith the amount of rainfall?A. more stomata, less rainfallB. no stomata, no rainfallC. more stomata, more rainfallD. fewer stomata, more rainfall

Nu

mb

er o

f st

om

ata/

leaf

Origin of leavesState A State B State C State D

Comparison of Numbers of Stomata

Rat

ing

Copper concentrationparts per million (ppm)

0

0 1 2 3 4

1

2

3

4

Stem Woodiness

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

21. Open Ended Sometimes foresters kill selected trees to reduce competition for limited environ-mental resources. They often use a process called girdling that involves removing a band of barkand some wood from around the trunk of a tree. Once this circle of material is removed, the treeeventually dies. Explain why this can happen.

22. Open Ended In the last decade, over three million acres of privately owned, forested land hasbeen converted to agricultural uses, real estate development, and other uses. Describe whatmight be the biological and ecological results of these changes.

CHAPTER 23 ASSESSMENT 631

http://bdol.glencoe.com/standardized_test

chapter 23: plant structure and...

Documents