Reading Essentials for Biology Glencoe (794133), страница 61
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Read the key terms and definitions in the Mini Glossary above. On the lines below, tell howthe key terms stomata and guard cells are related.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________READING ESSENTIALSChapter 23261NameDateClassSection23.1Plant Cells and Tissues, continued2.
Use the Venn diagram to compare and contrast the cells that make up xylem and phloem.Arrange the characteristics you list according to whether they are true for xylem only, truefor phloem only, or true for both.XylemPhloemBothCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about plant cells and tissues.262Chapter 23READING ESSENTIALSNameDateClassSection23.2 Roots, Stems, and LeavesSC.F.1.4.2 The student knows that body structures are uniquely designed and adapted for their function.◗ Before You ReadSalad ingredients include lettuce, tomatoes, radishes, cucumbers, onions, bean sprouts, carrots,and celery.
Botanists might look at a fresh salad and see plant organs. Think of salad ingredients youare familiar with. Decide whether they are roots, stems, or leaves and list them on the appropriateline below.Roots: ____________________________________________________________________________Stems: ____________________________________________________________________________Leaves: ___________________________________________________________________________◗ Read to LearnCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.RootsA root is a plant organ.
Roots anchor a plant, usually absorbwater and dissolved minerals, and contain vascular tissues that moveminerals to and from the stem. All roots are not alike. They can beshort or long, thick or thin, massive or threadlike. The surface areaof a plant’s roots can be as much as 50 times greater than the surface area of its leaves. Most roots grow in the soil, but some do not.There are two main types of root systems—taproots andfibrous roots. Carrots and beets are taproots. Taproots are single,thick structures with smaller branching roots. Taproots accumulate and store food. Fibrous roots have many small branchingroots that grow from a central point.TaprootsREADING ESSENTIALSFibrous rootsIdentifyDetails As you skim the section, highlight the structuresof roots, stems, and leaves inone color. Highlight the functions of roots, stems, andleaves in another color.Prop rootsChapter 23263NameDateClassSection23.2Roots, Stems, and Leaves, continuedOther types of roots include prop roots, aerial roots, and pneumatophores.
Corn has prop roots. They begin above ground andhelp support the plant. Some climbing plants have aerial roots.The aerial roots do not touch the ground. Instead, they cling towalls to provide support for the climbing stems. Bald cypress treesgrow mostly in swampy soils.
They produce modified roots calledpneumatophores, or “knees.” The knees grow upward from themud, and out of the water. They help supply oxygen to the roots.What is the structure of roots?A Dissolved minerals and waterenter root hairs and travelthrough and between thecells of the cortex.PericycleEndodermisCortexEpidermisRoot hairEndodermalcellsWaterproofsealB Minerals dissolved in water canflow between the parenchymacells, directly into the root cortex,then through the cells of the endodermis.264Chapter 23READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Locate the root hair on the diagram below, which shows across-section of a root. You can see that a root hair is an extensionof an epidermal cell.
The epidermal cells are in the epidermis, theoutermost layer of the root. Root hairs increase the surface areaof a root that touches the soil. Root hairs absorb water, oxygen,and dissolved minerals.Moving inward from the epidermis, the next layer of cells in theroot is the cortex. The cortex is ground tissue. It helps move waterand dissolved minerals into the vascular tissue. The cortex is madeup of parenchyma cells that sometimes store food and water.NameDateClassSection23.2Roots, Stems, and Leaves, continuedA The xylem in this dicot root is arranged in aXylemXylemPhloemPhloemParenchymaB In this monocot, there are alternatingCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.central star-shaped fashion. The phloem isfound between the points of the star.strands of xylem and phloem thatsurround a core of parenchyma cells.The next layer inward from the cortex is the endodermis.
Thisis a layer of waterproof cells that form a seal around the root’svascular tissues. The waterproof seal of the endodermis forcesthe water and dissolved minerals to pass through the cells ofthe endodermis. The endodermis controls the flow of water anddissolved minerals into the root.
Next to the endodermis is thepericycle. Lateral roots develop from the pericycle in older roots.Xylem and phloem are located in the center of the root. Oneof the major differences between monocots and dicots is thearrangement of the xylem and phloem tissue. In dicot plants,the xylem forms a central star with phloem cells between therays of the star. In monocot plants, strands of xylem alternatewith strands of phloem.
The alternating strands form a ring.XylemSometimes the center of the ring contains parenchyma cellscalled a pith.PhloemRoot hairsHow do roots grow?There are two areas of cell growth in roots. The root apicalmeristem produces cells that cause a root to increase in length.As these cells begin to mature, they develop into different typesof cells. In dicots, the area between the xylem and the phloembecomes vascular cambium. The additional cells in the vascularcambium add to the root’s diameter.Each layer of new cells produced by the root apical meristemis left farther behind as new cells are added.
The new cells causethe root to grow forward through the soil. A protective layer ofparenchyma cells called the root cap covers the tip of each root.As the root grows through the soil, the cells of the root capREADING ESSENTIALSPericycleEndodermisApical meristemRoot capChapter 23265NameDateClassSection23.2Roots, Stems, and Leaves, continuedwear away.
The apical meristemconstantly produces replacementcells so that the root tip is alwaysprotected.A A white potatoB This gladiolus cormis a thickened,underground stemfrom which roots,leaves, and flowerbuds arise.C The rhizome ofan iris growshorizontallyunderground.StemsMost often stems are theaboveground part of a plant thatsupports leaves and flowers.Stems have vascular tissues thatmove water, dissolved minerals,and sugars to and from roots andleaves. Some stems are thin andgreen, like those of pansies, basil,and carnations. Green stems aresoft and flexible.
They oftencarry out photosynthesis. Treesand shrubs have thick, woodystems. The stems of trees can bequite large. Woody stems arehard and rigid. They have corkand vascular cambiums.Some stems store food. Thishelps the plant to survive droughtor cold. Stems that act as foodstorage organs are corms, tubers,and rhizomes. All three areunderground stems. A corm is ashort thickened stem surroundedby leaf scales. A tuber is aswollen stem that has buds fromwhich new plants grow. Rhizomesalso store food. The figure at leftshows examples of stems that actas food-storage organs.What is inside a stem?The vascular tissue inside stems is arranged differently than thevascular tissue inside roots.
Stems have bundles of vascular tissuewithin parenchyma tissue. In monocots, the vascular bundles arescattered throughout the stem. In dicots, the vascular bundlesform a ring in the cortex.266Chapter 23READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.is a tuber.NameDateClassSection23.2Roots, Stems, and Leaves, continuedWoody Stems Many plants, such as conifers,produce thick, sturdy stems that last several yearsor decades. As the stems of woody plants growtaller, they also grow in thickness. This addedthickness, called secondary growth, is the resultof cell divisions in the vascular cambium of thestem.
The xylem tissue produced by secondarygrowth also is called wood. The annual growthrings in tree trunks are the layers of vascular tissue produced each year by secondary growth.As secondary growth continues, the outerportion of the woody stem develops bark.Bark is made of phloem cells and the corkcambium. Bark is a tough, corky tissue thatprotects the stem from damage by insects orplant-eating animals.Annual growth ringsCorkPhloemVascularcambiumXylemCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What does the stem do?The stem transports water, sugars, and other compounds. Xylem1. How does bark protect aplant?moves water and dissolved minerals from the roots to the leaves.Water that is lost as water vapor through the stomata is continually________________________replaced by water moving upward in the xylem.
Water forms anunbroken column within the xylem. As water moves up through________________________the xylem, it carries dissolved minerals to all living plant cells.________________________The phloem carries dissolved sugars. The sugars come fromthe photosynthetic tissues that are usually in the leaves. Any partof the plant that storesWater lostthese sugars is called aSourcethrough leavesSugarsink, such as theof sugarsparenchyma cells thatmake up the cortex inPhloemthe root.
The movement of sugars inthe phloem is calledtranslocationSinkXylem(trans lo KAY shun).The illustration at rightshows the movement ofmaterials in the vascularSieve platetissues of a carrot.WaterA The open ends of the xylem vesselcells form complete pipelike tubes.READING ESSENTIALSCompanion cellB Sugars in the phloem of thiscarrot plant are moving to sinks.Chapter 23267NameDateClassSection23.2Roots, Stems, and Leaves, continuedHow does a stem grow?2. Does primary growthlengthen or thicken astem?________________________Primary growth lengthens a stem.
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