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Name Date Class Section 23.2 Roots, Stems, and Leaves Indiana Standards Standard B.1.15 Understand and explain that, in biological systems, structure and function must be considered together. ◗ Before You Read Salad 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 you are familiar with. Decide whether they are roots, stems, or leaves and list them on the appropriate line below. Roots: ____________________________________________________________________________ Stems: ____________________________________________________________________________ Leaves: ___________________________________________________________________________ ◗ Read to Learn Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Roots A root is a plant organ. Roots anchor a plant, usually absorb water and dissolved minerals, and contain vascular tissues that move minerals to and from the stem. All roots are not alike. They can be short or long, thick or thin, massive or threadlike. The surface area of 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 and fibrous 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 branching roots that grow from a central point. Taproots READING ESSENTIALS Fibrous roots Identify Details As you skim the section, highlight the structures of roots, stems, and leaves in one color. Highlight the functions of roots, stems, and leaves in another color. Prop roots Chapter 23 263 Name Date Class Section 23.2 Roots, Stems, and Leaves, continued Other types of roots include prop roots, aerial roots, and pneumatophores. Corn has prop roots. They begin above ground and help support the plant. Some climbing plants have aerial roots. The aerial roots do not touch the ground. Instead, they cling to walls to provide support for the climbing stems. Bald cypress trees grow mostly in swampy soils. They produce modified roots called pneumatophores, or “knees.” The knees grow upward from the mud, and out of the water. They help supply oxygen to the roots. What is the structure of roots? A Dissolved minerals and water enter root hairs and travel through and between the cells of the cortex. Pericycle Endodermis Cortex Epidermis Root hair Endodermal cells Waterproof seal B Minerals dissolved in water can flow between the parenchyma cells, directly into the root cortex, then through the cells of the endodermis. 264 Chapter 23 READING ESSENTIALS Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Locate the root hair on the diagram below, which shows a cross-section of a root. You can see that a root hair is an extension of an epidermal cell. The epidermal cells are in the epidermis, the outermost layer of the root. Root hairs increase the surface area of 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 the root is the cortex. The cortex is ground tissue. It helps move water and dissolved minerals into the vascular tissue. The cortex is made up of parenchyma cells that sometimes store food and water. Name Date Class Section 23.2 Roots, Stems, and Leaves, continued A The xylem in this dicot root is arranged in a Xylem Xylem Phloem Phloem Parenchyma B In this monocot, there are alternating Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. central star-shaped fashion. The phloem is found between the points of the star. strands of xylem and phloem that surround a core of parenchyma cells. The next layer inward from the cortex is the endodermis. This is a layer of waterproof cells that form a seal around the root’s vascular tissues. The waterproof seal of the endodermis forces the water and dissolved minerals to pass through the cells of the endodermis. The endodermis controls the flow of water and dissolved minerals into the root. Next to the endodermis is the pericycle. Lateral roots develop from the pericycle in older roots. Xylem and phloem are located in the center of the root. One of the major differences between monocots and dicots is the arrangement of the xylem and phloem tissue. In dicot plants, the xylem forms a central star with phloem cells between the rays of the star. In monocot plants, strands of xylem alternate with strands of phloem. The alternating strands form a ring. Xylem Sometimes the center of the ring contains parenchyma cells called a pith. Phloem Root hairs How do roots grow? There are two areas of cell growth in roots. The root apical meristem produces cells that cause a root to increase in length. As these cells begin to mature, they develop into different types of cells. In dicots, the area between the xylem and the phloem becomes vascular cambium. The additional cells in the vascular cambium add to the root’s diameter. Each layer of new cells produced by the root apical meristem is left farther behind as new cells are added. The new cells cause the root to grow forward through the soil. A protective layer of parenchyma cells called the root cap covers the tip of each root. As the root grows through the soil, the cells of the root cap READING ESSENTIALS Pericycle Endodermis Apical meristem Root cap Chapter 23 265 Name Date Class Section 23.2 Roots, Stems, and Leaves, continued wear away. The apical meristem constantly produces replacement cells so that the root tip is always protected. is a tuber. B This gladiolus corm is a thickened, underground stem from which roots, leaves, and flower buds arise. C The rhizome of an iris grows horizontally underground. Stems Most often stems are the aboveground part of a plant that supports leaves and flowers. Stems have vascular tissues that move water, dissolved minerals, and sugars to and from roots and leaves. Some stems are thin and green, like those of pansies, basil, and carnations. Green stems are soft and flexible. They often carry out photosynthesis. Trees and shrubs have thick, woody stems. The stems of trees can be quite large. Woody stems are hard and rigid. They have cork and vascular cambiums. Some stems store food. This helps the plant to survive drought or cold. Stems that act as foodstorage organs are corms, tubers, and rhizomes. All three are underground stems. A corm is a short thickened stem surrounded by leaf scales. A tuber is a swollen stem that has buds from which new plants grow. Rhizomes also store food. The figure at left shows examples of stems that act as food-storage organs. What is inside a stem? The vascular tissue inside stems is arranged differently than the vascular tissue inside roots. Stems have bundles of vascular tissue within parenchyma tissue. In monocots, the vascular bundles are scattered throughout the stem. In dicots, the vascular bundles form a ring in the cortex. 266 Chapter 23 READING ESSENTIALS Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. A A white potato Name Date Class Section 23.2 Roots, Stems, and Leaves, continued Woody Stems Many plants, such as conifers, produce thick, sturdy stems that last several years or decades. As the stems of woody plants grow taller, they also grow in thickness. This added thickness, called secondary growth, is the result of cell divisions in the vascular cambium of the stem. The xylem tissue produced by secondary growth also is called wood. The annual growth rings in tree trunks are the layers of vascular tissue produced each year by secondary growth. As secondary growth continues, the outer portion of the woody stem develops bark. Bark is made of phloem cells and the cork cambium. Bark is a tough, corky tissue that protects the stem from damage by insects or plant-eating animals. Annual growth rings Cork Phloem Vascular cambium Xylem Copyright © 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. Xylem 1. How does bark protect a plant? 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 an unbroken 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 from the photosynthetic tissues that are usually in the leaves. Any part of the plant that stores Water lost these sugars is called a Source through leaves sink, such as the Sugar of sugars parenchyma cells that make up the cortex in Phloem the root. The movement of sugars in the phloem is called translocation Sink Xylem (trans lo KAY shun). The illustration at right shows the movement of materials in the vascular tissues of a carrot. Sieve plate Water A The open ends of the xylem vessel cells form complete pipelike tubes. READING ESSENTIALS Companion cell B Sugars in the phloem of this carrot plant are moving to sinks. Chapter 23 267 Name Date Class Section 23.2 Roots, Stems, and Leaves, continued How does a stem grow? 2. Does primary growth lengthen or thicken a stem? ________________________ Primary growth lengthens a stem. It is similar to primary growth in a root. The increase in length is due to the production of cells by the apical meristem at the tip of the stem. Meristems located along the stem, called nodes, are where leaves and branches develop. ________________________ Leaves The primary function of the leaves is photosynthesis. Most leaves have a relatively large surface area that receives sunlight. Sunlight passes into the photosynthetic tissues just beneath the leaf surface. The flat, broad, green surface of a leaf is called the leaf blade. Sizes, shapes, and types of leaves vary a great deal. Some leaves, such as those of the Victoria water lily, can be more than two meters in diameter. Other leaves, such as duckweed, are measured in millimeters. Some plants produce different forms of leaves on the same plant. A blade of grass is joined directly to the stem, but in other leaves, a stalk joins the leaf blade to the stem. The stalk is called a petiole (PE tee ohl) and is actually part of the leaf. The petiole contains vascular tissues that stretch out from the stem into the leaf and form veins. You can often see the veins if you look closely at a leaf. They look like lines or ridges. Leaves vary in shape. A simple leaf is one with a blade that is not divided. Maple leaves and tulip poplar leaves are examples of simple leaves. When a leaf blade is divided into leaflets it is called a compound leaf. The leaves of the walnut are an example of compound leaves. A The leaves of the walnut are compound with many leaflets. 268 Chapter 23 B The needlelike leaves of the evergreen yew can receive sunlight year round. C The tulip poplar is a deciduous tree with broad, distinctive, simple leaves. READING ESSENTIALS Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. ________________________ Name Date Class Section 23.2 Roots, Stems, and Leaves, continued Not all leaves grow on a stem in the same way. When two leaves grow directly opposite each other on a stem, the arrangement is called opposite. If the leaves are on opposite sides of the stem, but one is higher than the other, the arrangement is called alternate. When three or more leaves grow around a stem at the same position, the arrangement is called whorled. Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. What is the structure of a leaf? Use the illustration at right as you study the internal structure of a typical leaf. Notice the cuticle and the upper and lower epidermis. Between the epidermal layers are two layers of mesophyll. Mesophyll (MEH zuh fihl) is the photosynthetic tissue of a leaf. It is usually made up of two types of parenchyma cells— palisade mesophyll and spongy mesophyll. Most photosynthesis takes place in the palisade mesophyll. These cells receive maximum exposure to sunlight and have many chloroplasts. The spongy mesophyll cells are surrounded by many air spaces that allow carbon dioxide, oxygen, and water vapor to flow freely. Gases also can move in and out of the stomata. Cuticle Upper epidermis Palisade mesophyll Vascular bundle Xylem Phloem Lower epidermis Stomata Spongy mesophyll Guard cell What is transpiration? The waxy cuticle and stomata help reduce water loss. Guard cells control the size of a stoma. The loss of water through the stomata is called transpiration. The opening and closing of guard cells regulates transpiration. The proper functioning of guard cells is important because plants lose up to 90 percent of all the water they transport from the roots by transpiration. 3. What regulates transpiration? ________________________ ________________________ ________________________ What are some modifications in leaves? The pattern of veins in leaves is one way to distinguish different groups of plants. Veins are vascular tissues that run through the mesophyll of a leaf. The patterns can be parallel or netlike as shown on page 270. Parallel veins are found in many monocots. Many dicots have netlike veins. The leaves of ginkgoes are dichotomously veined. READING ESSENTIALS Chapter 23 269 Name Date Class Section 23.2 Roots, Stems, and Leaves, continued Leaf venation Parallel Many plants have leaves that have functions besides photosynthesis. Some leaves, when crushed or broken, release substances that cause pain or swelling. Animals and humans learn to avoid such leaves. Cactus spines are modified leaves that help reduce water loss and provide protection from plant-eaters. Carnivorous plants have leaves with adaptations that can trap insects or other small animals. Other leaf modifications include the curling tendrils in sweet peas, the colorful bracts on poinsettias, and the overlapping scales that enclose and protect buds. Leaves often act as water or food storage sites. The leaves of Aloe vera, for instance, store water. A bulb, on the other hand, provides an example of leaves storing food. A bulb consists of a shortened stem, a flower bud, and thick immature leaves. Food is stored in the base of the immature leaves. Onions, tulips, and narcissus grow from bulbs. ◗ After You Read Mini Glossary cortex: layer of ground tissue in the root that is involved in the transport of water and dissolved minerals into the vascular tissue of the root endodermis: single layer of cells that forms a waterproof seal around a root’s vascular tissue; controls the flow of water and dissolved minerals into the root mesophyll (MEH zuh fihl): photosynthetic tissue of a leaf pericycle: in plants, the layer of cells just within the endodermis that gives rise to lateral roots 270 Chapter 23 petiole (PE tee ohl): in plants, the stalk that joins the leaf blade to the stem root cap: tough, protective layer of parenchyma cells that covers the tip of a root sink: any part of a plant that stores sugars produced during photosynthesis translocation (trans loh KAY shun): movement of sugars in the phloem of a plant transpiration: in plants, the loss of water through stomata READING ESSENTIALS Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Netlike Name Date Class Section 23.2 Roots, Stems, and Leaves, continued 1. Read the key terms and definitions in the Mini Glossary on page 270. Circle any three of the terms that describe the structure or function of leaves. In the space provided explain in your own words how those three terms relate to leaves. ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ 2. For each plant part listed in the first column, write the name of one related structure in the second column and one function in the third column. Structure Function Roots Stems Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Leaves Visit the Glencoe Science Web site at science.glencoe.com to find your biology book and learn more about roots, stems, and leaves. READING ESSENTIALS Chapter 23 271