23 Plant Structure and Function, Slides of History

Download 23 Plant Structure and Function and more Slides History in PDF only on Docsity!

Plant Structure

and Function

The leaves of this sundew plant are adapted to capture and digest live prey.

662

THE HOLLOW TREE

As you hike through a Central American rain forest on a guided tour on the last day of your tropical vacation, you see many unusual plants and animals. A monkey calls from a distant tree, and a dense fog covers the landscape. Then you stumble on a root and look up. A massive tree stands before you. Its trunk seems to be made up of many intertwined woody branches. Edging closer, you nervously slip your head through one of the larger gaps and look straight up. Inside, you find that the tree is completely hollow. This tree, a species of fig, is truly unusual. What happened to the interior of the tree? And how did the tree grow to such a great height if it has no center? Your guide explains that the seeds of this fig species sprout high up in the branches of other forest trees, called hosts. The roots grow downward, through the air. She also explains that the tangled fig “branches” are not actually stems. But your tour ends before all your questions are answered. How does the fig seedling get nutrients? How does the growing fig tree affect its host? Look for clues that explain more about the structure and habits of this strange fig plant. Then, solve the mystery.

Never Stop Exploring Your World. Finding the solution to The Hollow Tree mystery is only the beginning. Take a video field trip with the ecogeeks of Untamed Science to see where the mystery leads.

TEKS

FO

CUSON

12A

Texas Essential Knowledge and Skills

READINESS TEKS: 10B Describe the interactions that occur among systems that perform the functions of transport, reproduction, and response in plants. Also covered: TEKS 4B.

SUPPORTING TEKS: 5B Examine specialized cells, including roots, stems, and leaves of plants; and animal cells such as blood, muscle, and epithelium. 11A Describe the role of internal feedback mechanisms in the maintenance of homeostasis. 11B Investigate and analyze how organisms, populations, and communities respond to external factors.

Plant Structure and Function 663

Leaf

Stem

Root

Dermal tissue Vascular tissue Ground tissue

Cross Section

What Parts of Plants Do We Eat?

1 Examine 2 3 4 Examine an onion, a 5 6 7 8 9 potato, and an artichoke. Record your observations as notes and labeled sketches. CAUTION: Do not eat the vegetables.

(^1 2) Use your observations to classify 3 4 5 6 7 8 9 each vegetable as a root, stem, leaf, or other plant part.

Analyze and Conclude

1. Classify How did you classify the onion? Explain what characteristics you used to make this decision. 2. Infer How did you classify the potato? How is its structure related to its function? 3. Infer How did you classify the artichoke? What does its inner structure tell you about its function?

Plant Tissue Systems What are the primary functions of the main tissue systems of seed plants? Within the roots, stems, and leaves of plants are specialized tissue systems, shown in Figure 23–1. Plants have three main tissue systems: dermal, vascular, and ground. Dermal tissue covers a plant almost like a skin. Vascular tissue forms a system of pipe like cells to support the plant and serve as its “bloodstream,” transporting water and nutrients. Ground tissue produces and stores food. All three systems interact to carry out essential functions such as transport, repro duction, and response to the environment.

Dermal Tissue Dermal tissue in young plants consists of a single layer of cells called the epidermis (ep uh dur mis). The outer surfaces of epidermal cells are often covered with a thick waxy layer called the cuticle, which protects against water loss. Some epidermal cells have tiny projections known as trichomes (try kohmz). Trichomes help protect the leaf and may give the leaf a fuzzy appearance. Dermal tissue is the protective outer covering of a plant. In older plants, dermal tissue may be many cell layers deep and may be covered with bark. In roots, dermal tissue includes root hair cells that absorb water, passing it along to ground and vascular tissue where it is carried to the rest of the plant.

Figure 23–1 Principal Organs of Plants These cross sections of the principal organs of seed plants show that all three organs contain dermal tissue, vascular tissue, and ground tissue. Interpret Visuals Which tissue type is found in the center of a root?

TEKS 5B, 10B

TEKS 5B

Plant Structure and Function 665

Tracheid

Vessel element

Xylem

Phloem

Cross Section of a Stem

Companion cell

Sieve tube element

LM 15

Vascular Tissue Vascular tissue includes xylem, a water-conducting tissue, and phloem, a tissue that carries nutrients. As you can see in Figure 23–2, xylem and phloem consist of long, slender cells that con- nect almost like sections of pipe. (^) Interactions between vascular tissues support the plant body and transport water and nutrients throughout the plant.  Xylem: Tracheids All seed plants have xylem cells called tracheids. Recall from Chapter 22 that tracheids are long and narrow, with tough cell walls that help to support the plant. As they mature, tracheids die, leaving only their cell walls. These cell walls contain lignin, a complex molecule that resists water and gives wood much of its strength. Openings in the walls connect neighboring cells and allow water to flow from cell to cell. Thinner regions of the wall, known as pits, allow water to diffuse from tracheids into surrounding ground tissue. These adaptations allow tracheids to carry water throughout the plant and distribute it to tissues where it is needed.

 (^) Xylem: Vessel Elements In addition to tracheids, angiosperms possess a second form of xylem tissue known as a vessel element. Vessel elements are wider than tracheids and are arranged end to end on top of one another like a stack of tin cans. After they mature and die, cell walls at both ends are left with slitlike openings through which water can move freely. In some vessel elements, the end walls disap- pear altogether, producing a continuous tube.

 (^) Phloem: Sieve Tube Elements Unlike xylem cells, phloem cells are alive at maturity. The main phloem cells are sieve tube elements, which are arranged end to end, forming sieve tubes. The end walls of sieve tube elements have many small holes through which nutri- ents move from cell to cell in a watery stream. As sieve tube elements mature, they lose their nuclei and most other organelles. The remain- ing organelles hug the inside of the cell wall and are kept alive by companion cells.  (^) Phloem: Companion Cells The cells that surround sieve tube elements are called companion cells. Companion cells keep their nuclei and other organelles through their lifetime. Companion cells support the phloem cells and aid in the movement of substances in and out of the phloem.

Figure 23–2 Vascular Tissue Xylem and phloem form the vascular transport system that moves water and nutrients throughout a plant. Compare and Contrast How are tracheids and sieve tube elements similar? How are they different?

666 Chapter 23 • Lesson 1

Long Section

Stem apical meristem

Root apical meristem

LM 60

LM 1200

BUILD Vocabulary

RELATED WORD FORMS Apex and apical are related word forms. Apex is a noun meaning the narrowed or pointed end, or tip, and apical is an adjective describing something related to or located at the apex.

Apical Meristems Because the tip of a stem or root is known as its apex, meristems in these rapidly growing regions are called apical meristems. Unspecialized cells produced in apical meristems divide rapidly as stems and roots increase in length. Figure 23–4 shows examples of stem and root apical meristems. At first, the new cells that are pushed out of meristems look very much alike: They are unspecialized and have thin cell walls. Gradu- ally, they develop into mature cells with specialized structures and functions. This process is called dif- ferentiation. As the cells differentiate, they produce each of the tissue systems of the plant, including dermal, vascular, and ground tissue.

Meristems and Flower Development The highly specialized cells found in cones and flowers (which are the reproductive organs of seed plants), are also produced in meristems. Flower or cone development begins when the pattern of gene expression changes in a stem’s apical meristem. These changes transform the apical meristem of a flowering plant into a floral meristem. Floral meri- stems produce the tissues of flowers, which include the plant’s reproductive organs as well as the color- ful petals that surround them.

Figure 23–4 Apical Meristems Apical meristems are found in the growing tips of stems and roots. Within these meristems, unspecialized cells are produced by mitosis.

1. a. Review What are the three main organs of seed plants? b. Interpret Diagrams Review Figure 23–1. How are the three main organs of seed plants similar in structure? 2. a. Review What are the three main tissue systems of plants? b. Compare and Contrast How do the main functions of a plant’s tissue systems differ? 3. a. Review What is the function of meristems? b. Form a Hypothesis How might the presence of meristems explain the ability of plants to regenerate from cuttings? 4. Compare and Contrast You prob- ably have some knowledge of the human circulatory system. Based on this knowledge, write a para- graph comparing and contrasting the structure and function of the vascular system of a plant to the human circulatory system. (Hint: Show how the systems are alike and different.)

23.1 Review Key Concepts TEKS 10B

668 Chapter 23 • Lesson 1

23.2 Roots

Key Questions What are the main tissues in a mature root? What are the different functions of roots?

Vocabulary root hair (^) • cortex (^) • endodermis (^) • vascular cylinder (^) • root cap (^) • Casparian strip

Taking Notes Outline Before you read, use the headings of the lesson to make an outline about plant roots. As you read, fill in phrases after each heading that provide key information.

Root Structure and Growth

What are the main tissues in a mature root?

As soon as a seed begins to sprout, it puts out its first root to draw water and nutrients from the soil. Other roots soon branch out from this first root, adding length and surface area to the root system. Rapid cell growth pushes the tips of the growing roots into the soil. The new roots provide raw materials for the developing stems and leaves before they emerge from the soil.

Types of Root Systems The two main types of root systems are tap- root systems and fibrous root systems, shown in Figure 23–5. Taproot systems are found mainly in dicots. Fibrous root systems are found mainly in monocots. Recall from Chapter 22 that monocots and dicots are two categories of flowering plants.

 (^) Taproot System In some plants, the primary

root grows long and thick and gives rise to smaller branch roots. The large primary root is called a taproot. Taproots of oak and hickory trees grow so long that they can reach water several meters down. Carrots, dandelions, and beets have short, thick taproots that store sugars and starches.

 (^) Fibrous Root System In other plants, such as

grasses, the system begins with one primary root. But it is soon replaced by many equally sized branch roots that grow separately from the base of the stem. These fibrous roots branch to such an extent that no single root grows larger than the rest. The extensive fibrous root systems produced by many plants help prevent topsoil from being washed away by heavy rain.

Figure 23–5 A Comparison of Two Root Systems Dandelions have a taproot system (left), while grasses have a fibrous root system (right).

THINK ABOUT IT Can you guess how large a typical plant’s root

system is? Get ready for a surprise if you think that roots are small and insignificant. In a 1937 study of a single rye plant, botanist Howard Dittmer showed that the length of all the branches in the rye plant’s root system was an astonishing 623 kilometers (387 miles). The sur- face area of these roots was more than 600 square meters—130 times greater than the combined areas of its stems and leaves!

In this lesson you will learn about the structure and function of roots, the specialized cells in roots (TEKS 5B), and how the root system interacts with other plant systems (TEKS 10B). Also covered: TEKS 4B.

TEKS 5B

669

Nutrient Some Roles in Plant (Chemical Symbol)

Result of Deficiency

Nitrogen (N)

Phosphorus (P)

Potassium (K)

Magnesium (Mg)

Calcium (Ca)

• Proper leaf growth and color
• Synthesis of amino acids, proteins, nucleic acids, and chlorophyll
• Synthesis of DNA
• Development of roots, stems, flowers, and seeds
• Synthesis of proteins and carbohydrates
• Development of roots, stems, and flowers
• Resistance to cold and disease
• Synthesis of chlorophyll
• Cell growth and division
• Cell wall structure
• Cellular transport
• Enzyme action

Essential Plant Nutrients

▲

▲

▲

• Stunted plant growth
• Pale yellow leaves
• Poor flowering
• Stunted growth
• Weak stems
• Stunted roots
• Edges of leaves turn brown
• Thin stems
• Mottled, pale leaves
• Stunted growth
• Curled leaves

Figure 23–7 Important Plant Nutrients Soil contains several nutrients that are essential for plant growth. Interpret Tables If you notice that the leaves of a plant are becoming paler and more yellow, what nutrient might it be lacking?

Root Functions

What are the different functions of roots?

How does a root go about the job of absorbing water and minerals from the soil? Although it might seem to, water does not just “soak” into the root from soil. It takes energy on the part of the plant to absorb water. Roots support a plant, anchor it in the ground, store food, and absorb water and dissolved nutrients from the soil.

Uptake of Plant Nutrients An understanding of soil helps explain how plant roots function. Soil is a complex mixture of sand, silt, clay, air, and bits of decaying animal and plant tissue. Soil in different places contains varying amounts of these ingredients. Sandy soil, for example, is made of large particles that retain few nutrients, whereas the finely textured silt and clay soils of the Midwest and southeastern United States are high in nutrients. The ingredients define the soil and determine, to a large extent, the kinds of plants that can grow in it. To grow, flower, and produce seeds, plants require a variety of inor- ganic nutrients in addition to carbon dioxide and water. The nutrients needed in largest amounts are nitrogen, phosphorus, potassium, mag- nesium, sulfur, and calcium. The functions of these essential nutrients within a plant are described in Figure 23–7. In addition to large amounts of these nutrients, small amounts of other nutrients, called trace elements, are just as important. These trace elements include iron, zinc, molybdenum, boron, copper, manganese, and chlorine. As important as they are, excessive amounts of any of these nutrients in soil can also be poisonous to plants.

TEKS 4B, 5B, 10B

Plant Structure and Function 671

BUILD Vocabulary

Active transport of minerals Movement of water by osmosis

Cortex

Epidermis

Endodermis Casparian strip

Root hairs

Phloem Vascular Cylinder

Xylem

Endodermis

ACAdEmIC WORdS The term accumulate means to increase gradually in quantity or number.

Figure 23–8 Water Passage Into a Root A root absorbs water and dissolved nutrients from the soil. Interpret Visuals What is the function of the Casparian strip?

Active Transport of dissolved Nutrients The cell membranes of root hairs and other cells in the root epidermis contain active trans- port proteins. As you know, active transport is a process that uses the energy of ATP to move ions and other materials across membranes. Active transport brings the mineral ions of dissolved nutrients from the soil into the plant. The high concentration of mineral ions in the plant cells causes water molecules to move into the plant by osmosis.

Water movement by Osmosis You may recall that osmosis is the movement of water across a membrane toward an area where the concentration of dissolved material is higher. By using active transport to accumulate mineral ions from the soil, cells of the root epidermis create conditions under which osmosis causes water to “follow” those ions and flow into the root. Note that the root does not actually pump water. But by pumping mineral ions into its own cells, the end result is almost the same—the water moves from the epidermis through the cortex into the vascular cylinder, as shown in Figure 23–8. In this way, cells in all three tissue systems of the plant interact to transport water into the root.

movement Into the Vascular Cylinder Next, the water and dis- solved minerals pass the inner boundary of the cortex and move toward the vascular cylinder. The cylinder itself is enclosed by a layer of cortex cells known as the endodermis. The cells of the endodermis are each shaped a bit like a brick. Where these cells meet, their cell walls form a special waterproof zone called a Casparian strip. Most of the time, water can diffuse through cell walls, but not here. The strip is almost like a layer of waterproof cement between the bricks in a wall. Imagine many of these bricks placed edge to edge to build a cylinder, with this waterproof cement surrounding each of the bricks. The only way that water and dissolved nutrients could enter that cylinder would be through the bricks themselves.

672 Chapter 23 • Lesson 2

23.3 (^) Stems

Key Questions What are three main functions of stems? How do primary growth and secondary growth occur in stems?

Vocabulary node (^) • bud (^) • vascular bundle (^) • pith (^) • primary growth (^) • secondary growth (^) • vascular cambium • cork cambium • heartwood • sapwood • bark

Taking Notes Preview Visuals Before you read, preview the art in Figure 23–14. Define any familiar terms in your own words, and list any unfamiliar ones. Revise and add to your definitions as you read.

Stem Structure and Function What are three main functions of stems? What do water chestnuts, bamboo shoots, asparagus, and potatoes all have in common? They are all types of stems. Stems vary in size, shape, and method of development. Some grow entirely underground; others reach high into the air. Above- ground stems have several important functions: Stems produce leaves, branches, and flowers; stems hold leaves up to the sun; and stems transport substances throughout the plant. Stems make up an essential part of the water and mineral transport systems of the plant. Xylem and phloem form continuous tubes from the roots through the stems to the leaves. These vascular tissues link all parts of the plant, allowing water, nutrients, and other compounds to be carried throughout the plant. In many plants, stems also function in storage and aid in the process of photosynthesis.

Figure 23–10 Cactus Stems Desert cacti have thick green stems that carry out photosynthesis and are adapted to store water.

THINK ABOUT IT While visiting the salad bar for lunch, you notice

an intriguing range of offerings. After making your basic salad, you decide to add some sliced water chestnuts and bamboo shoots on top. Then you serve yourself some asparagus and potato salad on the side. These good things are all from plants, of course, but can you think of something else that ties them together? They all come from the same part of the plant. Do you have any idea which part?

In this lesson you will learn about the specialized cells that are found in stems (TEKS 5B). You will also learn about the structure and function of stems in plants.

TEKS 5B

674

Bud Node

Node

Monocot (^) Dicot

Epidermis

Cortex

Pith

Vascular bundles

Ground tissue (^) LM 11 LM 15

Cross Section

 (^) Monocot Stems The cross section of a

young monocot stem shows all three tissue systems clearly. The stem has a distinct epidermis, which encloses ground tissue and a series of vascular bundles. In monocots, vascular bundles are scattered throughout the ground tissue. The ground tissue is fairly uni- form, consisting mainly of parenchyma cells.

Online Journal Create a Venn diagram in which to record similarities and differences in the stem structure of monocots and dicots.

 (^) Dicot Stems Young dicot stems have vascular bundles, too, but they are generally arranged in an organized, ringlike pattern. The parenchyma cells inside the ring of vascular tissue are known as pith, while those outside form the cortex of the stem. These relatively simple tissue patterns become more complex as the plant grows larger and the stem increases in diameter.

Anatomy of a Stem Stems contain the plant’s three tissue systems: dermal, vascular, and ground tissue. Stems are surrounded by specialized epidermal cells with thick cell walls and a waxy protective coating. Growing stems contain distinct nodes, where leaves are attached, as shown in Figure 23–11. Small buds are found where leaves attach to the nodes. Buds contain apical meristems whose cells produce new stems and leaves. In larger plants, stems develop woody tissue that helps support leaves and flowers.

Vascular Bundle Patterns The arrangement of tissues in a stem differs among seed plants. In mono- cots, clusters of xylem and phloem tissue, called vascular bundles, are scattered throughout the stem. In most dicots and gymnosperms, vascular bundles are arranged in a cylinder, or ring. For a comparison of monocot and dicot stems, look at Figure 23–12.

Figure 23–12 Comparing Monocots and Dicots These cross sections through a monocot and dicot stem show their similarities and differences. Examine How does the arrangement of the vascular bundles differ?

Figure 23–11 Anatomy of a Stem Stems produce leaves from their nodes and new branches from buds. They hold leaves up to the sunlight, where they carry out photosynthesis.

Plant Structure and Function 675

Secondary Growth

The vascular cambium divides, producing secondary xylem cells toward the center of the stem and secondary phloem cells toward the outside.

The mature stem develops.

Secondary xylem

Secondary phloem

Wood

Primary Growth

Cork cambium Cork

Bark

Pith

Year 3

Unlike monocots, most dicots have meristems within their stems and roots that can produce true secondary growth. This enables many dicots to grow to great heights because the increase in width supports the extra weight. In addition to showing primary growth, Figure 23– illustrates the pattern of secondary growth in a dicot stem. In conifers and dicots, secondary growth takes place in meristems called the vascular cambium and cork cambium. The vascular cambium produces vascular tissues and increases the thick- ness of stems over time. The cork cambium produces the outer covering of stems. Similar types of cambium tissue enable roots to grow. The addition of new tissue in these cambium layers increases the thickness of stems and roots.

Growth From the Vascular Cambium In a young dicot stem, bundles of xylem and phloem are arranged in a ring. Once secondary growth begins, the vascular cambium appears as a thin, cylindrical layer of cells between clusters of vascular tissue. This new meristem forms between the xylem and phloem of each vascular bundle. Divisions in the vascular cambium give rise to new layers of xylem and phloem cells. As a result, the stem becomes wider. Each year, the cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker.

Online Journal List in sequence all the tissues found in a mature woody stem. Start from the center and move outward.

Plant Structure and Function 677

C B

A

Reading a Tree’s History

The analysis of tree rings can help deter- mine information about a tree and the environment in which it grew. A tree’s age can be measured by counting its growth rings—each ring is produced by a year of growth. The specific environmental condi- tions for each year of growth can be inferred by examining the relative width and color of each ring. Use the photograph at left to answer the questions.

1. Calculate Approximately how old was this tree when it was cut down? 2. Infer Areas A and B were both produced by four years of growth, yet they are different widths. What climatic conditions might account for this difference? 3. Interpret Visuals The area at C is blackened from a fire that apparently affected only one side of the tree. Describe how the tree grew after this fire.

Formation of Wood Most of what we call “wood” is actually layers of secondary xylem produced by the vascular cambium. These cells build up year after year, layer on layer. As woody stems grow thicker, the older xylem near the center of the stem no longer conducts water and instead becomes what is known as heartwood. Heartwood usually darkens with age because it accumulates colored deposits. Heartwood is surrounded by sapwood, which is active in fluid transport and is, there- fore, usually lighter in color.

Tree Rings In most of the temperate zone, tree growth is seasonal. When growth begins in the spring, the vascular cambium begins to grow rapidly, producing large, light-colored xylem cells with thin cell walls. The result is a light-colored layer of early wood. As the grow- ing season continues, the cells grow less and have thicker cell walls, forming a layer of darker late wood. This alternation of dark and light wood produces what we commonly call tree rings. Each ring has light wood at one edge and dark wood at the other, making a sharp boundary between rings. Usually, a ring corresponds to a year of growth. By counting the rings in a cross section of a tree, you can estimate its age. The size of the rings may even provide information about weather conditions, such as wet or dry years. Thick rings indicate that weather conditions were favorable for tree growth, whereas thin rings indicate less-favorable conditions.

678 Chapter 23 • Lesson 3

23.4 Leaves

Key Questions How is the structure of a leaf adapted to make photosyn- thesis more efficient? What role do stomata play in maintaining homeostasis?

Vocabulary blade (^) • petiole (^) • mesophyll (^) • palisade mesophyll (^) • spongy mesophyll (^) • stoma (^) • transpiration (^) • guard cell

Taking Notes Preview Visuals Before you read the lesson, look at Figure 23–15. Locate the three main tissue systems and infer which tissue system makes up the leaf veins.

Leaf Structure and Function How is the structure of a leaf adapted to make photosynthesis more efficient? Recall from Chapter 8 that photosynthesis uses carbon dioxide and water to produce sugars and oxygen. Leaves, therefore, must have a way of obtaining carbon dioxide and water as well as distributing end products. The structure of a leaf is optimized to absorb light and carry out photosynthesis.

Anatomy of a Leaf To collect sunlight, most leaves have a thin, flattened part called a blade. The flat shape of a leaf blade maximizes the amount of light it can absorb. The blade is attached to the stem by a thin stalk called a petiole (pet ee ohl). Like roots and stems, leaves have an outer covering of dermal tissue and inner regions of ground and vascular tissues, as shown in Figure 23–15.  (^) Dermal Tissue Leaves are covered on their top and bottom surfaces by epidermis. Leaf epidermis is a specialized layer of tough, irregularly shaped cells with thick outer walls that resist tearing. The epidermis of nearly all leaves is also covered by a waxy cuticle. The cuticle is a waterproof barrier that protects tissues and limits the loss of water through evaporation.  (^) Vascular Tissue Cells in the vascular tissues of leaves are connected directly to the vascular tissues of stems, making them part of the plant’s fluid transport system. Xylem and phloem cells are bundled in leaf veins that run from the stem throughout the leaf.  (^) Ground Tissue The area between leaf veins is filled with specialized ground tissue cells known as mesophyll (mes uh fil), where photosyn- thesis occurs. The sugars produced in mesophyll move to leaf veins, where they enter phloem sieve tubes for transport to the rest of the plant.

THINK ABOUT IT We hear a lot these days about “green industry,”

such as biofuels and material recycling, but did you know that the most important manufacturing sites on Earth are already green? They are the leaves of plants. In a sense, plant leaves are the world’s most important manufacturers. Using the energy captured in their leaves, plants make the sugars, starches, and oils that feed virtually all land animals, including us.

In this lesson you will learn about the specialized cells that leaves contain (TEKS 5B), as well as the structure and function of leaves. In addition, you will learn how leaves help maintain homeostasis in a plant (TEKS 11A), as well as how plants respond to their environment (TEKS 11B).

TEKS 5B

ELPS 2.E. Read aloud or listen to the Quick Lab instructions on page 683. Notice words you know. Use them to help understand new words. Use the figure, caption, and labels on page 682 to explain the term stomata.

680

Cuticle

Vein

Spongy Mesophyll

Xylem Phloem

Epidermis

Stoma

Guard cells

Palisade Mesophyll

Cuticle

Chloroplasts

Veins

Blade

Petiole

Epidermis

BUILD Vocabulary

AnAtomy of A LeAf FigurE 23–15 Leaves absorb light and carry out most of the photosynthesis in a plant. Compare and Contrast Compare the structure of the two types of mesophyll cells in a leaf.

Photosynthesis The mesophyll tissue in most leaves is highly spe- cialized for photosynthesis. Beneath the upper epidermis is a layer of cells called the palisade mesophyll, containing closely packed cells that absorb light that enters the leaf. Beneath the palisade layer is a loose tissue called the spongy mesophyll, which has many air spaces between its cells. These air spaces connect with the exterior through stomata (singular: stoma). Stomata are small openings in the epider- mis that allow carbon dioxide, water, and oxygen to diffuse into and out of the leaf.

Transpiration The walls of mesophyll cells are kept moist so that gases can enter and leave the cells easily. The trade-off to this feature is that water evaporates from these surfaces and is lost to the atmos- phere. Transpiration is the loss of water through leaves. This lost water may be replaced by water drawn into the leaf through xylem vessels in the vascular tissue. Transpiration helps to cool leaves on hot days, but it may also threaten the leaf ’s survival if water is scarce.

Online Journal Make a two-column table in which you list structures found in a leaf cross section and describe their functions.

WOrd OrIgINS Mesophyll comes from two Greek words: meso , meaning “middle,” and phyllon , meaning “leaf.” Stomata comes from the Greek word meaning “mouths.”

Plant Structure and Function 681