Imagine that you are taking a course in engineering and are asked

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1 33 Leaf Structure and Function Red maple leaves. Note how the leaves of red maple (Acer rubrum) are arranged to efficiently capture light. David Sieren / Visuals Unlimited KEY CONCEPTS Leaf structure reflects its primary function of photosynthesis. Opening and closing of stomata affect carbon dioxide availability in the daily cycle of photosynthesis. Transpiration keeps leaves from getting overheated and promotes water transport in the plant. Leaf abscission is the seasonal removal of deciduous leaves. Some leaves are modified for special functions in addition to photosynthesis and transpiration. Imagine that you are taking a course in engineering and are asked to design an efficient solar collector that can convert the radiant energy it collects into chemical energy. Where would you start? It might be helpful to check in the library to see how solar collectors have been designed in the past. In this instance, it would also be wise to ask a biology student if anything comparable exists in nature. The answer, of course, is yes. Plants have organs that are effective solar collectors and energy converters: leaves. Plants allocate many resources to the production of leaves. Each year, a large maple tree (see photograph) may produce 47 m 2 (500 ft 2 ) of leaves, which may weigh more than 113 kg (250 lb). The metabolic cost of producing so many leaves is high, but leaves are essential to the tree s survival. Leaves gather the sunlight necessary for photosynthesis, the biological process that converts radiant energy into the chemical energy of carbohydrate molecules. Plants use these molecules as starting materials to synthesize all other organic compounds and as fuel to provide energy for metabolism. During a single summer, the leaves of a maple tree will fix about 454 kg (1000 lb) of carbon dioxide (CO 2 ) into organic compounds. The structure of a leaf is superbly adapted for its primary function of photosynthesis. Most leaves are thin and flat, a shape that allows optimal absorption of light energy and the efficient internal diffusion of gases such as CO 2 and O 2. As a result of their ordered arrangement on the stem, leaves efficiently catch the sun s rays. The leaves form an intricate green mosaic, bathed in sunlight and atmospheric gases. 715

2 To control water loss, a thin, transparent layer of wax covers the leaf surface. Such structural adaptations are compromises between competing needs, and some features that optimize photosynthesis promote water loss. For example, plants have minute pores that allow gas exchange for photosynthesis, but these openings also let water vapor escape into the atmosphere. Thus, leaf structure represents a trade-off between photosynthesis and water conservation. LEAF FORM AND STRUCTURE Learning Objectives 1 Discuss variation in leaf form, including simple versus compound leaves, leaf arrangement on the stem, and venation patterns. 2 Describe the major tissues of the leaf (epidermis, photosynthetic ground tissue, xylem, and phloem), and label them on a diagram of a leaf cross section. 3 Compare leaf anatomy in eudicots and monocots. 4 Relate leaf structure to its function of photosynthesis. Foliage leaves are the most variable of plant organs, so much so that plant biologists developed specific terminology to describe their shapes, margins (edges), vein patterns, and the way they attach to stems. Because each leaf is characteristic of the species on which it grows, many plants can be identified by their leaves alone. Leaves may be round, needlelike, scalelike, cylindrical, heart shaped, fan shaped, or thin and narrow. They vary in size from those of the raffia palm (Raphia ruffia), whose leaves often grow more than 20 m (65 ft) long, to those of water-meal (Wolffia), whose leaves are so small that 16 of them laid end to end measure only 2.5 cm (1 in) (see Fig. 32-1a). Blade Veins Petiole Axillary bud The broad, flat portion of a leaf is the blade; the stalk that attaches the blade to the stem is the petiole. Some leaves also have stipules, which are leaflike outgrowths usually present in pairs at the base of the petiole ( Fig. 33-1). Some leaves do not have petioles or stipules. Leaves may be simple (having a single blade) or compound (having a blade divided into two or more leaflets) ( Fig. 33-2a). Sometimes it is difficult to tell whether a plant has formed one compound leaf or a small stem bearing several simple leaves. One easy way to determine if a plant has simple or compound leaves is to look for axillary buds, so called because each develops in a leaf axil (the angle between the stem and petiole). Axillary buds form at the base of a leaf, whether it is simple or compound. However, axillary buds never develop at the base of leaflets. Also, the leaflets of a compound leaf lie in a single plane (you can lay a compound leaf flat on a table), whereas simple leaves usually are not arranged in one plane on a stem. Leaves are arranged on a stem in one of three possible ways ( Fig. 33-2b). Plants such as beeches and walnuts have an alternate leaf arrangement, with one leaf at each node, the area of the stem where one or more leaves are attached. In an opposite leaf arrangement, as occurs in maples and ashes, two leaves grow at each node. In a whorled leaf arrangement, as in catalpa trees, three or more leaves grow at each node. Leaf blades may possess parallel venation, in which the primary veins strands of vascular tissue run approximately parallel to one another (generally characteristic of monocots), or netted venation, in which veins are branched in such a way that they resemble a net (generally characteristic of eudicots; Fig. 33-2c). 1 Netted veins can be pinnately netted, with major veins branching off in succession along the entire length of the midvein (main or central vein of a leaf ), or palmately netted, with several major veins radiating out from one point. Leaf structure consists of an epidermis, photosynthetic ground tissue, and vascular tissue Stipules Stem The leaf is a complex organ composed of several tissues organized to optimize photosynthesis ( Fig. 33-3). The leaf blade has upper and lower surfaces consisting of an epidermal layer. The upper Figure 33-1 Parts of a leaf A geranium leaf consists of a blade, a petiole, and two stipules at the base of the leaf. Note the axillary bud in the leaf axil. 1 Recall that flowering plants, the focus of this chapter, are divided into two main groups, informally called eudicots and monocots (see Chapter 28). Examples of eudicots include beans, petunias, oaks, cherry trees, roses, and snapdragons; monocots include corn, lilies, grasses, palms, tulips, orchids, and bananas. 716 Chapter 33

3 Modified leaves of carnivorous plants capture insects Carnivorous plants are plants that capture insects. Most carnivorous plants grow in poor soil that is deficient in certain essential minerals, particularly nitrogen. These plants meet some of their mineral requirements by digesting insects and other small animals. The leaves of carnivorous plants are adapted to attract, capture, and digest their animal prey. Some carnivorous plants have passive traps. The leaves of a pitcher plant, for example, are shaped so that rainwater collects and forms a reservoir that also contains acid secreted by the plant ( Fig ). Some pitchers are quite large; in the tropics, pitcher plants may be large enough to hold 1 L (approximately 1 qt) or more of liquid. An insect attracted by the odor or nectar of the pitcher may lean over the edge and fall in. Although it may make repeated attempts to escape, the insect is prevented from crawling out by the slippery sides and the rows of stiff hairs that point downward around the lip of the pitcher. The insect eventually drowns, and part of its body disintegrates and is absorbed. Most insects are killed in pitcher plants. However, the larvae of several insects (certain flies, midges, and mosquitoes) and a large community of microorganisms live inside the pitchers. These insect species obtain their food from the insect carcasses, and the pitcher plant digests what remains. It is not known how these insects survive the acidic environment inside the pitcher. The Venus flytrap is a carnivorous plant with active traps. Its leaf blades resemble tiny bear traps (see Fig. 1-3). Each side of the leaf blade contains three small, stiff hairs. If an insect alights and brushes against two of the hairs, or against the same hair twice in quick succession, the trap springs shut with amazing rapidity about 100 milliseconds. The spines along the margins of the blades fit closely together to prevent the insect from escaping. After the leaf initially traps the insect, the leaf continues to slowly close for the next several hours. Digestive glands on the surface of the trap secrete enzymes in response to the insect pressing against them. Days later, after the insect has died and been digested, the trap reopens and the indigestible remains fall out. Bill Lea/Dembinsky Photo Associates Figure A common pitcher plant This species (Sarracenia purpurea), whose pitchers grow to 30.5 cm (12 in), is widely distributed in acidic bogs and marshes in eastern North America. Young pitchers are green but turn red as they age. Note the dead beetle in the pitcher. Review What are the primary functions of each of the following modified leaves: spines, tendrils, and bud scales? What are the functions of bulbs? Of succulent leaves? What are some of the specialized features of the leaves of carnivorous plants? SUMMARY WITH KEY TERMS Learning Objectives 1 Discuss variation in leaf form, including simple versus compound leaves, leaf arrangement on the stem, and venation patterns (page 716). Leaves typically consist of a broad, flat blade and a stalklike petiole. Some leaves also have small, leaflike outgrowths from the base called stipules. Leaves may be simple (having a single blade) or compound (having a blade divided into two or more leaflets). Leaf arrangement on a stem may be alternate (one leaf at each node), opposite (two leaves at each node), or whorled (three or more leaves at each node). Leaves may have parallel or netted venation. Netted venation may be palmately netted, with several major veins radiating from one point, or pinnately netted, with veins branching along the entire length of the midvein. 2 Describe the major tissues of the leaf (epidermis, photosynthetic ground tissue, xylem, and phloem), and label them on a diagram of a leaf cross section (page 716). Upper and lower surfaces of the leaf blade are covered by an epidermis. A waxy cuticle coats the epidermis, enabling the plant to survive the dry conditions of a terrestrial existence. Stomata are small pores in the epidermis that permit gas exchange needed for photosynthesis. Each pore is surrounded by two guard cells that are often associated with special epidermal cells called subsidiary cells. Subsidiary 728 Chapter 33

4 cells provide a reservoir of water and ions that move into and out of the guard cells as they change shape during stomatal opening and closing. Mesophyll consists of photosynthetic parenchyma cells. Mesophyll is divided into palisade mesophyll, which functions primarily for photosynthesis, and spongy mesophyll, which functions primarily for gas exchange. Leaf veins have xylem to conduct water and essential minerals to the leaf and phloem to conduct sugar produced by photosynthesis to the rest of the plant. Learn more about leaf tissues by clicking on the figure in ThomsonNOW. 3 Compare leaf anatomy in eudicots and monocots (page 716). Monocot leaves have parallel venation, whereas eudicot leaves have netted venation. Some monocots (corn and other grasses) do not have mesophyll differentiated into distinct palisade and spongy layers. Some monocots (grasses, reeds, and sedges) have guard cells shaped like dumbbells, unlike the more common bean-shaped guard cells. 4 Relate leaf structure to its function of photosynthesis (page 716). Leaf structure is adapted for its primary function of photosynthesis. Most leaves have a broad, flattened blade that is quite efficient in collecting the sun s radiant energy. Stomata generally open during the day for gas exchange needed during photosynthesis and close at night to conserve water when photosynthesis is not occurring. The transparent epidermis allows light to penetrate into the middle of the leaf, where photosynthesis occurs. Air spaces in mesophyll tissue permit the rapid diffusion of CO 2 and water into, and oxygen out of, mesophyll cells. 5 Explain the role of blue light in the opening of stomata (page 722). Blue light, which is a component of sunlight, triggers the activation of proton pumps located in the guard cell plasma membrane. Blue light also triggers the synthesis of malic acid and the hydrolysis of starch. 6 Outline the physiological changes that accompany stomatal opening and closing (page 722). Protons (H ) are pumped out of the guard cells. The protons are produced when malic acid ionizes. As protons leave the guard cells, an electrochemical gradient (a charge and concentration difference) forms on the two sides of the guard cell plasma membrane. The electrochemical gradient drives the uptake of potassium ions through voltage-activated potassium channels into the guard cells. Chloride ions are also taken into the guard cells through ion channels. These osmotically active ions increase the solute concentration in the guard cell vacuoles. The resulting osmotic movement of water into the guard cells causes them to become turgid, forming a pore. As the day progresses, potassium ions slowly leave the guard cells and starch is hydrolyzed to sucrose, which increases in concentration in the guard cells. Stomata close when water leaves the guard cells as a result of a decline in the concentration of sucrose, an osmotically active solute. The sucrose is converted to starch, which is osmotically inactive. Watch stomata in action by clicking on the figure in ThomsonNOW. 7 Discuss transpiration and its effects on plants (page 724). Transpiration is the loss of water vapor from aerial parts of plants. Transpiration occurs primarily through the stomata. The rate of transpiration is affected by environmental factors such as temperature, wind, and relative humidity. Transpiration appears to be both beneficial and harmful to the plant that is, transpiration represents a trade-off between the CO 2 requirement for photosynthesis and the need for water conservation. 8 Distinguish between transpiration and guttation (page 724). Guttation, the release of liquid water from leaves of some plants, occurs through special structures when transpiration is negligible and available soil moisture is high. In contrast, transpiration is the loss of water vapor and occurs primarily through the stomata. 9 Define leaf abscission, explain why it occurs, and describe the physiological and anatomical changes that precede it (page 725). Leaf abscission is the loss of leaves that often occurs as winter approaches in temperate climates or at the beginning of the dry period in tropical climates with wet and dry seasons. Abscission is a complex process involving physiological and anatomical changes that occur prior to leaf fall. An abscission zone develops where the petiole detaches from the stem. Sugars, amino acids, and many essential minerals are transported from the leaves to other plant parts. Chlorophyll breaks down, and carotenoids and anthocyanins become evident. 10 List at least four examples of modified leaves, and give the function of each (page 726). Spines are leaves adapted to deter herbivores. Some tendrils are leaves modified for grasping and holding on to other structures (to support weak stems). Bud scales are leaves modified to protect delicate meristematic tissue or dormant buds. Bulbs are short, underground stems with fleshy leaves specialized for storage. Many plants adapted to arid conditions have succulent leaves for water storage. Carnivorous plants have leaves modified to trap insects. TEST YOUR UNDERSTANDING 1. Plants with an alternate leaf arrangement have (a) blades divided into two or more leaflets (b) major veins that radiate out from one point (c) one leaf at each node (d) major veins branching off along the entire length of the midvein (e) two leaves at each node 2. The photosynthetic ground tissue in the middle of the leaf is called (a) cutin (b) mesophyll (c) the abscission zone (d) subsidiary cells (e) palisade and spongy stomata Leaf Structure and Function 729

5 3. The primary function of the spongy mesophyll is (a) reducing water loss from the leaf surface (b) changing the shape of the guard cells (c) supporting the leaf to prevent it from collapsing under its own weight (d) diffusing gases within the leaf (e) deterring herbivores 4. Gas exchange occurs through microscopic pores formed by two (a) subsidiary cells (b) abscission cells (c) mesophyll cells (d) guard cells (e) stipules 5. Most stomata are usually located in the of the leaf. (a) upper epidermis (b) lower epidermis (c) cuticle (d) spongy mesophyll (e) palisade mesophyll 6. The thin, noncellular layer of wax secreted by the epidermis of leaves is the (a) stoma (b) subsidiary cell (c) trichome (d) bundle sheath (e) cuticle 7. The encircles a vein. (a) palisade mesophyll (b) guard cell (c) bundle sheath (d) blade (e) cuticle 8. The of a leaf vein transports water and dissolved minerals, whereas the transports sugars produced by the leaf during photosynthesis. (a) xylem; phloem (b) xylem; bundle sheath (c) phloem; xylem (d) phloem; vein (e) vascular bundle; bundle sheath 9. Which of the following is not an adaptation of pine needles to conserve water? (a) less surface area exposed to the air than thin-bladed leaves (b) a relatively thick cuticle (c) sunken stomata (d) netted veins instead of parallel veins (e) both c and d are not adaptations of pine needles 10. Most of the water that a plant absorbs from the soil is lost by the process of (a) guttation (b) circadian rhythm (c) abscission (d) transpiration (e) photosynthesis 11. When transpiration is negligible, plants such as grasses exude excess water by (a) guttation (b) circadian rhythm (c) abscission (d) pumping H out of and K into guard cells (e) photosynthesis 12. At sunrise, the accumulation in the guard cells of the osmotically active substance causes an inflow of water and the opening of the pore. (a) protons (b) starch (c) ATP synthase (d) sucrose (e) potassium ions 13. Stomatal opening is most pronounced in response to light. (a) green (b) yellow (c) blue (d) ultraviolet (e) infrared 14. The seasonal detachment of leaves is known as (a) forest decline (b) transpiration (c) abscission (d) guttation (e) dormancy 15. Anatomically, the abscission zone where a petiole detaches from a stem consists of (a) thin-walled parenchyma cells with few fibers (b) thick-walled cork parenchyma cells (c) clusters of fibers and collenchyma strands (d) hard, pointed stipules (e) epidermal cells with sunken stomata 16. Modified leaves that enable a stem to climb are called, whereas modified leaves that cover the winter buds of a dormant woody plant are called. (a) spines; bud scales (b) bud scales; tendrils (c) tendrils; bud scales (d) tendrils; spines (e) carnivorous leaves; spines 17. There is a trade-off between photosynthesis and transpiration in leaves because (a) numerous stomatal pores provide both gas exchange for photosynthesis and openings through which water vapor escapes (b) a waxy layer, the cuticle, reduces water loss (c) blue light triggers an influx of potassium ions (K ) into the guard cells (d) leaves of deciduous plants abscise as winter approaches in temperate climates (e) stomata are closed at night, although water continues to move into the roots by osmosis CRITICAL THINKING 1. Suppose that you are asked to observe a micrograph of a leaf cross section and distinguish between the upper and lower epidermis. How would you make this decision? 2. Given that (a) xylem is located toward the upper epidermis in leaf veins and phloem is toward the lower epidermis and (b) the vascular tissue of a leaf is continuous with that of the stem, suggest one possible arrangement of vascular tissues in the stem that might account for the arrangement of vascular tissue in the leaf. 3. What might be some of the advantages of a plant having a few large leaves? What might be some disadvantages? What might be some advantages of having many small leaves? What disadvantages might this entail? How would your answers differ for plants growing in a humid environment compared to those in a desert? 4. Briefly explain why research on the molecular mechanism of stomatal closure might be of future use in agriculture. 5. Evolution Link. Why did natural selection favor the evolution of seasonal leaf abscission in woody flowering plants living in colder climates? What adaptations enable conifers to survive these climates without leaf abscission? Additional questions are available in ThomsonNOW at login 730 Chapter 33