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33
Leaf Structure and Function
Red maple leaves. Note how
the leaves of red maple ( Acer
rubrum) are arranged to efficiently capture light.
I
magine 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
David Sieren / Visuals Unlimited
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 m2
(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 radi-
K EY C ONCE P TS
ant energy into the chemical energy of carbohydrate molecules.
Leaf structure reflects its primary function of photosynthesis.
Plants use these molecules as starting materials to synthesize all
Opening and closing of stomata affect carbon dioxide
availability in the daily cycle of photosynthesis.
tabolism. During a single summer, the leaves of a maple tree will
Transpiration keeps leaves from getting overheated and
promotes water transport in the plant.
compounds.
Leaf abscission is the seasonal removal of deciduous leaves.
tion of photosynthesis. Most leaves are thin and flat, a shape that
Some leaves are modified for special functions in addition
to photosynthesis and transpiration.
allows optimal absorption of light energy and the efficient internal
other organic compounds and as fuel to provide energy for mefix about 454 kg (1000 lb) of carbon dioxide (CO2) into organic
The structure of a leaf is superbly adapted for its primary func-
diffusion of gases such as CO2 and O2. 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.
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To control water loss, a thin, transparent layer of wax covers
pores that allow gas exchange for photosynthesis, but these open-
the leaf surface. Such structural adaptations are compromises be-
ings also let water vapor escape into the atmosphere. Thus, leaf
tween competing needs, and some features that optimize pho-
structure represents a trade-off between photosynthesis and water
tosynthesis promote water loss. For example, plants have minute
conservation.
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LEAF FORM AND STRUCTURE
Learning Objectives
Discuss variation in leaf form, including simple versus compound leaves, leaf arrangement on the stem, and venation
patterns.
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.
Compare leaf anatomy in eudicots and monocots.
Relate leaf structure to its function of photosynthesis.
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2
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4
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
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.
Petiole
Axillary bud
Stipules
Stem
Leaf structure consists of an epidermis,
photosynthetic ground tissue, and
vascular tissue
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
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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.
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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.
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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. 33-14). 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
Modified leaves of carnivorous
plants capture insects
Figure 33-14
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?
❚
❚
S UM M A RY WI T H KE Y TE RM S
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
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Chapter 33
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
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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.
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.
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
CO2 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
3
Watch stomata in action by clicking on the
figure in ThomsonNOW.
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 CO2 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.
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T E ST Y OU R UN D E RS TA ND ING
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
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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
C R I TI C AL TH I N KI N G
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
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Chapter 33
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 www.thomsonedu.com/
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