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34
Stems and Transport in Vascular Plants
Baobab tree. Baobabs (Adansonia digitata), which are native
to Africa, Madagascar, and
Australia, store large volumes of
water and starch in their massive
trunks. Baobab trees are relatively short (growing to 18 m, or
60 ft), but their trunks can be as
much as 9 m (30 ft) in diameter.
Photographed in Namibia, with
children around the tree as a size
reference.
A
vegetative (not sexually reproductive) vascular plant has three
parts: roots, leaves, and stems. As discussed in Chapter 32,
roots serve to anchor the plant and absorb materials from the soil,
Michael Fairchild/Peter Arnold, Inc.
whereas leaves are primarily for photosynthesis, converting radiant energy into the chemical energy of carbohydrate molecules.
Stems, the focus of this chapter, link a plant’s roots to its leaves and
are usually located aboveground, although many plants have underground stems. Stems exhibit varied forms, ranging from ropelike vines to massive tree trunks. They can be either herbaceous,
with soft, nonwoody tissues; or woody, with extensive hard tissues
of wood and bark.
Stems perform three main functions in plants. First, stems of
K EY C ONCE P TS
Primary tissues (epidermis, cortex, pith, xylem, and phloem)
of stems develop from shoot apical meristems.
Secondary tissues (wood and bark) of stems develop
from two lateral meristems, vascular cambium and cork
cambium.
The concept of water potential explains the direction
of water flow into, through, and out of a plant.
most species support leaves and reproductive structures. The upright position of most stems and the arrangement of the leaves
on them allow each leaf to absorb light for use in photosynthesis.
Reproductive structures (flowers and fruits) are located on stems in
areas accessible to insects, birds, and air currents, which transfer
pollen from flower to flower and help disperse seeds and fruits.
Second, stems provide internal transport. They conduct water
and dissolved minerals (inorganic nutrients) from the roots, where
According to the tension–cohesion model, transpiration
pulls water up through the stem as water evaporates from
leaves by transpiration.
these materials are absorbed from the soil, to leaves and other
According to the pressure–flow hypothesis, sucrose is
transported in phloem sap from the source, where the
sugar is loaded into phloem, to the sink, where the sugar
is removed from phloem.
however, that stems are not the only plant organs that conduct
plant parts. Stems also conduct the sugar produced in leaves by
photosynthesis to roots and other parts of the plant. Remember,
materials. The vascular system is continuous throughout all parts
of a plant, and conduction occurs in roots, stems, leaves, and reproductive structures.
731
Third, stems produce new living tissue. They continue to grow
stem tissues, stems of some species are modified for asexual re-
throughout a plant’s life, producing buds that develop into stems
production (see Chapter 36) or, if green, to manufacture sugar by
with new leaves and/or reproductive structures. In addition to the
photosynthesis. Also, some stems are specialized to store starch
main functions of support, conduction, and production of new
(see photograph on previous page).
■
EXTERNAL STEM STRUCTURE IN WOODY TWIGS
Learning Objective
Describe the external features of a woody twig.
1
Although stems exhibit great variation in structure and growth,
they all have buds, which are embryonic shoots. A terminal bud
is the embryonic shoot located at the tip of a stem. The dormant
(not actively growing) apical meristem of a terminal bud is covered and protected by an outer layer of bud scales, which are
modified leaves (see Fig. 33-13c). Axillary buds, also called lateral buds, are located in the axils of a plant’s leaves (see Fig. 33-1).
An axil is the upper angle between a leaf and the stem to which
it is attached. When terminal and axillary buds grow, they form
Bud scale
Terminal
bud
One year's
growth
branches that bear leaves and /or flowers. The area on a stem
where each leaf is attached is called a node, and the region between two successive nodes is an internode.
To demonstrate certain structural features of the stem, we can
use a woody twig of a deciduous tree that has shed its leaves, as
shown in ❚ Figure 34-1. Bud scales cover the terminal bud and
protect its delicate apical meristem during dormancy. When the
bud resumes growth, the bud scales covering the terminal bud
fall off, leaving bud scale scars on the stem where they were attached. Because temperate-zone woody plants form terminal
buds at the end of each year’s growing season, the number of sets
of bud scale scars on a twig indicates its age. A leaf scar shows
where each leaf was attached on the stem; the pattern of leaf scars
can be used to determine leaf arrangement on a stem— alternate,
opposite, or whorled (see Fig. 33-2b). The vascular (conducting)
tissue that extends from the stem out into the leaf forms bundle
scars within a leaf scar. Axillary buds may be found above the leaf
scars. Also, the bark of a woody twig has lenticels, sites of loosely
arranged cells that allow oxygen to diffuse into the interior of
the woody stem. Lenticels look like tiny specks on the bark of
a twig.
Review
❚
❚
❚
Terminal bud scale scars
What is the difference between terminal and axillary buds?
What is the function of bud scales? Of lenticels?
How can you tell the age of a woody twig?
Node
Internode
STEM GROWTH AND STRUCTURE
Axillary bud
Leaf scar
Node
Learning Objectives
2
3
Lenticels
Terminal bud
scale scars
Bundle scars
Figure 34-1 Animated External structure of a woody
twig in its winter condition
The age of a woody twig can be determined by the number of sets
of bud scale scars (do not count side branches). How old is this twig?
732
Chapter 34
4
Label cross sections of herbaceous eudicot and monocot
stems, and describe the functions of each tissue.
Name the two lateral meristems, and describe the tissues
that arise from each.
Outline the transition from primary growth to secondary
growth in a woody stem.
You may recall from Chapter 32 that plants have two different
types of growth. Primary growth is an increase in the length of a
plant and occurs at apical meristems located at the tips of roots
and shoots and also within the buds of stems. Secondary growth
is an increase in the girth (thickness) of a plant as a result of the
activity of lateral meristems located within stems and roots. The
new tissues formed by the lateral meristems are called secondary
tissues to distinguish them from primary tissues produced by apical meristems.
All plants have primary growth; some plants have both primary and secondary growth. Stems with only primary growth
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Epidermis
Vascular
bundles
Cortex
Phloem
fiber cap
Cortex
Phloem
Epidermis
Vascular
cambium
Xylem
Ed Reschke
Ed Reschke
Vessel
element
500 µm
(a) Cross section of a sunflower (Helianthus
annuus) stem. Note the vascular bundles
arranged in a circle around a central core
of pith.
Figure 34-2
Animated
Pith
250 µm
(b) Close-up of a vascular bundle. In each
bundle, xylem is located toward the stem’s
interior, and phloem toward the exterior.
Each vascular bundle is “capped” by a
batch of fibers for additional support.
LMs of a herbaceous eudicot stem
are herbaceous, whereas those with both primary and secondary
growth are woody.1 A woody plant increases in length by primary
growth at the tips of its stems and roots, whereas its older stems
and roots farther back from the tips increase in girth by secondary
growth. In other words, at the same time that secondary growth
is adding wood and bark, thereby causing the stem to thicken,
primary growth is increasing the length of the stem.
Herbaceous eudicot and monocot
stems differ in internal structure
Although considerable structural variation exists in stems, they
all possess an outer protective covering (epidermis or periderm),
one or more types of ground tissue, and vascular tissues (xylem
and phloem). Let us first consider the structure of herbaceous
eudicot stems and then of monocot stems.
Vascular bundles of herbaceous eudicot stems
are arranged in a circle in cross section
A young sunflower stem is a representative herbaceous eudicot
stem that exhibits primary growth (❚ Fig. 34-2). Its outer covering, the epidermis, provides protection in herbaceous stems,
as it does in leaves and herbaceous roots (see Table 32-4 and
1
Vascular bundle
Pith
Certain herbaceous stems, such as geranium and sunflower, also have a
limited amount of secondary growth.
Fig. 32-6). The cuticle, a waxy layer of cutin, covers the stem epidermis and reduces water loss from the stem surface. Stomata
permit gas exchange. (Recall from Chapter 33 that a cuticle and
stomata are also associated with the leaf epidermis.)
Inside the epidermis is the cortex, a cylinder of ground tissue
that may contain parenchyma, collenchyma, and sclerenchyma
cells (see Table 32-2 and Fig. 32-4). As might be expected from
the various types of cells that it contains, the cortex in herbaceous
eudicot stems can have several functions, such as photosynthesis,
storage, and support. If a stem is green, photosynthesis occurs in
chloroplasts of cortical parenchyma cells. Parenchyma in the cortex also stores starch (in amyloplasts) and crystals (in vacuoles).
Collenchyma and sclerenchyma in the cortex confer strength and
structural support for the stem.
The vascular tissues provide conduction and support. In herbaceous eudicot stems, the vascular tissues are located in bundles
that, when viewed in cross section, are arranged in a circle. However, viewed lengthwise, these bundles extend as long strands
throughout the length of a stem and are continuous with vascular
tissues of both roots and leaves.
Each vascular bundle contains both xylem, which transports
water and dissolved minerals from roots to leaves, and phloem,
which transports dissolved sugar (see Table 32-3 and Fig. 32-5).
Xylem is located on the inner side of the vascular bundle, and
phloem is found toward the outside. Sandwiched between xylem
and phloem in some herbaceous stems is a single layer of cells
called the vascular cambium, a lateral meristem responsible for
secondary growth (discussed later).
Stems and Transport in Vascular Plants
733
Phloem
Ground tissue
Vascular
bundles
Sieve
tube
element
Companion
cell
Xylem
Vessel
element
Epidermis
500 µm
(a) Cross section of a corn ( Zea mays) stem
shows vascular bundles scattered throughout
ground tissue.
Figure 34-3
Animated
Bundle sheath
(surrounds the
vascular bundle)
Ed Reschke
Ed Reschke
Air space
100 µm
(b) Close-up of a vascular bundle. The air space is the
site where the first xylem elements were formed and
later disintegrated. The entire bundle is enclosed in a
bundle sheath of sclerenchyma for additional support.
LMs of a monocot stem
Because most stems support the aerial plant body, they are
much stronger than roots. The thick walls of tracheids and vessel elements in xylem help support the plant. Fibers also occur
in both xylem and phloem, although they are usually more extensive in phloem. These fibers add considerable strength to the
herbaceous stem. In sunflowers and certain other herbaceous eudicot stems, phloem contains a cluster of fibers toward the outside of the vascular bundle, called a phloem fiber cap, that helps
strengthen the stem. The phloem fiber cap is not present in all
herbaceous eudicot stems.
The pith is a ground tissue at the center of the herbaceous
eudicot stem that consists of large, thin-walled parenchyma cells
that function primarily in storage. Because of the arrangement
of the vascular tissues in bundles, there is no distinct separation
of cortex and pith between the vascular bundles. The areas of
parenchyma between the vascular bundles are often referred to
as pith rays.
baceous eudicots, however, vascular bundles of monocots are
not arranged in a circle but are scattered throughout the stem
(❚ Fig. 34-3). Each vascular bundle is enclosed in a bundle sheath
of supporting sclerenchyma cells. The monocot stem does not
have distinct areas of cortex and pith. The ground tissue in which
the vascular tissues are embedded performs the same functions
as cortex and pith in herbaceous eudicot stems.
Monocot stems do not possess lateral meristems (vascular
cambium and cork cambium) that give rise to secondary growth.
Monocots have primary growth only and do not produce wood
and bark. Although some treelike monocots (such as palms) attain considerable size, they do so by a modified form of primary
growth in which parenchyma cells divide and enlarge. Stems of
some monocots (such as bamboo and palm) contain a great deal
of sclerenchyma tissue, which makes them hard and woodlike in
appearance.
Vascular bundles are scattered
throughout monocot stems
Woody plants have stems
with secondary growth
An epidermis with its waxy cuticle covers monocot stems, such
as the herbaceous stem of corn. As in herbaceous eudicot stems,
the vascular tissues run in strands throughout the length of a
stem. In cross section the vascular bundles contain xylem toward
the inside and phloem toward the outside. In contrast with her-
Woody plants undergo secondary growth, an increase in the girth
of stems and roots. Secondary growth occurs as a result of the
activity of two lateral meristems: vascular cambium and cork
cambium. Among flowering plants, only woody eudicots (such as
apple, hickory, and maple) have secondary growth. Cone-bearing
734
Chapter 34
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Most water loss from transpiration takes place through stomata, the numerous microscopic pores present on leaf and stem
surfaces. The tension extends from leaves, where most transpiration occurs, down the stems and into the roots. It draws water up
stem xylem to leaf cells that have lost water as a result of transpiration and pulls water from root xylem into stem xylem. As water
is pulled upward, additional water from the soil is drawn into the
roots. Thus, the pathway of water movement is as follows:
Soil ¡ root tissues (epidermis, cortex, and so forth) ¡
root xylem ¡ stem xylem ¡ leaf xylem ¡ leaf
mesophyll ¡ atmosphere
This upward pulling of water is possible only as long as there
is an unbroken column of water in xylem throughout the plant.
Water forms an unbroken column in xylem because of the cohesiveness of water molecules. Recall from Chapter 2 that water
molecules are cohesive, that is, strongly attracted to one another,
because of hydrogen bonding. In addition, the adhesion of water
to the walls of xylem cells, also the result of hydrogen bonding,
is an important factor in maintaining an unbroken column of
water. Thus, the cohesive and adhesive properties of water enable
it to form a continuous column that can be pulled up through
the xylem.
The movement of water in xylem by the tension – cohesion
mechanism can be explained in terms of water potential. The
atmosphere has an extremely negative water potential. For example, air with a relative humidity of 50% has a water potential
of 100 MPa; even moist air at a relative humidity of 90% has a
negative water potential of 13 MPa. Thus, there is a water potential gradient from the least negative (the soil) up through the plant
to the most negative (the atmosphere). This gradient literally pulls
the water from the soil up through the plant.
Although the tension – cohesion model was first proposed
toward the end of the 19th century, conclusive experimental evidence to support this mechanism was not obtained until the late
1990s and early 2000s. At that time, several research groups made
direct measurements of the large negative pressure that exists in
xylem, indicating that the water potential gradients in root, stem,
and leaf xylem are adequate to explain the observed movement
of water.
Is the tension – cohesion model powerful enough to explain
the rise of water in the tallest plants? Plant biologists have studied this question for years. For example, a 2004 study of five of
the eight tallest trees in the world (all redwoods, or Sequoia sempervirens) concluded that the tension produced by transpiration
is strong enough to pull water upward to a maximum height of
130 m (422 ft). Because the height of the tallest known living
tree, located in the Humboldt Redwoods State Park in California,
is about 113 m (367 ft), the tension – cohesion model easily accounts for the transport of water. Currently, most botanists consider the tension – cohesion model to be the dominant mechanism of xylem transport in most plants.
Root pressure pushes water from the root up a stem
In the less important mechanism for water transport, known as
root pressure, water that moves into roots from the soil is pushed
up through xylem toward the top of the plant. Root pressure
742
Chapter 34
occurs because mineral ions that are actively absorbed from the
soil are pumped into the xylem, decreasing its water potential.
This accumulation of ions has an osmotic effect, causing water
to move into xylem cells from surrounding root cells. In turn,
water moves into roots by osmosis because of the difference in
water potential between the soil and root cells. The accumulation
of water in root tissues produces a positive pressure (as high as
0.2 MPa) that forces the water up through the root xylem into
the shoot.
Guttation, a phenomenon in which liquid water is forced out
through special openings in the leaves (see Chapter 33), results
from root pressure. However, root pressure is not strong enough
to explain the rise of water to the tops of coastal redwoods and
other tall trees. Root pressure exerts an influence in smaller
plants, particularly in the spring when the soil is quite wet, but it
clearly does not cause water to rise 100 m (330 ft) or more in the
tallest plants. Furthermore, root pressure does not occur to any
appreciable extent in summer (when water is often not plentiful
in soil), yet water transport is greatest during hot summer days.
Sugar in solution is translocated in phloem
The sugar produced during photosynthesis is converted into
sucrose (common table sugar), a disaccharide composed of one
molecule of glucose and one of fructose (see Fig. 3-8b), before
being loaded into phloem and translocated to the rest of the
plant. Sucrose is the predominant photosynthetic product carried in phloem. Phloem sap also contains much smaller amounts
of other materials, such as amino acids, organic acids, proteins,
hormones, certain minerals, and sometimes disease-causing
plant viruses. Translocation of phloem sap is not as rapid as xylem transport (see Table 34-1).
Fluid within phloem tissue moves both upward and downward. Sucrose is translocated in individual sieve tubes from a
source, an area of excess sugar supply (usually a leaf ), to a sink,
an area of storage (as insoluble starch) or of sugar use, such as
roots, apical meristems, fruits, and seeds.
The pressure–flow hypothesis explains
translocation in phloem
Current experimental evidence supports the translocation of
dissolved sugar in phloem by the pressure –flow hypothesis,
which was first proposed in 1926 by the German scientist Ernst
Münch. The pressure –flow hypothesis states that solutes (such
as dissolved sugars) move in phloem by means of a pressure
gradient — that is, a difference in pressure. The pressure gradient
exists between the source, where the sugar is loaded into phloem,
and the sink, where the sugar is removed from phloem.
At the source, the dissolved sucrose is moved from a leaf ’s
mesophyll cells, where it was manufactured, into the companion
cells, which load it into the sieve tube elements of phloem. This
loading occurs by active transport, a process that requires adenosine triphosphate (ATP) (❚ Fig. 34-12). The ATP supplies energy to pump protons out of the sieve tube elements, producing
a proton gradient that drives the uptake of sugar through specific
channels by the cotransport of protons back into the sieve tube
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Key Point
In phloem, solutes move
from sources to sinks.
XYLEM
PHLOEM
At SOURCE (Leaf cell)
Sucrose actively
loaded into sieve tube
elements (requires
ATP).
Water diffuses
from xylem as a result
of decreased (more
negative) water
potential in sieve tube.
elements (an example of a linked cotransport system, discussed
in Chapter 5). The sugar therefore accumulates in the sieve tube
element. The increase in dissolved sugars in the sieve tube element at the source — a concentration that is 2 to 3 times as great
as in surrounding cells — decreases (makes more negative) the
water potential of that cell. As a result, water moves by osmosis
from the xylem cells into the sieve tubes, increasing the turgor
pressure (hydrostatic pressure) inside them. Thus, phloem loading at the source occurs as follows:
Proton pump moves H out of sieve tube element ¡ sugar
is actively transported into sieve tube element ¡ water diffuses from xylem into sieve tube element ¡ turgor pressure
increases within sieve tube
Companion cell
Sieve tube element
Direction of
water movement
Direction of
sucrose movement
At SINK (Root cell)
Vessel
running through
length of plant
Sucrose actively
and passively unloaded
into sink cell, such as
parenchyma cell in the
Sieve tube
running through root cortex. (Active
length of plant unloading requires
ATP.)
Water diffuses
from phloem to xylem
as a result of increased
(less negative) water
potential in sieve tube.
Figure 34-12
Animated
The pressure–flow hypothesis
Sugar is actively loaded into the sieve tube element at the source. As
a result, water diffuses from the xylem into the sieve tube element. At
the sink, the sugar is actively or passively unloaded, and water diffuses
from the sieve tube element into the xylem. The pressure gradient
within the sieve tube, from source to sink, causes translocation from
the area of higher turgor pressure (the source) to the area of lower
turgor pressure (the sink).
At its destination (the sink), sugar is unloaded by various
mechanisms, both active and passive, from the sieve tube elements. With the loss of sugar, the water potential in the sieve tube
elements at the sink increases (becomes less negative). Therefore,
water moves out of the sieve tubes by osmosis and into surrounding cells where the water potential is more negative. Most of this
water diffuses back to the xylem to be transported upward. This
water movement decreases the turgor pressure inside the sieve
tubes at the sink. Thus, phloem unloading at the sink proceeds
as follows:
Sugar is transported out of sieve tube element ¡ water
diffuses out of sieve tube element and into xylem ¡ turgor
pressure decreases within sieve tube
The pressure –flow hypothesis explains the movement of dissolved sugar in phloem by means of a pressure gradient. The difference in sugar concentrations between the source and the sink
causes translocation in phloem as water and dissolved sugar flow
along the pressure gradient. This pressure gradient pushes the
sugar solution through phloem much as water is forced through
a hose.
The actual translocation of dissolved sugar in phloem does
not require metabolic energy. However, the loading of sugar at
the source and the active unloading of sugar at the sink require
energy derived from ATP to move the sugar across cell membranes by active transport.
Although the pressure –flow hypothesis adequately explains
current data on phloem translocation, much remains to be
learned about this complex process. Phloem translocation is difficult to study in plants. Because phloem cells are under pressure,
cutting into phloem to observe it releases the pressure and causes
the contents of the sieve tube elements (the phloem sap) to exude and mix with the contents of other severed cells that are also
unavoidably cut. In the 1950s, scientists developed a unique research tool to avoid contaminating the phloem sap: aphids, which
are small insects that insert their mouthparts into phloem sieve
tubes for feeding (❚ Fig. 34-13). The pressure in the punctured
phloem drives the sugar solution through the aphid’s mouthpart
into its digestive system. When the aphid’s mouthpart is severed
from its body by a laser beam, the sugar solution continues to
flow through the mouthpart at a rate proportional to the pressure
in phloem. This rate can be measured, and the effects on phloem
Stems and Transport in Vascular Plants
743
Key Experiment
QUESTION: How can phloem sap be studied without cutting nonphloem cells that would
contaminate the sap?
HYPOTHESIS: An aphid mouthpart can be used to penetrate a single sieve tube.
EXPERIMENT: After allowing aphids to insert their mouthparts into phloem of a stem, researchers
Mouthpart
Sieve tube
element
Dwight Kuhn
Mouthpart
25 µm
(a) Aphid feeding on a stem.
(b) LM of phloem cells, showing an aphid mouthpart
penetrating a sieve tube element.
RESULTS AND CONCLUSION: About 1 mm3 of phloem sap exuded from each severed
mouthpart per hour for several days, so researchers were able to collect and analyze the
composition of the sap.
Figure 34-13
Collecting and analyzing phloem sap
This method, first used in the 1950s by insect physiologists studying aphids, was quickly adopted
by plant physiologists studying phloem translocation.
transport of different environmental conditions —varying light
intensities, darkness, and mineral deficiencies, for example — can
be ascertained.
The identity and proportions of translocated substances can
also be determined using severed aphid mouthparts. This technique has verified that in most plant species the sugar sucrose is
the primary carbohydrate transported in phloem; however, some
species transport other sugars, such as raffinose, or sugar alcohols, such as sorbitol.
Review
❚
❚
❚
❚
How does the direction of transport differ in xylem and
phloem?
What is water potential? How is the movement of water
related to water potential?
How does the tension–cohesion model explain the rise
of water in the tallest trees?
How does the pressure–flow hypothesis explain sugar movement in phloem? Include in your answer the activities at
source and sink.
S UM M A RY WI T H KE Y TE RM S
Learning Objectives
1
744
Describe the external features of a woody twig (page 732).
❚ Woody twigs demonstrate the external structure of stems.
Buds are undeveloped embryonic shoots. A terminal
Chapter 34
bud is located at the tip of a stem, whereas axillary buds
(lateral buds) are located in leaf axils. A dormant bud is
covered and protected by bud scales. When the bud rewww.thomsonedu.com/biology/solomon
M. H. Zimmermann, SCIENCE, Vol. 133, pp. 73 –79 (Fig. 4), 13 Jan. 1961. © 1961 by the
American Association for the Advancement of Science
anesthetized the feeding aphids with CO2 and used a laser to cut their bodies away from their
mouthparts. The mouthparts remained in the phloem and functioned like miniature pipes.
sumes growth, bud scales covering the bud fall off, leaving
bud scale scars.
❚ The area on a stem where each leaf is attached is called
a node, and the region of a stem between two successive nodes is an internode. A leaf scar shows where each
leaf was attached to the stem. Bundle scars are the areas
within a leaf scar where the vascular tissue extended from
the stem to the leaf. Lenticels are sites of loosely arranged
cells that allow oxygen to diffuse into the interior of a
woody stem.
2 Label cross sections of herbaceous eudicot and monocot
stems, and describe the functions of each tissue (page 732).
❚ Herbaceous stems possess an epidermis, vascular tissue,
and either ground tissue or cortex and pith. The epidermis is a protective layer covered by a water-conserving
cuticle. Stomata permit gas exchange. Xylem conducts
water and dissolved minerals, and phloem conducts dissolved sugar. The cortex, pith, and ground tissue function
primarily for storage.
❚ All herbaceous stems have the same basic tissues, but
their arrangement varies. Herbaceous eudicot stems have
the vascular bundles arranged in a circle (in cross section)
and have a distinct cortex and pith. Monocot stems have
vascular bundles scattered in ground tissue.
Learn more about eudicot and monocot stems
by clicking on the figures in ThomsonNOW.
Name the two lateral meristems, and describe the tissues
that arise from each (page 732).
❚ Vascular cambium is the lateral meristem that produces
secondary xylem (wood) and secondary phloem (inner
bark).
❚ Cork cambium produces periderm, which consists of cork
parenchyma and cork cells. Cork cells are the functional
replacement for epidermis in a woody stem. Cork parenchyma functions primarily for storage in a woody stem.
4 Outline the transition from primary growth to secondary
growth in a woody stem (page 732).
❚ Secondary growth (the production of the secondary
tissues, wood and bark) occurs in some flowering plants
(woody eudicots) and in all cone-bearing gymnosperms.
During secondary growth, the vascular cambium divides in
two directions to form secondary xylem (to the inside) and
secondary phloem (to the outside). The primary xylem and
primary phloem in the original vascular bundles become
separated as secondary growth proceeds.
3
Learn more about secondary growth by clicking on the figures in ThomsonNOW.
5
Describe the pathway of water movement in plants
(page 739).
❚ Water and dissolved minerals move from the soil into root
tissues (epidermis, cortex, and so forth). Once in root xylem, water and minerals move upward, from root xylem to
stem xylem to leaf xylem. Much of the water entering the
leaf exits leaf veins and passes into the atmosphere.
Define water potential (page 739).
❚ Water potential is a measure of the free energy of
water. Pure water has a water potential of 0 megapascals,
whereas water with dissolved solutes has a negative water
potential. Water moves from an area of higher (less negative) water potential to an area of lower (more negative)
water potential.
7 Explain the roles of tension–cohesion and root pressure as
mechanisms responsible for the rise of water and dissolved
minerals in xylem (page 739).
❚ The tension–cohesion model explains the rise of water in
even the tallest plants. The evaporative pull of transpiration causes tension at the top of the plant. This tension is
the result of a water potential gradient that ranges from
the slightly negative water potentials in the soil and roots
to the very negative water potentials in the atmosphere.
As a result of the cohesive and adhesive properties of
water, the column of water pulled up through the plant
remains unbroken.
6
Learn more about the tension–cohesion model
by clicking on the figure in ThomsonNOW.
❚ Root pressure, caused by the movement of water into
roots from the soil as a result of the active absorption of
mineral ions from the soil, helps explain the rise of water
in smaller plants, particularly when the soil is wet. Root
pressure pushes water up through xylem.
8 Describe the pathway of sugar translocation in plants
(page 739).
❚ Dissolved sugar is translocated upward or downward in
phloem, from a source (an area of excess sugar, usually a
leaf) to a sink (an area of storage or of sugar use, such as
roots, apical meristems, fruits, and seeds). Sucrose is the
predominant sugar translocated in phloem.
9 Discuss the pressure–flow hypothesis of sugar translocation
in phloem (page 739).
❚ Movement of materials in phloem is explained by the
pressure–flow hypothesis. Companion cells actively load
sugar into the sieve tubes at the source; ATP is required
for this process. The ATP supplies energy to pump protons
out of the sieve tube elements. The proton gradient drives
the uptake of sugar by the cotransport of protons back
into the sieve tube elements. Sugar therefore accumulates
in the sieve tube element, causing the movement of water
into the sieve tubes by osmosis.
Learn more about how sugar travels by clicking on the figure in ThomsonNOW.
❚ Companion cells actively (requiring ATP) and passively (not
requiring ATP) unload sugar from the sieve tubes at the
sink. As a result, water leaves the sieve tubes by osmosis,
decreasing the turgor pressure (hydrostatic pressure)
inside the sieve tubes.
❚ The flow of materials between source and sink is driven
by the turgor pressure gradient produced by water entering phloem at the source and water leaving phloem at
the sink.
T E ST Y OU R UN D E RS TA ND ING
1. The three main functions of stems are (a) support, conduction, and photosynthesis (b) support, anchorage in soil, and
production of new living tissues (c) conduction, production
of new living tissues, and sexual reproduction (d) conduction,
asexual reproduction, and sexual reproduction (e) support,
conduction, and production of new living tissues
Stems and Transport in Vascular Plants
745
2. Stems have undeveloped embryonic shoots called (a) lenticels
(b) buds (c) vines (d) phloem fiber caps (e) periderm
3. Axillary buds are located (a) at the tips of stems (b) in unusual places, such as on roots (c) in the region between two
successive nodes (d) in the upper angle between a leaf and the
stem to which it is attached (e) within the loosely arranged
cells of lenticels
4. The protective outer layer of cells covering herbaceous stems
is the (a) periderm (b) cork cambium (c) lateral meristem
(d) epidermis (e) bud scale
5. Ground tissue in monocot stems performs the same functions
and
in herbaceous
as
eudicot stems. (a) phloem; xylem (b) cork cambium; vascular
cambium (c) epidermis; periderm (d) primary xylem; secondary xylem (e) cortex; pith
6. Which of the following statements is false? (a) primary growth
is an increase in the length of a plant (b) primary growth occurs at both apical and lateral meristems (c) all plants have primary growth (d) herbaceous stems have primary growth,
whereas woody stems have both primary and secondary growth
(e) buds are embryonic shoots that contain apical meristems
7. The two lateral meristems responsible for secondary growth
are (a) phloem and xylem (b) cork cambium and vascular
cambium (c) epidermis and periderm (d) primary xylem and
secondary xylem (e) cortex and pith
8. Cork cambium and the tissues it produces are collectively
called (a) periderm (b) lenticels (c) cortex (d) epidermis
(e) wood
9. Horizontal movement of materials in woody plants occurs
in (a) bud scales (b) cortex (c) rays (d) lenticels (e) pith rays
10. The older, darker wood in the center of a tree trunk is
commonly called (a) hardwood (b) softwood (c) sapwood
(d) heartwood (e) cork
11. Each annual ring in a section of wood represents 1 year’s
growth of (a) primary xylem (b) secondary xylem (c) primary
xylem or secondary xylem in alternate years (d) primary
phloem (e) secondary phloem
12. Water potential is (a) the formation of a proton gradient
across a cell membrane (b) the transport of a watery solution
of sugar in phloem (c) the transport of water in both xylem
and phloem (d) the removal of sucrose at the sink, causing
water to move out of the sieve tubes (e) the free energy of
water in a particular situation
13. Which of the following is a mechanism of water movement
in xylem that is responsible for guttation: (a) pressure –
flow hypothesis (b) tension – cohesion (c) root pressure
(d) active transport of potassium ions into guard cells
(e) transpiration
14. Which of the following is a mechanism of water movement
in xylem that combines the evaporative pull of transpiration
with the cohesive and adhesive properties of water? (a) pressure –flow (b) tension – cohesion (c) root pressure (d) active
transport of potassium ions into guard cells (e) guttation
15. Which of the following is a mechanism of phloem transport in which dissolved sugar is moved by means of a pressure gradient that exists between the source and the sink?
(a) pressure –flow (b) tension – cohesion (c) root pressure (d) active transport of potassium ions into guard cells
(e) guttation
16. How does increasing solute concentration affect water potential? (a) water potential becomes more positive (b) water
potential becomes more negative (c) water potential becomes
more positive under certain conditions and more negative
under other conditions (d) water potential is not affected by
solute concentration (e) water potential is always zero when
solutes are dissolved in water
17. Label the various tissues, give at least one function for each
tissue, and identify the stem as a herbaceous eudicot, monocot, or woody plant. Use Figure 34-2 to check your answers.
Ed Reschke
Image not available due to copyright restrictions
(b)
746
Chapter 34
www.thomsonedu.com/biology/solomon
C R I TI C AL TH I N KI NG
1. When secondary growth is initiated, certain cells become
meristematic and begin to divide. Could a mature tracheid
ever do this? A sieve tube element? Why or why not?
2. Why does the wood of many tropical trees lack annual rings?
Why does the wood of other tropical trees possess annual
rings?
3. Why is hardwood more desirable than softwood for making
furniture? Explain your answer, based on the structural differences between hardwood and softwood.
4. Why should you cut off a few inches from the stem ends of
cut flowers before placing them in water? Base your answer on
what you have learned about the tension – cohesion model.
5. When a strip of bark is peeled off a tree branch, what tissues
are usually removed?
6. Could a tree grow to a height of 150 m (488 ft)? Why or
why not?
7. Evolution Link. Like stems in general, some vines are herbaceous and others are woody. Tropical rain forests have a
greater diversity of vines than in any other environment on
Earth, and most of these vines are woody. Develop a hypothesis to explain why natural selection has favored the evolution
of more species of woody vines (as opposed to herbaceous
vines) in tropical rain forests.
Additional questions are available in
ThomsonNOW at www.thomsonedu.com/
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Stems and Transport in Vascular Plants
747