Mastering Concepts
21.1
1. How do stems, leaves, and roots support one another?
Stems hold leaves up to light, where they can photosynthesize and produce
carbohydrates. Sugars move via the stem from leaves to roots to nourish root cells. The
roots anchor the plant and absorb water and minerals. These substances move through the
stem to the leaves, where they are used in photosynthesis.
2. What is the relationship between the node and the internode of a stem?
The node of a stem is a place where leaves are attached. An internode is a length of stem
located between nodes.
3. Give examples of roots, stems, and leaves with specialized functions.
The following table gives examples of stems, leaves, and roots with specialized
functions:
Organ
Example
Specialized function
Stem
Tuber
Food storage
Rhizome
Produces new roots and new shoots
Thorny stems
Protection
Tendrils
Support of climbing stems
Leaves
Cactus spines
Protection
Carnivorous
Attract, capture, digest prey
leaves
Onion bulb
Nutrient storage
Roots
Beets, carrots
Store carbohydrates
Cactus roots
Store water
Buttress & prop Support plant
roots
Pneumatophores Obtain air
21.2
1. What is the difference between determinate and indeterminate growth?
A plant with determinate growth stops growing after it reaches its mature size. In
contrast, a plant with indeterminate growth can keep growing as long as the environment
can support it.
2. What are the locations and functions of meristems?
Apical meristems are located at the tips of roots and shoots. Apical meristems allow roots
and shoots to grow in length. Lateral meristems are located in an internal cylinder of cells
that extends along most of the length of the plant. Lateral meristems allow an increase in
girth of roots and stems. Intercalary meristems occur in grasses and other monocots
between the nodes of a mature stem, often at the base of an internode. These meristems
allow a plant to tolerate repeated grazing and mowing because they regrow leaves from
the base when the top of the leaf is clipped off.
21.3
1. Describe the structures and functions of the cell types that make up a plant body.
Parenchyma cells–the most numerous of cells, these cells are unspecialized with thin
walls and retain the ability to divide. These cells carry out major functions such as
photosynthesis, repair and storage.
Collenchyma cells–elongated cells with walls of various thickness providing elastic
support during growth
Sclerenchyma cells—thick walled cells of variable shape and length with lignin for
inelastic support
Tracheid cells—narrow, dead cells with overlapping walls that conduct water and
minerals through pits
Vessel elements—wide, dead cells arranged end to end that conduct water and minerals
through end walls
Sieve tube elements—living cells aligned end to end with sieve plates in the end walls
that conduct dissolved organic materials
Companion cells—living cells connected to sieve tube elements by plasmodesmata,
which transfer material in and out of the sieve tubes
2. Where in the plant does ground tissue occur?
Ground tissue fills the spaces between more specialized cell types inside stems, leaves,
fruits, and seeds. Most of the body of a plant is made of ground tissue.
3. What are the functions of vascular tissue?
The functions of vascular tissues are to move water, minerals, carbohydrates, and other
dissolved substances within the plant.
4. How does the structure of dermal tissue contribute to its functions?
Dermal tissue consists of a single layer of flat, transparent, tightly packed cells that cover
the leaf, stem, and roots of the plant. Dermal tissue secretes the cuticle, which retards
loss of water from deeper plant tissues. Stomata, which are surrounded by guard cells, are
pores that allow gases to move into and out of a leaf. When stomata are closed, they
prevent water loss from leaves.
21.4
1. Name the cell layers that occur in the stem of a monocot and a eudicot, moving from
the epidermis to the innermost tissues.
Moving from the epidermis of a eudicot stem toward its center, the tissues are: epidermis;
cortex; vascular bundles composed of fibers, phloem, and xylem; and pith. Moving from
the epidermis of a monocot stem toward its center, the tissues are epidermis; vascular
bundles made of xylem, phloem, and fibers; and ground tissue. Vascular bundles are
arranged in a ring in a eudicot stem but scattered throughout a monocot stem.
2. List the parts of a simple and a compound leaf.
Both types of leaves have a blade, petiole, and axillary bud. Simple leaves have a single
blade while compound leaves have a divided blade.
3. Describe the internal anatomy of a leaf.
Inside a leaf, mesophyll cells are surrounded by air spaces that promote gas exchange as
the cells carry out photosynthesis. Veins contain xylem and phloem, which deliver water
and minerals and carry off sugars produced in photosynthesis. A waxy cuticle covers the
epidermis, and stomata are pores through which gases enter and leave the leaf.
4. Compare and contrast the development of taproot and fibrous root systems.
Both roots begin development with a primary root. At the tip is an apical meristem,
which is covered with a root cap that protects against abrasion. In both root systems, root
tips explore the soil seeking out water and minerals. In taproots, the primary root persists
through life, growing fast and deep and producing lateral branches. In fibrous root
systems, slender adventitious roots from the base of the stem replace the primary root,
producing a shallower root system.
5. How do the roots of monocots and eudicots differ?
Roots of monocots are fibrous, while eudicots usually have taproots. Internally, the roots
of eudicots have a central core of vascular tissue surrounded by a cortex and epidermis.
Monocot roots have a ring of vascular bundles surrounding a central pith. The ring is
surrounded by cortex and an epidermis.
21.5
1. What are the two lateral meristems in a woody stem or root, and which tissues does
each meristem produce?
The two lateral meristems are the vascular cambium and the cork cambium. Vascular
cambium produces the secondary xylem to the inside and secondary phloem to the
outside. The cork cambium produce parenchyma cells to the inside and cork to the
outside.
2. What are the functions of wood and bark?
Wood, or secondary xylem, produces the girth of the plant. The sapwood conducts water
and minerals. Bark protects the plant; in addition, secondary phloem in the bark conducts
carbohydrates within the plant .
3. How do softwoods differ from hardwoods?
Softwoods are composed mainly of tracheids; they lack the tough fibers found in
hardwoods.
4. Explain the origin of tree rings.
Vascular cambium cells, dormant through the winter, divide and produce wood during
the spring and summer. During the wet spring months the water conducting cells are
larger and the wood is lighter. During the drier summer the cells are smaller and form
darker rings.
21.6
1. What is the overall question the researchers were asking in this study? Summarize the
data they used to arrive at their conclusion.
The overall question asked by the researchers was, “Why would natural selection select
for plants that expend energy on homes for ants (domatia), and do domatia increase the
reproductive fitness of the plant?” The data came from several related experiments. The
researchers used H. brunonis as a model organism because the same species provides
control groups without domatia and experimental groups with domatia. The researchers
used as a measure of reproductive fitness the number of fruits produced on a tree. Their
first experiment revealed that trees with domatia produce more fruit. Additional
experiments revealed that tramp ants, living in the domatia, quickly recognize herbivores
and actively attack them, thereby increasing the number of leaves available for
photosynthesis. From these data the researchers were able to conclude that the domatia
indirectly improved reproductive fitness in H. brunonis.
2. Propose an explanation for the observation that the trees secrete nectar only on young
leaves and not on tougher, more mature leaves.
Answers will vary, but one possible explanation is that herbivorous caterpillars prefer
young leaves. The plant would therefore waste energy by attracting defenders to mature
leaves.
Write It Out
1. List the main vegetative organs of a plant, and explain how each relies on the others.
The vegetative (non-reproductive) organs of a typical plant include the roots, stems, and
leaves. The stem and leaves constitute the shoot. The stem supports the leaves, the main
sites of photosynthesis. The products of photosynthesis, in turn, nourish the non-green
plant parts, including the roots. Most roots exist below ground, anchoring the plant and
absorbing water and minerals from the soil. These resources move via stems to the
leaves and other aboveground plant parts.
2. Describe a stem specialization and a leaf specialization that provide protection.
Thorns form on stems; spines are specialized leaves in cacti.
3. How can you tell whether a plant’s growth is determinate or indeterminate?
Plants that stop growing at maturity exhibit determinate growth; plants that grow
indefinitely by adding modules exhibit indeterminate growth.
4. Many biology labs use preserved slides of root tips to demonstrate the stages of
mitosis. Why is this a better choice than using a slide of a mature leaf?
The tips of roots consist of rapidly dividing meristematic cells that show all the stages of
mitosis; the leaf does not contain meristematic tissue.
5. Compare and contrast tracheids, vessel elements, and sieve elements.
Tracheids, vessel elements, and sieve tubes are similar in that they are structures
dedicated for conduction. Tracheids and vessel elements are part of the xylem and are
used in the conduction of water, whereas sieve tubes are part of the phloem and are used
in the conduction of sugars. Tracheids and vessel elements are dead at functional
maturity; sieve tube members are alive at functional maturity. Vessel elements lack end
walls; tracheids have pits; sieve tube members have sieve plates through which strands of
cytoplasm pass from cell to cell.
6. Which plant tissues have cells that are dead at maturity? Why is it advantageous to the
plant for these cells to die?
Xylem cells are dead at maturity; if cytoplasm were present, it would interfere with water
movement. Cork also has cells that are dead at maturity. Since they provide
waterproofing and insulation, both of which can be done without metabolic cellular
activity, it is advantageous for the plant not to waste energy keeping these cells alive.
7. Corn is a monocot and sunflower is a eudicot. Make a chart that compares the stems,
leaves, and roots of these plants.
Stem structure: Monocots have vascular bundles that are scattered throughout the ground
tissue of the stem; eudicots have a single ring of vascular bundles in the stem. Leaf
venation: Monocot leaves have parallel veins; eudicot leaves have netted veins. Root
organization: Monocots have a ring of vascular tissue surrounding a central core (pith) of
parenchyma cells; eudicots have a vascular cylinder consisting of a solid core of xylem,
with ridges that project into the pericycle. The phloem strands are generally located
between the “arms’ of the xylem core.
8. Thorns, spines, and tendrils are so highly modified that it can be difficult to tell
whether they derive from leaves or stems. How could a biologist use knowledge of
internal plant anatomy to determine the origin of these structures?
The structures would have to be dissected for the examination of internal anatomy. A
leaf should have a network of veins and some mesophyll cells; a stem should have the
internal anatomy typical of a monocot or eudicot.
9. Compare and contrast the formation of branches in stems and roots.
Branches greatly increase the surface area of both stems and roots. In stems, branches
come from apical meristem tissue contained in buds. In roots, branches are produced at
the pericycle.
10. Describe why and how leaves and roots maximize surface area.
Most leaves have a broad, flat blade; the great number of leaves increases the total
surface area, maximizing sunlight capture. Roots have numerous branches in the soil; in
addition, root hairs maximize the surface area to absorb water and minerals.
11. Where do parenchyma cells occur in stems, leaves, and roots?
Parenchyma cells are located in the ground tissue of the stem, the mesophyll of the
leaves, and the cortex of a root.
12. Fossils show that eudicots appeared about 125 million years ago. What structures
would definitively identify a fossilized plant as a eudicot?
Definitive structures would include a taproot, vascular bundles in a single ring in the
stem, pollen with three pores or furrows, netted veins in the leaves, and flower parts
occurring in multiples of four or five.
13. Mammals exchange gases in the alveoli of the lungs (see figure 30.8). How do the
structures and functions of leaf mesophyll compare with those of alveoli?
The mesophyll cells in the lower part of the leaf are separated by air spaces; when the
stomata are open, air can enter and gas exchange can occur across the cell membranes of
the mesophyll cells. Similarly, when the respiratory tract delivers air to the alveoli, gas
exchange can occur across the cell membranes of the alveoli and red blood cells.
14. Explain how conditions in the terrestrial environment that selected for each of the
following adaptations: cuticle, stomata, vascular tissue, roots, stems, and leaves.
The cuticle decreases water loss; the stomata allow gas exchange in the presence of the
cuticle; vascular tissue provides support and a distribution system in large plants; roots
both anchor the plant and maximize the surface area for water and nutrient absorption;
stems support the leaves, providing an advantage in the competition for light; thin, broad
leaves maximize the surface area for photosynthesis.
15. Predict the anatomical differences you might expect to see among a desert plant, a
rain forest plant, and an aquatic plant.
Desert—the scarcity of water is the main limiting factor. Plants probably have shallow,
broad root systems that quickly absorb scarce water before it evaporates; thick cuticles;
greatly reduced leaf surface area; water storage adaptations in roots and stems; spines that
protect against herbivores.
Rain forest—competition for sunlight is the main limiting factor. Plants probably are tall
and fast-growing, with broad leaves.
Aquatic—reduced access to gases is the main limiting factor, but water is abundant.
Plants require less xylem and a less developed root system. Stomata are concentrated on
the upper side of the leaf, not the lower side. Plants may also have adaptations that keep
the leaves floating on the water’s surface.
16. Starting at the outside and moving into a eudicot’s stem, what tissues do you
encounter?
From the outside: epidermis; cortex; vascular bundles with fibers, xylem and phloem;
pith
17. Girdling is cutting away or severing the living bark in a ring around a tree’s trunk.
Which part of a girdled tree do you expect to die first, the roots or the shoot? Why?
Would the tree be harmed as much by a vertical gash? Why or why not?
The roots should die first, because living bark consists of the phloem that delivers sugars
to the belowground parts of the plant. A vertical gash would not be as harmful because
the phloem would remain intact.
18. A palm tree is a monocot with a thick trunk. The woody tissues come from enlarged
parenchyma cells in the stem. How is this arrangement different from the secondary
growth that occurs in gymnosperms and eudicots?
In gymnosperms and eudicots the secondary growth is from the vascular cambium, which
produces xylem to the inside and phloem to the outside. Most of the bulk of a woody
gymnosperm or eudicot comes from secondary xylem.
19. Heartwood often contains secondary metabolites that inhibit microbial growth. How
are these secondary metabolites adaptive?
By inhibiting microbial growth in heartwood secondary xylem, the integrity of the stem is
maintained, so the stem doesn’t hollow out and collapse.
20. Suppose you drive a metal spike from the outermost bark layer to the center of a
tree’s trunk. Which tissues does your spike encounter as it moves through the stem, and
what type of meristem produced each type?
The spike will strike the cork and a thin layer of parenchyma, both produced by the cork
cambium. It will then strike the secondary phloem and enter the secondary xylem, both
produced by the vascular cambium.
21. List a function of each organ, tissue, and cell type described in this chapter, and then
list at least one feature that facilitates that function.
Organs:
Roots – Anchoring; a network of roots holds the plant in the soil.
Stems – Support; tough fibers help keep a plant upright
Leaves – Photosynthesis; mesophyll cells possess chloroplasts
Flowers – Reproduction; the petals of many flower types attract pollinators
Tissues:
Ground tissue – Storage; presence of plastids in some ground tissues
Dermal tissue – Protection; tough, waterproof periderm physically protects roots and
stems.
Vascular tissue – Transportation; xylem and phloem are adjacent to each other, allowing
xylem to contribute water to phloem sap
Cell Types:
Parenchyma – Storage; cells have vacuoles that store acids and other substances
Collenchyma – Support; thick, elastic cell walls
Sclerenchyma – Support; thick, tough cell walls (as in sclereids and fibers)
Vessel elements and tracheids – Transportation; cells lack cytoplasm that would
otherwise interfere with movement of water and nutrients.
Sieve tube elements – Transportation; cells have sieve plates that allow the movement of
materials from cell to cell.
Pull It Together
1. What are the three types of meristems, and how do they differ from one another?
The three types of meristem are the apical meristem, which lengthens the stems and
roots; the lateral meristem, which increases the girth of plants with secondary growth;
and the intercalary meristem, found in some monocots, which grows new stems and
leaves when the top of the plant is removed.
2. How do fibers and sclereids fit into this concept map?
Fibers and sclereids are two types of sclerenchyma cells.
3. Add the terms cortex, pith, mesophyll, endodermis, and Casparian strip to this concept
map.
Cortex and pith are composed of ground tissue that fills the interior of stems and roots;
mesophyll is ground tissue inside leaves; the Casparian strip is a waxy coating on the
endodermis of a root.
4. Describe a location in which you might find each of the three tissue types.
Ground tissue fills much of the interior of stems, roots, and leaves. Vascular tissue occurs
in the veins of leaves and in the vascular bundles of stems and roots. Dermal tissue coats
the exterior of shoots and roots.