Download 17 PLANT FORM AND FUNCTION

23 PLANT FORM AND FUNCTION
CHAPTER OUTLINE
LEARNING OBJECTIVES
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Understand that all vascular plants use the same basic body structures.
List the three major types of plant tissues and their functions.
Know the different types of meristems and where they are located.
Describe the various cell types that comprise ground tissue.
List the types of cells found in dermal tissue and give their functions.
Understand the nature of the cell types comprising xylem and phloem, the vascular tissue of the plant.
Describe the structure of the photosynthetic organs, the leaves.
Explain how stem structure relates to function.
Understand how primary growth differs from secondary growth in stems.
Know what cell types comprise wood and bark.
Describe the structure and function of roots.
Explain how branching in stems differs from branching in roots.
Know how water enters roots and moves through plants according to the cohesion-adhesion-tension
theory.
Describe how carbohydrates move through plants.
List the major essential plant nutrients.
Structure and Function of Plant Tissues (p. 416)
23.1
23.2
Organization of a Vascular Plant (p. 416; Fig. 23.1)
A. Most plants possess the same fundamental architecture.
B. A vascular plant is organized along a vertical axis, with a root, and a shoot consisting of a
stem and leaves.
C. Meristems are zones of undifferentiated cells whose sole purpose is to divide rapidly to
increase the size of the plant.
1. At the growing tips of the plant, apical meristems are found that are responsible for the
primary growth, or growth in length, of the plant.
2. Secondary growth is due to lateral meristems found around the plant periphery.
3. Two kinds of lateral meristems add to the girth of the plant: vascular cambium gives rise
to secondary phloem and xylem, and cork cambium produces layers of bark on the stem
and roots of the plant.
Plant Tissue Types (p. 417; Figs. 23.2, 23.3, 23.4, 23.5, 23.6, 23.7)
A. Plant organs are composed of different types of tissues.
B. A tissue is a functional unit made up of a group of cells that work together to serve a purpose
for an organism.
1. The three major types of plant tissues are ground tissue, dermal tissue, and vascular
tissue.
C. Each type of tissue is composed of distinctive kinds of cells.
D. Ground Tissue
1. Ground tissue is made up of several types of cells.
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2.
Parenchyma cells are very common and make up the bulk of roots, leaves, and stems;
they are alive at maturity and have only primary cell walls.
3. Collenchyma cells form continuous chains of cells under the epidermis of stems or leaf
stalks as well as along the veins in leaves; they provide support for any plant organ that
has not yet undergone secondary growth.
4. Sclerenchyma cells have thick secondary walls and are nonliving at maturity; their
purpose is to lend strength to the tissues in which they are found.
5. Two types of sclerenchyma exist: fibers, long cells that form strands, and sclereids, which
are often branched and variably shaped and make up the walls of the “stones” in peaches
and nutshells.
E. Dermal Tissue
1. Dermal tissue is made up of numerous flattened epidermal cells covered by a thick, waxy
coat called the cuticle.
2. Guard cells are pairs of cells scattered between epidermal cells.
3. The openings between the guard cells, called stomata, are where gas exchange occurs
between the atmosphere and the interior tissue of the plant.
4. Trichomes are extensions of the epidermal cells that help to regulate the heat and water
balance of the leaves.
5. Some trichomes are glandular and secrete substances that seem to deter predators.
6. Root hairs are epidermal extensions near root tips and take up water and nutrients from
the soil.
F. Vascular Tissue
1. Xylem and phloem are the two principal types of vascular tissue of plants.
2. Xylem conducts water in a continuous column throughout the plant.
3. Xylem is made up of tracheids and vessel elements, neither of which have living
cytoplasm at maturity.
4. Tracheids are characteristic of more primitive vascular plants, and have pits in their
secondary walls through which water flows.
5. Vessel elements have both pits and perforations in their end walls through which water
moves.
6. Phloem is made up of sieve-tube members and sieve cells, both of which are alive but
without nuclei at maturity.
7. Sieve-tube members are more evolutionarily advanced, are found in angiosperms, and are
arranged in an end-to-end fashion, forming a sieve tube through which carbohydrates and
other nutrients may pass.
8. Companion cells often assist sieve-tube members.
9. Sieve cells are more primitive, less efficient, are found in other vascular plants, and lack
companion cells.
The Plant Body (p. 420)
23.3
Roots (p. 420; Figs. 23.8, 23.9, 23.10)
A. Roots are organized in a manner similar to stems except that dicot roots contain no pith;
instead, a central column of xylem can be found with rays radiating outward.
B. Roots have a pericycle, a layer of thick-walled cells around the outer boundary of the vascular
tissue.
C. Lateral roots arise from the pericycle.
D. Just outside the pericycle lies the endodermis, a layer of tissue that regulates the flow of water
between the outer portion of the root and its vascular tissue.
E. A thickened, waxy band, the Casparian strip, surrounds the endodermis.
F. Distal to the apical meristem of the root is the root cap, a layer of undifferentiated cells that
protects the apical meristem as the root pushes its way downward.
G. Extensions of epidermal cells, called root hairs, are responsible for the absorption of water
and nutrients from the soil.
H. In the pericycle of woody plants, the cork cambium produces cork cells to the outside and
phelloderm parenchyma to the inside.
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23.4
23.5
I. The tissue associated with the cork cambium is called the periderm.
Stems (p. 422; Figs. 23.11, 23.12, 23.13, 23.14)
A. The stem is the part of the plant that serves as a framework for positioning the leaves.
B. Primary Growth
1. The points where leaves arise along stems are called nodes.
2. Between nodes are portions of stem called internodes.
3. As the leaves grow larger, a small bud forms in the axil near the point from which the leaf
arises from the stem.
4. The bud sometimes remains dormant or sometimes develops into side branches with
another set of leaves.
5. Whether or not this bud develops depends on hormonal signals from the terminal bud of
the shoot.
6. During primary growth, vascular tissue forms a cylinder close to the periphery of the stem
in dicots, and is scattered throughout the stem in bundles in monocots.
7. When only primary growth has occurred, the center of the stem contains pith, and outside
the pith is the cortex.
C. Secondary Growth
1. The vascular cambium initiates secondary growth in stems.
2. The vascular cambium develops from parenchyma cells that lie between the primary
xylem and primary phloem.
3. Cells that divide on the bark side become secondary phloem, while those that divide more
toward the center become secondary xylem.
4. While secondary growth is occurring, a second kind of secondary cambium, the cork
cambium, develops in the outer layers of the stem.
5. The cells forming to the outside of the cork cambium are waterproof, densely packed cork
cells that are nonliving at maturity.
6. The cork cambium together with the thick layer of cork cells make up the protective outer
periderm of the stem.
7. The term bark refers to any tissues of the stem outside the vascular cambium.
8. Inside the vascular cambium is the secondary xylem, which is commonly referred to as
wood.
9. Rings are formed in wood when a spurt of growth occurs during the growing season,
producing a light-colored area, followed by a period of slower or no growth in which the
cells are darker in color.
Leaves (p. 424; Figs. 23.15, 23.16, 23.17)
A. Leaves are the major light-capturing organs of plants and are structurally very diverse.
B. Leaves grow by means of marginal meristems which cease to function once the leaf blade is
fully expanded.
C. Most leaves have a slender petiole, which may be accompanied by stipules.
D. Dicot leaves have a net of reticulate venation while monocot leaves have parallel veins.
E. Leaf arrangement may be alternate, opposite, or whorled.
F. Masses of palisade parenchyma cells make up much of the mesophyll of the leaf.
G. The remaining mesophyll is composed of spongy parenchyma, the spaces which function in
gas exchange.
Plant Transport and Nutrition (p. 426)
23.6
Water Movement (p. 426; Figs. 23.18, 23.19, 23.20, 23.21, 23.22)
A. Vascular plants have a means of transporting fluids and nutrients throughout the plant.
B. Cohesion-Adhesion-Tension Theory
1. How water moves from the root to the shoot tip through xylem in plants can be explained
by the cohesion-adhesion-tension theory.
2. Air moving across the leaves of plants causes fluids to evaporate; when fluids evaporate,
there is a pull on the water within the plant, drawing it upward.
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3.
23.7
23.8
When xylem cells are narrow, water adheres to the insides of the tubes (adhesion) and can
be pulled upward toward the top of the plant.
4. At the same time, water molecules tend to stick to each other, or are cohesive, due to their
hydrogen bonding.
5. Tensile strength is added by making the inner diameter of the column very small, so the
water flow is uninterrupted.
C. Transpiration
1. Transpiration, or loss of water from the leaves, occurs because of the factors that drive
water up the plant.
D. Regulation of Transpiration: Open and Closed Stomata
1. Water losses from transpiration can be minimized by having a cuticle and by closing
stomata during times of water stress.
2. The stomata open and close because of changes in the water pressure of their guard cells.
3. Very high temperatures also tend to cause stomata to close.
E. Water Absorption by Roots
1. Water moves into the root hairs because of the greater concentration of dissolved
minerals inside root epidermal cells; thus, water is moving in passively by osmosis.
2. Membrane ion channels actively pump ions into root cells, even against strong
concentration gradients.
Carbohydrate Transport (p. 429; Fig. 23.23)
A. Carbohydrates move through phloem by mass flow.
B. Where carbohydrates are manufactured in the leaves, they are loaded into sieve tubes, and
water also enters the tubes due to osmosis.
C. This material flows through the plant to the points where carbohydrates are needed. Water
exits the phloem at the same time.
Essential Plant Nutrients (p. 430; Fig. 23.24)
A. Aside from carbohydrates, plants need a number of other nutrients to thrive, such as nitrogen,
potassium, calcium, magnesium, phosphorus, and sulfur.
B. Each of these elements plays a specific role within the plant.
C. Some plants use other organisms directly as a source of food and are called carnivorous plants
(i.e., Venus flytrap and pitcher plants).
KEY TERMS
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root (p. 416)
stem (p. 416)
leaves (p. 416)
meristems (p. 416) Plant meristems are made up of cells whose function is to divide.
vascular cambium (p. 416)
ground tissue (p. 417)
dermal tissue (p. 417)
stomata (p. 418) “Stoma” is singular, “stomata” is plural.
xylem (p. 418)
phloem (p. 418)
wood (p. 423)
mesophyll (p. 425) Mesophyll means “middle leaf.”
cohesion (p. 426) It may be necessary to review hydrogen bonding to help students understand
cohesion.
transpiration (p. 426)
root hairs (p. 428)
translocation (p. 429)
turgor pressure (p. 430)
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LECTURE SUGGESTIONS AND ENRICHMENT TIPS
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Economically Important Plants. Anyone can imagine the economic importance of grain crops or of
forests planted for logging interests. What about the economic importance of plants for medicinal
purposes or for clothing or writing papers? For housing or rubber tires? Introduce to your students a
number of the following examples of the economic importance of plants. Wheat, corn, and rice provide
the bulk of the world's food staples. Other plants contribute significantly to our enjoyment of these
basic foods. For example, most spices are derived from plant parts. Pepper is the ground fruit (peppers)
of a variety of pepper plants. Cinnamon, oregano, cilantro, basil, lemon grass, and many other spices
are all derived from plants. Coffee, tea, and colas are beverages that use extracts of beans or leaves.
Certainly to the satisfaction of many a sweet tooth, sugarcane and sugar beets produce table sugar.
Cotton is used for clothing and many other textile products. Houses around the world are often
constructed of plant parts. Many medications have been derived from plant parts, and ongoing
pharmaceutical research is occurring at a breakneck pace to find plant-based compounds that can cure
cancer and other major human diseases. Plant derivatives are the basis for most hallucinogenic and
other illegal street drugs. With the rapid destruction of the world's tropical forests, many plants will
undoubtedly go extinct before their medicinal, much less ecological, values can be fully determined.
There Is No Such Thing as Vegetables. Botanically speaking, vegetables do not exist. Show students a
variety of what they might consider to be vegetables, such as carrots, beets, corn, tomatoes, green
beans, radishes, cabbage, broccoli, cauliflower, and celery. Then ask them to determine what plant part
each of these items represents in botanical terms—i.e., roots, seeds, fruit, leaves, flowers, or stems.
They will soon understand that “vegetables” is a nonbotanical, all-inclusive term.
Edible Wild Plants and Herbs. Certain types of wildflowers, herbs, and portions of trees and shrubs are
edible. Locate a guidebook for your local area and find a variety of edible plant parts, berries, flowers,
and so forth for demonstration in the classroom. Point out the hallmark traits that help identify each
plant. Make an herbal tea from mint leaves (the smallest or youngest leaves taste best) and pass
samples around to your students. Mint leaves work well since few people are allergic to this group of
plants. Camomile, on the other hand, can trigger allergic responses in individuals who have problems
with ragweed. Be sure students understand that some herbal remedies are tried and true, but with many
others, caution must be used because claims can be exaggerated. Edible plant parts, on the other hand,
are mostly nutritious counterparts of cultivated plants. Emphasize that students must learn to properly
identify plants before foraging on their own. Similar poisonous species must be recognized as well.
CRITICAL THINKING QUESTIONS
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How is the growth of marginal meristems in leaves different than that of the apical meristems of stems
and roots?
Is there any advantage to one type of leaf arrangement as compared to another? Can you think of any
situations in which one type of arrangement is more beneficial to plant survival?
Describe how branching differs in roots and shoots and speculate why this is so.
How does water move in through root hairs?
Why would very high temperatures cause stomata to close?
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