Chapter 17 Lecture Outline
Introduction
A Gentle Giant
A. Plants are unique organisms.
Review: Basic characteristics of the plant kingdom (Module 17.1).
1. Examining the giant sequoia, Sequoiadendron gigantea, helps underscore the unique
capabilities and adaptations of plants. The tree known as General Sherman is the largest
individual plant on Earth: 84 m tall, 10 m in diameter at the base, first branch at 40 m,
weighing about 1,400 tons, and alive for about 2,500 years (chapter-opening photos).
2. Humans depend on plants for a variety of needs (e.g., lumber, fabric, paper, and food), and
many other organisms depend on them for nutrition and shelter.
NOTE: Despite the importance of plants, on a worldwide basis, slightly more
photosynthesis is carried out by photosynthetic protists (algae) and bacteria of aquatic
habitats.
3. Giant sequoias are gymnosperms (naked seeds; Modules 17.7 and 17.8). Because
angiosperms (covered seeds; Modules 17.9–17.13) make up 90% of the world’s plant
species, they are the focus of this chapter.
Review: Module 17.3 discusses the basic differences between gymnosperms and
angiosperms.
Module 31.1 Talking About Science: Plant scientist Natasha Raikhel studies the Arabidopsis plant as a
model biological system.
A. Dr. Natasha Raikhel is the Distinguished Professor of Plant Cell Biology at the University of
California, Riverside. She recently received an award from the American Society of Cell
Biology for her work on the mustard seed plant, Arabidopsis (Figure 31.1B).
B. Raikhel uses the mustard seed plant to study biological systems and the structure-function
relationship in plants. Raikhel is a leader in the field of plant cell biology, and she also loves to
share her enthusiasm for plants with her students (Figure 31.1A).
I. Plant Structure and Function
Module 31.2 The two major groups of angiosperms are the monocots and the dicots.
A. Monocots include orchids, bamboos, palms, lilies, and grasses (including most of the
agricultural, grain-producing plants). They are distinguished by having one seed leaf
(cotyledon) but also usually have parallel-veined leaves, scattered vascular bundles in stems,
floral parts in multiples of three, and fibrous root systems (Figure 31.2).
B. Most angiosperms are dicots. Dicots include most shrubs and trees (except conifers) and many
herbaceous plants, including many food plants and plants domesticated for their fibers. They
are distinguished by having two cotyledons but also usually have net-veined leaves, vascular
bundles in a ring in stems, floral parts in multiples of four or five, and taproot systems.
C. Careful analysis of plant structure often reveals its function. Conversely, function provides
insight into the logic of a plant’s structure.
Module 31.3 A typical plant body consists of roots and shoots.
A. Plants, like animals, have a hierarchical structure; plants are made of organs, organs are made
of tissues, and tissues are made of cells.
B. Plant structural adaptations allow them to function in terrestrial habitats without drying out.
Functionally, plants need to absorb water and minerals from the soil, CO2 from the air, and
light from the sun. Plants must then create the necessary body parts from the raw materials and
from the products of photosynthesis.
C. The root system anchors the plant, absorbs and transports minerals and water, and stores food.
The fibrous roots of monocots, and the taproot plus secondary roots of dicots, are effective in
anchoring and absorption. The focal point of absorption is the root hair, an outgrowth of the
epidermal cells that increases the surface area for the absorption of water and minerals (Figure
31.3).
Preview: Most plants are also aided in nutrient uptake by mycorrhizal fungi (Module 32.12,
Chapter 17 Opening Essay).
D. The shoot system consists of supporting stems, photosynthetic leaves, and reproductive
structures (in angiosperms, flowers). Stems are composed of nodes, where leaves, flowers, or
other stems are attached, and internodes. Leaves are composed of photosynthetic blades and
short stalks that join the blade to the stem’s nodes.
E. Buds are undeveloped shoots that contain potential nodes, internodes, and leaves. Two types
occur: The terminal bud at the plant apex is the source of growth in height. The axillary buds,
one in each angle formed by a leaf and the stem, are usually dormant but can produce new
branches that add to a plant’s width.
F. Apical dominance results from the release of hormones from terminal buds that inhibits the
growth of the axillary buds. One can cause a plant to be bushier by removing the terminal bud
(pinching back), thereby stimulating (removing the inhibition of) the development of the
axillary buds.
Module 31.4 Many plants have modified roots, stems, and leaves.
A. In many dicots (e.g., carrots, turnips, beets, and sweet potatoes), food is stored in modified
taproots (Figure 31.4A). Plants store carbohydrates, in the form of starch, as a source of food in
these structures. Plants use the stored starch for flowering and fruit production.
NOTE: Animals store carbohydrate in the form of glycogen (Module 21.11).
B. Stems can be modified for several purposes. Strawberries make runners (or stolon) that provide
a mean for asexual reproduction. Rhizomes (e.g., iris and ginger) and tubers (e.g., potatoes) are
underground stems that store starch (Figure 31.4B).
C. Leaves may also be modified from their photosynthetic function. Some leaf bases (e.g., celery)
store food. Tendrils (e.g., vetch) are modified for grasping and climbing. Spines (e.g., cactus)
are modified for protection (Figure 31.4C).
Module 31.5 Plant cells and tissues are diverse in structure and function.
A. Plant cells have many of the same features as animal cells. Plants have three unique features.
1. Many are photosynthetic and contain chloroplasts.
2. They often have a large, central vacuole that helps support the cell (and plant tissues) by
maintaining cell turgor (Module 5.17).
3. Plant cells have a cell wall composed mainly of cellulose that surrounds the plasma
membrane.
B. Most plant cells that provide support have an additional, stronger secondary wall hardened with
lignin that is laid down inside the primary wall. Pits with cytoplasmically continuous
plasmodesmata (Module 4.19) often interconnect adjacent cells (Figure 31.5A).
C. Plant cells can be grouped into five types based upon the structure and function of the cells.
1. Parenchyma cells are the most abundant, unspecialized cells, with only primary walls that
are thin and flexible. They function in food storage, photosynthesis, and aerobic respiration
(Figure 31.5B). Parenchyma cells can differentiate into any other plant cell type.
2. Collenchyma cells resemble parenchyma cells but lack secondary walls and have thicker
primary walls that are uneven. They provide flexible support for young parts of plants that
are still growing (Figure 31.5C).
3. Sclerenchyma cells have thick, rigid primary walls, hardened with lignin, and uneven
secondary walls. They function in support and protection. Lignin is the main chemical
component of wood. There are two types of sclerenchymal cell. Fibers are long, slender,
and arranged in bundles. Hemp fiber is a good example. Sclereids are short cells with thick,
irregular, and very hard secondary walls. Sclereids are in many seed coats, nutshells, and
the gritty texture of the soft tissue of pears (Figure 31.5D).
4. Water-conducting cells have rigid secondary walls containing lignin, and function only
when dead and connected end to end. Tracheids are long, thin cells with tapered ends,
covered with open pits. Vessel elements are wider, shorter, less tapered, and have
completely open ends (Figure 31.5E). Either type of cell, as the name implies, forms a
system of tubers that transports water from the roots to the stems and leaves.
5. Food-conducting cells (sieve-tube members) are also arranged end to end but have
relatively thin primary walls and no secondary walls. They are alive but lack nuclei and
ribosomes. The end of each cell forms a sieve plate containing numerous pits with
plasmodesmata. Each food-conducting cell is found in association with at least one
companion cell that makes certain proteins for it.
D. Plant cells are grouped into tissues. Two types of vascular tissues are as follows:
Xylem tissue is largely composed of water-conducting cells. Phloem tissue is largely composed
of these food-conducting cells. Both types of vascular tissue have other cells, for example,
sclerenchyma cells for support and parenchyma cells for storage.
Preview: Xylem and phloem function (Modules 32.3 and 32.5).
Module 31.6 Three tissue systems make up the plant body.
A. Plants have three tissue systems, each made of one or more tissue type. The plant organs (roots,
stems, and leaves) are made of all three tissue systems.
1. The dermal tissue system is the skinlike first defense against damage or infection. In many
plants, this tissue system is composed of a single surrounding layer of cells called an
epidermis. The epidermis of leaves and most stems is covered with a cuticle (waxy coating)
to prevent water loss.
2. The vascular tissue system, composed of xylem and phloem tissues, conducts water and
nutrients throughout the plant. This tissue system also provides support.
3. The ground tissue system fills the spaces between the epidermis and vascular tissue system
in young plants and functions variously in photosynthesis, storage, and support.
B. Each tissue system is continuous from organ to organ throughout the plant (Figure 31.6A).
C. Roots are surrounded by epidermal cells with root hairs and without a cuticle. Water and
minerals must be absorbed through the epidermis. The ground tissue system forms the cortex,
which functions in conducting materials from the root surface into the central vascular tissue
and in food storage. The inner layer of the cortex is the endodermis. It provides a selective
barrier, regulating flow into the vascular tissue (Figure 31.6A).
Preview: Solute uptake is controlled by the plasma membranes of root cells (Module 32.2).
D. Stems of dicots and monocots differ in the relative distributions of ground tissue and vascular
tissue systems. However, both types of plants have their vascular tissue systems arranged in
vascular bundles. Monocots have vascular bundle tissues scattered in a more uniform ground
tissue. A dicot has bundles of vascular tissue in an outer ring supported in a ring of ground
tissue cortex that surrounds a food storage region called the pith (Figure 31.6A).
E. Leaves have a complex arrangement of the three tissue types that perfectly fit their function of
photosynthesis. The epidermis contains guard cells that form a pore or stomata (singular, stoma)
(Module 32.4).
F. The ground tissue system is the layer between two epidermal layers and is called the mesophyll.
The mesophyll contains mostly photosynthetic parenchyma cells. The lower mesophyll is
loosely arranged, particularly near the stomata, where most gas exchange takes place.
G. Tiny branches, or veins, make up the vascular tissue system of leaves and are continuous with
the vascular bundles of stems. Each vein has xylem and phloem and is in close contact with
photosynthetic cells providing minerals and water from the soil and removing the sugars to be
transported to the rest of the plant.
II. Plant Growth
Module 31.7 Primary growth lengthens roots and shoots.
A. Unlike animals that have a determinate growth and only grow to a certain size, plants have
indeterminate growth; that is, they continue to grow during their entire lives. Animals can move
through their surroundings (Chapter 30), but individual plants must grow to reach other places
in their environments.
B. Three seasonal growth patterns occur in plants. Annuals complete their life cycle in one year.
Biennials complete their life cycle in two years. Perennials continue to live and reproduce for
many years.
C. Indeterminate growth is the result of plants having meristems, unspecialized cells that continue
to give rise to new cells. Apical meristems are found at the root and shoot tips and in axillary
buds. The lengthwise growth produced by these regions is primary growth (Figure 31.7A). The
mechanism of primary growth differs between roots and shoots.
D. The apical meristem in the root tip is covered by a root cap that is sloughed off during growth
through abrasive soil. Just above the meristem, these tissues elongate, pushing the root tip
downward.
E. The cells behind the meristem grow through three successive stages of primary growth but with
no distinct boundaries:
1. Zone of cell division—includes cells of the root apical meristem and cells derived from the
meristem; cells of the root cap and new root cells come from this zone.
2. Zone of elongation—root cells elongate up to ten times their original length. Elongation by
water uptake is responsible for the downward progression of the root. Expansion in width is
limited by the parallel arrangement of cellulose fibers.
3. Zone of maturation—the three tissue systems (dermal, ground, and vascular) develop here
based on differential gene expression. The cells of the vascular cylinder differentiate into
primary xylem and primary phloem (Figure 31.7B).
F. The apical meristem of the shoot is a dome-shaped mass of dividing cells at the end of the
terminal bud. Elongation occurs below the terminal bud, forcing it upward. Some of the apical
meristem cells remain in lateral positions and develop the apical meristems in axillary buds
(Figure 31.7C).
Preview: The hormonal basis of shoot growth is discussed in Module 33.2.
Module 31.8 Secondary growth increases the girth of woody plants.
A. Secondary growth involves growth in stems and roots that causes thickening. It is most evident
in trees, shrubs, and vines. The increase in diameter is due to another type of meristem called
lateral meristems, which are composed of two types of dividing cells.
B. The vascular cambium is a cylindrical meristem that develops from two layers of parenchyma
cells between the xylem and phloem of shoots. As cells in this vascular cambium divide inward,
they form new layers of secondary xylem to the outside of the primary xylem. As cells of the
vascular cambium divide outward, they form new layers of secondary phloem inside the
primary phloem. The increase in thickness of the stem is mostly layers of secondary xylem,
which form the wood of a tree, shrub, or vine (Figure 31.8A).
C. In regions that have distinct seasons, secondary xylem cells are larger in diameter during
periods of favorable growth (early wood, growth during the spring) and smaller at other times
(late summer). This results in distinct annual growth rings (Figure 31.8B).
D. The new layers of phloem external to the vascular cambium do not accumulate but are sloughed
off in bark at about the same rate they are produced. Bark includes the secondary phloem, the
cork cambium, and the cork. Cork cambium (a meristem tissue) is derived from parenchyma
cells in the cortex and makes cork. Mature cork cells are dead and have waxy, thick walls that
protect the stem surface from damage and infection, much like epidermal cells. However, they
eventually slough off as new cork cambium develops within deeper layers (Figure 31.8B).
NOTE: This also provides a way for a woody plant to increase the circumference of its
protective layer as the diameter of the stem increases.
E. Wood itself can be divided into central heartwood, xylem that no longer functions in transport
because it is plugged with resins, and sapwood, younger secondary xylem that actually conducts
water and minerals (xylem sap). Wood rays are collections of parenchyma cells that extend
laterally from heartwood into the sapwood, providing channels between these two regions. The
heartwood of trees acts as an endoskeleton, providing a strong, rigid, yet flexible core upon
which the living plant substance is supported.
F. Wood is the source of many products useful to humans. As a building material, it is unmatched
for its combination of strength, hardness, lightness, insulating properties, durability,
workability, and beauty.
III. Reproduction of Flowering Plants
Review: Features of angiosperms (Modules 17.9–17.12).
Module 31.9 Overview: The sexual life cycle of a flowering plant.
A. A flower is the reproductive shoot of an angiosperm composed of modified leaves (sepals,
petals, stamens, and carpels) (Figure 31.9A).
B. Sepals are usually green and protect flower buds. Petals are usually large and showy and attract
pollinators.
C. Stamens are male structures with pollen-bearing anthers at the tip. Pollen grains deliver sperm
nuclei to females.
D. Carpels are female structures composed of stigma and an ovary. Inside the ovary are the ovules,
which carry the developing egg and supporting cells.
E. Pollination occurs when pollen is delivered to the stigma of another flower. Fertilization occurs
in the ovule. The fertilized egg develops into an embryo, and the ovule develops into a seed that
holds the embryo. The ovary develops into a fruit that aids in seed dispersal. The seed
germinates in a favorable environment to complete the life cycle (Figure 31.9B).
Module 31.10 The development of pollen and ovules culminates in fertilization.
A. All plants, including angiosperms, alternate between diploid sporophytes that produce spores by
meiosis (spores then divide mitotically and develop into a gametophyte) and haploid
gametophytes that produce gametes by mitosis. The gametes unite by fertilization to form a
diploid zygote, which is the first cell of the next sporophyte generation.
Review: Diploid and haploid generations of plants (Module 17.4).
B. The mature plant we see is the sporophyte. Angiosperm gametophytes are microscopic and are
found inside the flower parts (Figure 31.10).
C. The male gametophyte is the two-celled pollen grain. It develops into spores following meiosis
of cells in the anther. Each resulting spore divides mitotically to produce two haploid cells, a
tube cell and a generative cell. The outer wall of the pollen grain is thick and resistant.
D. The female gametophyte develops inside the ovule, a central cell surrounded by a coating of
smaller cells. The central cell undergoes meiosis, but only one of the resulting haploid nuclei
develops into a spore. The nucleus in the haploid spore enlarges and divides mitotically,
forming the embryo sac. The embryo sac, housed in and protected by the sporophyte, is the
female gametophyte. The embryo sac contains a large central cell with two haploid nuclei.
Another of the cells is the haploid egg. All this happens in specialized ovary tissue at the base
of the carpel.
E. Pollination is the delivery of pollen to a stigma. Pollen is usually wind- or animal-dispersed
(Module 17.13).
F. After pollination, the pollen grain germinates and the tube cell grows its pollen tube downward
into the stigma and ovary. The generative cell divides mitotically, forming two sperm nuclei. At
the base of the ovule, the pollen tube releases both sperm nuclei.
G. The double fertilization that follows is a hallmark of the angiosperms. One sperm nucleus
fertilizes the egg, forming the zygote (2n, diploid) that will develop into the embryo. The other
sperm nucleus fuses with the two central nuclei, forming a triploid (3n) nucleus that will
develop into the endosperm, tissue that nourishes the embryo.
Module 31.11 The ovule develops into a seed.
A. Within the ovule, the triploid cell develops into a nutrient-rich endosperm, and repeated
division of the diploid zygote leads to the development of an embryo (Figure 31.11A).
NOTE: Be sure to point out that the ovule includes everything inside the original coating of
small cells that surrounded the cell that underwent meiosis to form, ultimately, the eight haploid
nuclei, including the egg. All nuclei other than the zygote and endosperm do not develop
further.
B. The embryo develops when the zygote first divides into two cells. One of the cells becomes the
embryo. The other cell divides and becomes a thread that forces the embryo into the endosperm.
Near the end of maturation, the seed loses water and develops a resistant seed coat.
C. Seed dormancy occurs when the seed develops up to a point and then further growth and
development are suspended. This allows time for dispersal and for the seasonal occurrence of
conditions favorable for independent growth.
D. Seeds of dicots (common bean) have two fleshy cotyledons that have absorbed the endosperm nutrients and taken over
the role of nourishment. The seed of a monocot is actually a fruit with one seed. The fruit of maize (a kernel) has one
cotyledon, a protective sheath over the embryonic root and shoot, and contains a large endosperm (Figure 31.11B).
Module 31.12 The ovary develops into a fruit.
A. The ovary of a flower changes into a fruit when hormones are released during fertilization. A
fruit houses and protects seeds and helps disperse them. During development, hormonal
changes make the ovary grow and thicken.
B. Pod formation follows several steps:
1. Pollination must take place first.
2. Petals drop from the flower, the ovaries start to grow, and the walls thicken.
3. The ovaries form a pod (or a fruit).
C. Fruits are highly varied in organization, depending on how many ovules, how many ovaries,
how many carpels, or how many flowers are involved in the formation, and on the ultimate
means of dispersal (wind, water, or animal).
D. A peach or peapod are examples of a simple fruit (Figures 31.12A and B).
E. A raspberry is an example of an aggregate fruit, fruit that develops from many united carpels.
F. A pineapple is an example of a multiple fruit, fruit that develops from many united flowers
(Figure 31.12C).
NOTE: Plums and avocados are fruits with single seeds. The winged maple “seed” and plumed
dandelion “seed” are examples of fruits modified for wind dispersal. The seed of each is at the
heavy end of the fruit.
Module 31.13 Seed germination continues the life cycle.
A. New plant life does not start with seed germination. Seed dormancy is discontinued when
conditions are acceptable for growth to resume. A previously developed embryo starts
developing again when the seed takes up water, expands, and ruptures its coat. Endosperm or
cotyledons begin to enzymatically digest stored nutrients, and the nutrients are transported to
the growing parts.
B. In dicots, the embryonic root emerges first, followed by young shoots that exit the seed in a hooked shape that protects the
terminal meristem from abrasion by soil particles. Once the shoot clears the soil surface, light stimulates the hook to
straighten, and the first foliage leaves develop at the tip and begin photosynthesis (Figure 31.13A). In the pea, the
cotyledons, having provided food for the germinating embryo, remain in the soil and decompose.
C. In monocots, the embryonic root emerges first, followed by young shoots that do not develop a
hook. However, the shoots are protected from abrasion by a sheath that surrounds them until
they break through the soil surface. The maize cotyledon remains in the soil and decomposes
(Figure 31.13B).
D. The process of reproduction is a perilous journey for a seedling. Production of seeds in great
quantities helps ensure the survival of the next generation of plants.
Module 31.14 Asexual reproduction produces plant clones.
Review: Asexual and sexual reproduction (Chapter 8 Opening Essay and Module 27.1).
A. Vegetative propagation (or asexual reproduction) is the production of offspring from a single
parent. The offspring from asexual reproduction are called clones and are genetically identical
to the original plant.
Review: Animal reproduction by fragmentation is discussed in Module 27.1.
B. Asexual reproduction is an extension of a plant’s ability to grow throughout its life. The
meristem tissue and parenchyma cells are capable of sustaining life indefinitely.
C. Asexual reproduction often involves fragmentation (e.g., garlic) into separate parts; each
regenerates into a new plant. A garlic bulb is an underground stem that will fragment into
cloves giving rise to several new plants (Figure 31.14A). A root sprout of a coast redwood will
grow to take the place of the parent, if the parent is lost (Figure 31.14B). The creosote bush of
southwestern deserts reproduces vegetatively from its roots, forming very old clones. (The one
in Figure 31.14C is estimated at 12,000 years old.) Dune grass propagates by underground
runners (Figure 31.14D).
D. There are several advantages to asexual reproduction. Offspring are well suited to their
immediate environments. Early life for vegetative offspring is less hazardous than for seedlings
because they are less fragile.
Module 31.15 Connection: Asexual reproduction is a mainstay of modern agriculture.
A. Many ornamental trees, shrubs, and houseplants are propagated by stem or leaf cuttings.
B. Plant tissue culture provides another way to grow offspring from a few meristematic cells
(Figure 31.15), and this technique has been adapted to propagate genetically engineered plant
cells. The addition of a foreign gene into a plant cell (parenchyma cell) results in a genetically
modified (GM) plant. There are some environmental and health concerns related to GM plants
(see Module 12.19).
C. Vegetative propagation has one main disadvantage: Crop plants developed from cloning
processes have inherently low levels of genetic diversity, which exposes them to potential
devastation from disease, especially when planted in monoculture (single plant species in a
large area).