Structure and Function of Plant Tissues (p. 682)
33.1
33.2
Organization of a Vascular Plant (p. 682; Fig. 33.1)
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
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. 683)
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.
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
(sometimes called “stone cells”), which are often branched and variably shaped and form
the gritty texture of pears.
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.
8.
9.
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.
Companion cells often assist sieve-tube members.
Sieve cells are more primitive, less efficient, are found in other vascular plants, and lack
companion cells.
The Plant Body (p. 686)
33.3
33.4
Roots (p. 686; Figs. 33.2, 33.3)
A. Roots have a simpler pattern of development than stems do.
B. Dicot roots have a central column of xylem with rays radiating outward; alternating with the
radiating arms of xylem are strands of phloem.
C. Roots have a pericycle, a layer of thick-walled cells around the outer boundary of the vascular
tissue.
D. Lateral roots arise from the pericycle.
E. 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.
F. A thickened, waxy band, the Casparian strip, surrounds the endodermis.
G. The outer layer of the root is the epidermis.
H. 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.
I. Extensions of epidermal cells, called root hairs, are responsible for the absorption of water
and nutrients from the soil.
Stems (p. 688; Figs. 33.4-33.7)
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 bundles around the periphery of the stem in
dicots, while in monocots the bundles are scattered throughout the stem.
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. A lateral meristem called the vascular cambium initiates secondary growth in stems.
2. The vascular cambium develops from parenchyma cells that lie between the xylem and
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 lateral meristem, 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; inwardly, the cork cambium produces a layer of
parenchyma cells.
6. The cork cambium together with the thick layer of cork cells and the inner layer of
parenchyma 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.
33.5
Rings are formed in wood when a spurt of growth occurs during the growing season,
producing a light-colored area of larger cells, followed by a period of slower or no
growth in which the cells are smaller and appear darker in color.
Leaves (p. 690; Figs. 33.8, 33.9, 33.10)
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 stalk called the petiole, which may be accompanied by stipules.
D. Dicot leaves have a pattern of reticulate venation, while monocot leaves have parallel veins.
E. Leaves can be simple or compound, and 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, in which intercellular spaces
function in gas exchange.
Plant Transport and Nutrition (p. 692)
33.6
33.7
Water Movement (p. 692; Figs. 33.11, 33.12 ,33.13, 33.14)
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.
3. 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. 695)
A. Most of the carbohydrates manufactured in plant leaves and other green parts are moved
through the phloem to other parts of the plant, a process called translocation.
B. Carbohydrates move through phloem by mass flow: carbohydrates produced as a result of
photosynthesis are loaded into sieve tubes, and water also enters the tubes due to osmosis; the
water pressure forces the carbohydrates down the plant.
C. This material flows through the plant to the points where carbohydrates are needed. Water
exits the phloem at the same time.