CHAPTER OUTLINE
9.1 Plant Organs
Most flowering plants, or angiosperms, possess a root system and a shoot system. The root
system simply consists of the roots, whereas the shoot system consists of the stem and leaves.
Root System
The root system in the majority of plants is located underground. Extensive root systems
of plants help anchor them in the soil and provide support. The root system absorbs water
and minerals from the soil for the entire plant.
Shoot System: Stems
The stem is the main axis of a plant and terminates in tissue that allows the stem to
elongate and produce leaves. A stem supports the leaves and contains vascular tissue that
transports water and minerals from the roots to the leaves and transports the products of
photosynthesis.
Shoot System: Leaves
Leaves are the major component of the plant that require H2O, CO2, and sunlight to carry
on photosynthesis.
9.2 Cells and Tissues of Plants
Plants have levels of biological organization similar to animals. Meristematic tissue allows a
plant to grow its entire life because it retains cells that ever have the ability to divide and produce
more tissues.
Meristematic Tissue
Meristematic tissue is present in the shoot tip and root tip, where it is called the apical
meristem.The apical meristem produces three types of primary meristem, which develop
into three types of specialized tissues in the body of a plant: epidermal tissue, ground
tissue, and vascular tissue.
Epidermal Tissue
Epidermal tissue forms the outer protective covering of a plant. It is modified in
roots, stems, and leaves.
Ground Tissue
Ground tissue forms the bulk of a plant and contains three different types of
specialized cells: parenchyma, collenchyma, and sclerenchyma.
Vascular Tissue
There are two types of vascular tissue: xylem, which transports water and
minerals from the roots to the leaves, and phloem, which transports sugar and
other organic compounds throughout the plant.
9.3 Monocot Versus Eudicot Plants
Flowering plants are divided into two groups, depending on the number of cotyledons, or seed
leaves, in the embryonic plant. There are several differences in structure between monocots,
which have one cotyledon, and eudicots, which have two.
9.4 Organization of Roots
Primary growth, which causes a plant to grow lengthwise, is centered in the apex (tip) of the
shoot and the root systems. Other areas of the root include the zone of cell division, zone of
elongation, and zone of maturation. The zone of cell division is protected by the root cap, which
is composed of parenchyma cells and protected by a slimy sheath.
Anatomy of a Eudicot Root
The following specialized tissues are identifiable in a eudicot root:
Epidermis
The epidermis forms the outer layer of the root and consists of only a single layer
of cells.
Cortex
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Underneath the epidermis, large, thin-walled parenchyma cells make up the cortex of the
root. The cells contain starch granules, which function in food storage.
Endodermis
The endodermis is a single layer of cells that forms a boundary between the cortex and
the vascular cylinder.
Vascular Tissue
The first layer of cells within the vascular cylinder is the pericycle, which can become
meristematic and start the development of branch roots. The main portion of the vascular
cylinder contains xylem and phloem.
Anatomy of Monocot Roots
Monocot roots have the same growth zones as eudicot roots, but they do not undergo
secondary growth as many eudicot roots do. A monocot root contains pith, a type of
ground tissue, that is centrally located and surrounded by a vascular ring composed of
alternating xylem and phloem bundles.
Root Diversity
Some plants have a primary root (taproot) with lateral roots for water absorption, while
other plants have a large number of slender roots that make up a fibrous root system.
Root Specializations
Specializations of roots include adventitious roots, epiphytes, root
nodules and mycorrhizae.
9.5 Organization of Stems
The terminal bud contains the shoot tip protected by bud scales, it includes the shoot apical
meristem and young leaves. The shoot apical meristem produces everything in a shoot: leaves,
axillary buds, additional stem, and sometimes flowers. It gives rise to the same primary
meristems as in the root, which develop into the differentiated tissues of a shoot system.
Herbaceous Stems
Mature nonwoody stems are called herbaceous stems and exhibit only primary growth.
Woody Stems
A woody plant has both primary and secondary tissues. Primary tissues are those new
tissues formed each year from primary meristems. Secondary tissues develop during the
first and subsequent years of growth from lateral meristems: vascular cambium and cork
cambium. Trees and shrubs undergo secondary growth due to activities of the vascular
cambium.
Bark
The bark of a tree contains both periderm (cork, cork cambium, and a single
layer of cork cells filled with suberin) and phloem.
Wood
Wood is secondary xylem that builds up year after year, thereby increasing the
girth of trees.
Woody Plants
In certain habitats, it is more advantageous for a plant to put most of its
energy into simply reproducing rather than being woody.
Stem Diversity
Stems exist in diverse forms, such as stolons, rhizomes, bulbs, and so on.
9.6 Organization of Leaves
The functions of leaves are to carry on photosynthesis, regulate water loss, and be protective
against parasites and predators. Foliage leaves are generally flat and thin and contain branching
leaf veins to transport materials. Bundle sheaths, layer of cells surrounding vascular tissue, help
regulate the entrance and exit of materials into and out of the leaf vein.
Leaf Diversity
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The blade of a leaf can be simple or compound and leaves can be arranged on a stem in
three ways: alternate, opposite, or whorled. Leaves are adapted to environmental
conditions. The leaves of a few plants are specialized to catch insects.
9.7 Uptake and Transport of Nutrients
Water and minerals are transported through a plant in xylem, while the products of
photosynthesis are transported in phloem.
Water Uptake and Transport
Water and minerals enter a plant at the root, primarily through the root hairs. Water
entering root cells creates a positive pressure called root pressure that tends to push
xylem sap upward.
Cohesion-Tension Model of Xylem Transport
Vascular tissue of xylem contains tracheids and vessel elements, which are
pipelines to move water from the roots to the leaves.
Explanation of the Model
Plants utilize a passive means of transport to move water in xylem. The
cohesion-tension model relies on the properties of water. Cohesion refers
to the tendency of water molecules to cling together. Adhesion refers to
the ability of water, a polar molecule, to interact with the molecules
making up the walls of the vessels in xylem. Once water reaches the
leaves it may evaporate, a process called transpiration.
Opening and Closing of Stomata
Each stoma is a small pore in the leaf epidermis bordered by guard cells, which fill with
water, opening the stoma. When water exits the guard cells, the stomata close. Potassium
ions, K+ accumulate within guard cells when stomata open. Stomata opening is
influenced by environmental conditions.
Organic Nutrient Transport
Plants transport water and minerals from the roots to the leaves and organic nutrients to
the parts of plants that need them.
Role of Phloem
Phloem is the tissue that transports sugars and other organic nutrients.
Pressure-Flow Model of Phloem Transport
According to the pressure-flow model, sugars move from photosynthesizing
leaves, the source, to the roots and other growth areas, the sink.
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