Unit 7 Plants
Ch. 23 Roots, Stems, & Leaves
Seed Plant Structure
• 3 of the principal organs of seed plants are
roots, stems, & leaves
• The root system of a plant absorbs water &
dissolved nutrients
Seed Plant Structure
- A stem has a support system for the plant
body, a transport system that carries
nutrients, & a defense system
that protects the plant against
predators & disease
- Leaves are the plant’s main
photosynthetic systems
Plant Tissue Systems
• Plants consist of 3 main tissue systems:
dermal tissue, vascular tissue, & ground
tissue
Dermal Tissue
• The outer covering of a plant, that consists
of a single layer of epidermal cells
• The outer surfaces are often covered with
thick waxy layer to protect against water
loss & injury
Vascular Tissue
• Contains several types of specialized cells
• Xylem consists of tracheids & vessel
elements
• Phloem consists
of sieve tube
elements &
companion cells
Vascular Tissue - Xylem
• All seed plants have a type of xylem cell
called a tracheid, cells that resist water
pressure
• Vessel elements - wide cells that form
hollow tubes, which water can move
through freely
Vascular Tissue - Phloem
• Sieve tube elements - main cells in phloem,
form tubes through which materials, like
sugars & other foods, are carried in a watery
stream
• Companion cells - phloem cells that
surround sieve tube elements, help
movement of substances in & out of the
phloem
Ground Tissue
• Cells that lie between dermal & vascular
tissues
• Consists mainly of parenchyma - cells that
are packed with chloroplasts & are the site
of most of a plant’s photosynthesis
Ground Tissue
• Collenchyma - cells that have strong,
flexible cell walls that help support larger
plants (makes up “strings” of a stalk of
celery)
• Sclerenchyma - rigid cell walls that make
ground tissue tough & strong
Plant Growth & Meristematic
Tissue
• Meristems - clusters of tissue that are
responsible for continuing growth
throughout a plant’s lifetime
• Meristematic tissue - undifferentiated cells,
they have not yet become specialized for
specific functions, such as transport
Plant Growth & Meristematic
Tissue
• Apical meristem - group of undifferentiated
cells that divide to produce increased length
of stems & roots
Plant Growth & Meristematic
Tissue
• Differentiation - cells develop into mature
cells with specialized structures & functions
• As cells differentiate, they produce each of
the tissue systems of the plant: dermal,
ground, & vascular tissue
Types of Roots
• The 2 main types of roots are taproots,
found mainly in dicots, & fibrous roots,
found mainly in monocots
• Taproot - primary root
– Ex.) carrots, dandelions, beets, etc.
Types of Roots
• Fibrous roots - roots that branch to such an
extent that no single root grows larger than
the rest
– Ex.) grass
Root Structure & Growth
• A mature root has an outsider layer, the
epidermis, & a central cylinder of vascular
tissue
• Between these
2 tissues, lies a
large area of
ground tissue
Root Structure & Growth
• Root hairs - tiny cellular projections that
penetrate the spaces between soil particles
& produce a large surface area through
which water can enter the plant
Root Structure & Growth
• Cortex - spongy layer of ground tissue just
inside the epidermis
• Endodermis - another layer of cells that
completely encloses the root’s vascular
subsystem in a region called the vascular
cylinder
Root Structure & Growth
• Root cap - protects the root as it forces its
way through the soil
Root Functions
• Roots anchor a plant in the ground & absorb
water & dissolved nutrients from the soil
Stem Structure & Function
• Stems have 3 important functions: they
produce leaves, branches, & flowers; they
hold leaves up to the sunlight; & they
transport substances between roots & leaves
Stem Structure & Function
• Nodes - where leaves are attached
• Internode - regions between the nodes
• Buds - contain undeveloped tissue that can
produce new stems & leaves
Monocot & Dicot Stems
• In monocots, vascular bundles are scattered
throughout the stem
• In dicots & most gymnosperms, vascular
bundles are arranged in a cylinder
Monocot & Dicot Stems
• Vascular bundles - contains xylem &
phloem tissue
• Pith - the parenchyma cells inside the ring
of vascular tissue
Primary Growth of Stems
• Primary growth - growth occurring only at
the ends of a plant
• Its produced by cell divisions
in the apical meristem, & takes
place in all seed plants
Secondary Growth of Stems
• Secondary growth - method of growth
where stems increase in width
• In conifers & dicots, secondary growth
takes place in lateral meristematic tissues
called the vascular cambium & cork
cambium
Secondary Growth of Stems
• Vascular cambium - produces vascular
tissues & increases the thickness of stems
over time
• Cork cambium - produces the outer
covering of stems
Formation of Wood
• Heartwood - older xylem near the center of
the stem that no longer conducts water
• Sapwood - surrounds heartwood, active in
fluid transport
Formation of Bark
• Bark - includes all of the tissues outside the
vascular cambium, includes: phloem, the
cork cambium, & cork
Leaf Structure
• The structure of a leaf is optimized for
absorbing light & carrying out
photosynthesis
Leaf Structure
• Blades - flattened section, attached to the
stem by a thin stalk - petiole
Leaf Functions
• A leaf can be considered a system
specialized for photosynthesis
• Subsystems of the leaf include tissues that
bring gases, water, & nutrients to the cells
that carry out photosynthesis
Leaf Functions
• Guard cells - specialized cells in the
epidermis that control the opening &
closing of stomata (air spaces) by
responding to changes in water pressure
Leaf Functions
• Plants keep their stomata open just enough
to allow photosynthesis to take place, but
not so much that they lose an excessive
amount of water
Leaf Functions
• Transpiration - the loss of water through its
leaves