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Roots, Stems & Leaves
Chapter 23
23.1- Specialized Tissues in
Plants
• Do you wonder if plants are really
alive?
• They don’t do much.
• They outnumber animals and
make up far more of the Earth’s
biomass
• The cells of plants work together
to ensure the plant’s survival
Seed Plant Structure
• Cells of seed plants are organized
into different tissues and organs.
• The three principal organs are
roots, stems and leaves
• The organs are linked by systems
and subsystems that run the
length of the plant
Roots
• Absorbs water and dissolved
nutrients
• Anchor plant in the ground,
preventing erosion of soil
• Protect plant from soil bacteria and
fungi
• Hold plant upright in wind and rain
Stems
• Support system for plant body strong
enough to hold up leaves and branches
• Transport system to lift water from
roots to leaves and carry
photosynthesis products from leaves
down to roots
• Defense system protects from predators
and disease.
Leaves
• Main photosynthetic system of the
plant.
• Broad flat surface of many leaves
increase the amount of sunlight
the plant absorbs
• Contain subsystems to protect
plant from water loss
Plant Tissue Systems
• Within roots, stems and leaves are
specialized tissue systems.
– Dermal tissue-the skin of the plant
– Vascular tissue-the plants
bloodstream
– Ground tissue-everything else in the
plant
Dermal Tissue
• Outer covering of the plant
• Usually made of a single layer of epidermal
cells whose outer layer is often covered with a
cuticle.
• Some have projections known as trichomes to
protect the leaf and give it a fuzzy
appearance.
• In roots this includes the root hairs that aid in
water absorption
• Underside of leaves they contain guard cells to
regulate water loss and gas exchange
Vascular tissue
• Forms transport system to move
water and nutrients throughout the
plant
• Main subsystems are xylem and
phloem
– Xylem has specialized cells called
tracheids and vessel elements
– Phloem has sieve tube elements and
companion cells
Xylem
• All seed plants have tracheids
– Long narrow cells with walls impermeable to
water.
– Walls pierced by openings that connect
neighbors to each other
– When they mature they die and the
cytoplasm disintegrates and then can
conduct water
• Vessel elements are in angiosperms
– Wider than tracheids and stacked on top of
each other like stacked cans
Phloem
• Main cells are sieve tube elements
– End to end like vessel elements.
– Different in that end walls don’t
disintegrate but have small holes in
them to allow materials between the
cells
– Mature cells lose nuclei and most
other organelles.
– Rest of organelles hug the cell wall
Phloem-con.
• Companion cells
– Surround sieve tube elements that
keep nuclei and organelles.
– Support the phloem cells and aid in
the movement of substances in and
out of the phloem
Ground Tissue
• Cells between the dermal and vascular
tissues
• Three main types of G.T.
– Parenchyma-makes up most plants, thin cell
walls and large vacuoles. Site of
photosynthesis in leaves
– Collenchyma-strong, flexible cell walls to
support larger plants, celery strings
– Schlerenchyma-extreme thick and rigid cell
walls that make g.t. tough and strong
Plant Growth and
Meristematic Tissue
• Most plants grow by an indeterminate
type of growth.
– They grow and produce new cells at the tips
of their roots and stems for as long as they
live
• Cells are produced in meristems.
Clusters of tissues responsible for
continuing growth through plant’s life.
• Cells are undifferentiated-not
specialized
P.G. and Meristematic Tissue
• Apical meristem is near the tip of growing
stem and root
• Produce length of stems and roots
• Meristematic tissue is the only tissue that
produces new cells by mitosis
• The cells that originate look alike at first and
then change by differentiation to produce
dermal, ground and vascular tissues
• Flowers also produced in meristems by turning
on in a shoot apical meristem to transform the
apical meristem into a floral meristem.
23.2-Roots
• Seed grows it puts out a root to
draw water and nutrients from the
soil
• Overall size of a root system can
be huge.
– Rye plant-600 square meters, 130x
more than stems + leaves
Types of Roots
• Two main types of roots
– Taproots-found in dicot
• Primary root grows long and thick while
secondary roots grow small
• Store sugars, starches
– Fibrous roots-found in monocots
• Branch to large extent so one root isn’t
too much larger than others
• Help to keep soil in place
Root Structure and Growth
• Contain cells from dermal, vascular and
ground tissues
• Mature root has epidermis, central
cylinder of vascular tissue and between
these layers is the ground tissue
• Main functions are water and mineral
transport
• Lots of subsystems help carry out these
functions
Structure and Growth
• Epidermis has dual function
– Protection and absorption
– Root hairs-tiny projections that penetrate
soil spaces and make large surface area so
water can enter the plant
– Root cap is tough and secretes a slippery
substance to move root through the soil.
Protects the apical meristem.
Structure and Growth
• Ground tissue-spongy layer called
the cortex between epidermis and
endodermis that surrounds
vascular cylinder.
• Most of increase in root size comes
just behind the apical meristem
where the new cells are elongating
– At a later stage these new cells
differentiate and become specialized
Root Functions
• Anchor the plant in the ground
• Absorb water from soil
• Absorb dissolved nutrients from
soil
– Takes energy of the plant to absorb
water and nutrients. How??
Uptake of Plant Nutrients
• Understanding soil helps explain how plants
work.
• Soil is a mixture of
– Sand, silt, clay, air, decaying material
• Soil is different in different spots and has
differing amounts of nutrients
• Main nutrients are nitrogen, phosphorus,
potassium, magnesium and calcium in addition
to co2 and water
• Trace elements also needed in small
quantities. Large amounts can kill plants
Active Transport of Minerals
• Cell membranes of root hairs and other
cells in the epidermis of roots contain
active transport proteins to move
mineral ions into the plant using ATP.
• Creates a high concentration of
minerals inside the plant root which
causes water to move by osmosis into
the cells. Water follows the minerals
Movement Into the Vascular
Cylinder
• Both Active Transport and Osmosis cause
water and minerals to move from the
epidermis, through the cortex and into the
endodermis
• Endodermis encloses the vascular cylinder
(xylem and phloem)
• Casparian strip surrounds cells of endodermis
and is waterproof.
– Keeps water from back flowing between the cells
– Concentration of water in cells prevents backflow
from cell to cell
Osmosis
• Water moves into the vascular
cylinder by osmosis.
• They can’t get through the
Casparian strip backwards and
can’t go back through the cells
because of concentration levels
they become trapped in the
vascular cylinder.
Root Pressure
• Result of all of this is that water and
mineral flow in a one way direction into
the vascular cylinder
• Roots build up pressure from all the
water flowing into the V. C.
– The pressure causes the water to move up
because the Casparian strip keeps the water
from filling up the cortex
– Pressure comes from active transport
• Root pressure is start of how transport
takes place in the entire plant.
23.3 Stems
• Vary in size, shape and method of
development
• Some are underground, some high
in the air
• Vary in structure and internal
arrangement of cells
Stem Structure and Function
• Three functions in plants
– Produce leaves, branches and flowers
– Hold leaves up to the sunlight
– Transport substances between the roots
and the leaves
• They make an essential part of
transport systems of the plant
• Made of three types of tissues just like
rest of the plant
Stem structure and function
• Most plants contain distinct node where
the leaves attach
• Internode-place between nodes
• Buds-contain undeveloped tissue that
can produce new stems and leaves
– Found where leaves attach to nodes
• Larger plants stems develop woody
tissue to help support leaves and
flowers
Monocot and Dicot Stems
• Arrangement of tissues in stems
differs in seed plants
• Monocots-vascular tissue is
scattered in bundles throughout
the stem
• Dicots/most gymnospermsarranged in a cylinder
Monocot Stems
• Stem has distinct epidermis
• Within epidermis are series of
vascular bundles with xylem facing
inside and phloem facing outside
of stem
• Ground tissue inbetween vascular
bundles mainly parenchyma
Dicot Stems
• Young dicots have vascular bundles in
an organized ringlike pattern
• Parenchyma cells inside the bundles are
known as pith and those outside form
the cortex of the stem
• Patterns become more complex as the
plant grows larger and stem increases
in diameter.
Primary Growth of Stems
• Primary growth is the growth that
occurs at the tips of roots and
shoots in the apical meristems
• Produces growth in length
• All seed plants have this type of
growth
Secondary Growth of Stems