23–2 Roots
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23–2 Roots
Types of Roots
Types of Roots
• Taproots: found mainly in dicots
• Fibrous roots: found mainly in monocots
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23–2 Roots
Types of Roots
In some plants, the primary root grows long and thick.
This primary root is called a taproot.
Ex: oak and hickory trees, carrots, dandelions, beets,
and radishes.
Fibrous Roots
Taproot
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23–2 Roots
Types of Roots
Fibrous roots
branch to such an
extent that no
single root grows
larger than the
rest.
Fibrous roots are
found in grasses.
Fibrous Roots
Helps prevent top
soil from being
washed away.
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23–2 Roots
Root Structure and Growth
Root Structure and Growth
Roots contain cells from dermal, vascular, and
ground tissue.
A mature root has an outside layer, the epidermis,
and a central cylinder of vascular tissue.
Between these two tissues lies a large area of
ground tissue.
The root system plays a key role in water and
mineral transport.
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23–2 Roots
Root Structure and Growth
The epidermal subsystem
provides protection and
absorption.
The root’s surface is
covered with cellular
projections called root
hairs.
Root
hairs
Root hairs provide a large
surface area through which
water can enter the plant.
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23–2 Roots
Inside the epidermis
is a layer of ground
tissue called the
cortex.
Root Structure and Growth
Ground
tissue
(cortex)
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23–2 Roots
Root Structure and Growth
The cortex extends to
another layer of cells, the
endodermis.
Endodermis
The endodermis
completely encloses the
vascular cylinder.
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23–2 Roots
Root Structure and Growth
The vascular cylinder is the
central region of a root that
includes the xylem and phloem.
Vascular cylinder
Phloem
Xylem
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23–2 Roots
Root Structure and Growth
Roots grow in length as their apical
meristem produces new cells near
the root tip.
These new cells are covered by the
root cap that protects the root as it
forces its way through the soil.
It secretes a slippery substance that
helps to progress the root through
Apical meristem
the soil.
Root cap
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23–2 Roots
Root Functions
Root Functions
Roots anchor a plant in the ground and absorb water
and dissolved nutrients from the soil.
It takes energy on the part of the plant to absorb
water.
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23–2 Roots
Root Functions
Uptake of Plant Nutrients
To grow, flower, and produce seeds, plants need a
variety of inorganic nutrients in addition to carbon
dioxide and water.
The most important nutrients plants need include
nitrogen, phosphorus, potassium, magnesium, and
calcium.
In addition to these essential nutrients, trace
elements are required in small quantities to maintain
proper plant growth including sulfur, iron, zinc,
molybdenum, boron, copper, manganese, and
chlorine.
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23–2 Roots
Root Functions
Active Transport of Minerals
The cell membranes of root hairs and other cells in
the root epidermis contain active transport proteins.
The root does not actually pump water, but by
pumping dissolved minerals into its own cells, the
end result is almost the same: water moves from
the epidermis through the cortex into the vascular
cylinder.
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23–2 Roots
Root Functions
Transport proteins use ATP to pump mineral ions
from the soil into the plant.
Root hairs
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23–2 Roots
Root Functions
The high concentration of mineral ions in the plant
cells causes water molecules to move into the plant
by osmosis.
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23–2 Roots
Root Functions
Movement Into the Vascular Cylinder
Osmosis and active transport move water and
minerals from the root epidermis into the cortex.
The water and dissolved minerals pass the inner
boundary of the cortex and enter the endodermis.
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23–2 Roots
Root Functions
The endodermis stretches up and down the entire
length of root, like a cylinder, and is composed of
many individual cells.
Endodermis
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23–2 Roots
Root Functions
Each cell is surrounded on four sides by a waterproof
strip called a Casparian strip.
Casparian strip
Casparian
strip
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23–2 Roots
Root Functions
The Casparian strip prevents the backflow of water
out of the vascular cylinder into the root cortex.
Water moves into the vascular cylinder by osmosis.
Because water and minerals cannot pass through the
Casparian strip, once they pass through the
endodermis, they are trapped in the vascular cylinder.
As a result, there is a one-way passage of materials
into the vascular cylinder in plant roots.
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23–2 Roots
Root Functions
Root Pressure
As minerals are pumped into the vascular cylinder,
more and more water follows by osmosis, producing a
strong pressure.
This root pressure forces water through the vascular
cylinder and into the xylem.
As more water moves from the cortex into the
vascular cylinder, more water in the xylem is forced
upward through the root into the stem.
Root pressure is the starting point for movement of
water through the vascular system of the entire plant.
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