Slide 1

Report
Plant Organs: Roots
Chapter 6
KEY TERMS
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TAPROOT SYSTEM
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A root system consisting of one prominent
main root with smaller lateral roots branching
from it
FIBROUS ROOT SYSTEM
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A root system consisting of several
adventitious roots of approximately equal size
that arise from the base of the stem
Root Systems
LEARNING OBJECTIVE 1
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Describe the functions of roots
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Describe two features of roots that
shoots lack
Functions of Roots
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Anchorage
Absorption
Conduction
Storage
Storage Roots
KEY TERMS
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ROOT CAP
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A covering of cells over the root tip that
protects delicate meristematic tissue directly
behind it
ROOT HAIR
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An extension of an epidermal cell of a root
that increases absorptive capacity of the root
Root Cap
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Each root tip has a root cap
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A protective thimblelike layer
Many cells thick
Covers delicate root apical meristem
May orient root so it grows downward
Root Hairs
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Short-lived, unicellular extensions of
epidermal cells near the growing root tip
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Increase surface area of root in contact with
moist soil, increasing root’s absorptive
capacity
Structures Unique to Roots
Root apical
meristem
(area of
cell division)
Root cap
(a) The root cap of an onion (Allium cepa)
root. The root cap protects the root’s apical
meristem.
Fig. 6-3a, p. 114
Root hairs
Soil air
Soil water
Soil particles
Epidermis
(b) Root hairs on a radish (Raphanus sativus) seedling. Each delicate hair is a
unicellular extension of the root epidermis. Root hairs increase the surface area
in contact with the soil.
Fig. 6-3b, p. 114
LEARNING OBJECTIVE 2
•
Contrast the structure of a primary
eudicot root and a monocot root
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Describe the functions of each tissue
Primary Eudicot Roots 1
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Outer protective covering
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Ground tissues
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Epidermis
Cortex
Pith (in certain roots)
Vascular tissues
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Xylem
Phloem
Primary Eudicot Roots 2
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Epidermis
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Protects the root
Root hairs help absorb water and dissolved
minerals
Cortex
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Consists of parenchyma cells
Usually stores starch
KEY TERMS
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ENDODERMIS
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Innermost layer of the cortex of the root that
prevents water and dissolved materials from
entering the xylem by passing between cells
KEY TERMS
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CASPARIAN STRIP
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A band of waterproof material around the
radial and transverse cells of the endodermis
Ensures that water and minerals enter the
xylem only by passing through the
endodermal cells
Endodermis and Mineral Uptake
Endodermis
Cortex
Epidermis
Casparian
strip
Endodermis
Root cross section
Casparian strip
Movement of
water through
the endodermis
to the center
of the root
Fig. 6-5, p. 118
KEY TERMS
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PERICYCLE
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A layer of cells just inside the endodermis of
the root
Gives rise to lateral roots
Lateral Root
Fig. 6-7, p. 120
Ruptured epidermis
Lateral root
Cortex
Pericycle
Willow
Fig. 6-7, p. 120
Primary Eudicot Roots 3
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Xylem
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conducts water and dissolved minerals
Phloem
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conducts dissolved sugar
Herbaceous Eudicot Root
Buttercup
Epidermis
Cortex
Stele
(a) Cross section of a buttercup (Ranunculus) root.
Note that the bulk of the root is the cortex.
Fig. 6-4a, p. 116
Cortex cells
filled with
amyloplasts
Endodermis
cell
Pericycle cell
Phloem cell
Xylem vessel
elements
Intercellular
space
(b) A close-up of the stele of the buttercup root.
Note the solid core of vascular tissues.
Fig. 6-4b, p. 116
Comparing Monocot and Eudicot
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Monocot roots often have a pith in the
center of the root
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In herbaceous eudicot roots, xylem and
phloem form a solid mass in center of root
Monocot roots lack a vascular cambium
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Do not have secondary growth
Monocot
Root
Monocot Root
Greenbrier
Epidermis
Cortex
Stele
Fig. 6-8a, p. 121
Endodermis
Pericycle
Phloem
Vascular
tissues
Xylem
Pith
Fig. 6-8b, p. 121
LEARNING OBJECTIVE 3
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Trace the pathway of water from the
soil through the various root tissues
Water Movement 1
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In a primary eudicot root, water moves
from soil into center of root:
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Root hair → epidermis → cortex (symplast or
apoplast pathway) → endodermis → pericycle
→ xylem of root
Water is transported upward through root
xylem into stem xylem and rest of plant
KEY TERMS
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SYMPLAST
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A continuum consisting of the cytoplasm of
many plant cells, connected from one cell to
the next by plasmodesmata
APOPLAST
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A continuum consisting of the interconnected,
porous plant cell walls, along which water
moves freely
Symplast and Apoplast
Movement upward
Endodermis
Xylem
vessels
Casparian
strip
Cortex
Epidermis
Symplast:
interconnected
cytoplasm of
living cells
Phloem Pericycle
cells
Plasma membrane
Plasmodesma
Cell wall
Apoplast:
interconnected
cell-wall spaces
Water and
dissolved
nutrient
minerals
Root hair
Fig. 6-6, p. 119
LEARNING OBJECTIVE 4
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Describe several roots that are
modified to perform unusual functions
KEY TERMS
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PROP ROOT
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An adventitious root that arises from the stem
and provides additional support for the plant
CONTRACTILE ROOT
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A specialized root, often found on bulbs or
corms, that contracts and pulls the plant to a
desirable depth in the soil
Prop Roots
Contractile Roots
Corm
Contractile
roots
(a) Plants that produce corms or bulbs often have
contractile roots that lose much of their length as
root cells shorten and broaden.
Fig. 6-13a, p. 124
Contractile
roots
Fig. 6-13b, p. 124
KEY TERMS
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PNEUMATOPHORE
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A specialized aerial root produced by certain
trees living in swampy habitats
May facilitate gas exchange between the
atmosphere and submerged roots
Pneumatophores
Other Modified Roots 1
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Buttress roots
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Swollen bases or braces that hold trees upright
Aid in extensive distribution of shallow roots
Found in some tropical rainforest trees
Buttress
Roots
Other Modified Roots 2
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Suckers
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Aboveground stems that develop from
adventitious buds on the roots
Asexual reproduction method of some roots
Certain epiphytes have roots that are
modified to photosynthesize
Parasitic
Epiphytes
Juniper
bark
Mistletoe
root
Juniper
wood
(b) Micrograph of a parasitized juniper (Juniperus)
branch, showing a mistletoe root penetrating the
wood (secondary xylem) of the juniper.
Fig. 6-12b, p. 123
LEARNING OBJECTIVE 5
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Discuss the significance of roots to
humans
Important Foods
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Roots which store the products of
photosynthesis are important sources of
food for human consumption
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Some roots are used as flavorings
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Example: root beer flavoring (dried greenbrier
roots)
Root Crops
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Predominantly taproots
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carrots, beets, sugar beets, parsnips, turnips,
rutabagas, radishes
Some fibrous roots
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sweet potatoes, cassava
KEY TERMS
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MYCORRHIZA
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A mutually beneficial association between a
fungus and a root that helps the plant absorb
essential minerals from the soil
NODULE
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A small swelling on the root of a leguminous
plant in which beneficial nitrogen-fixing
bacteria (Rhizobium) live
Mycorrhizae
Sheath of
fungal hyphae
encircles root
Fungal hypha
between plant
cells
(a) Cross section of root showing ectomycorrhizae,
fungal associations that form a sheath around the
root. The fungal hyphae penetrate the root between
cortical cells but do not enter the cells.
Fig. 6-14a, p. 124
Fungal hyphae
within plant
cortical cells
(b) Cells of a root cortex showing endomycorrhizae,
fungal associations in which the fungal hyphae penetrate
root cells of the cortex to aid in delivering and receiving
nutrients. Endomycorrhizae colonize roots of most vascular
plant species.
Fig. 6-14b, p. 124
Animation: Root Systems
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Animation: Water Absorption
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Animation: Root Functioning
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Animation: Root Crosssection
Root
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