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Introductory Plant Biology
Ninth Edition
Kingsley Stern
Chapter 5
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Chapter 5
Roots
Root function
• Anchor plant to soil
• absorb water and minerals in
solution
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How roots develop
• Embryo- an immature plant
• Radicle- first root that grows out of
an embryo
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2 types of mature roots
1. Taproot- large major root
- pine trees, carrots
2. Fibrous roots- many branched
roots;
Adventitious roots- roots that
develop from stem, leaf, etc.
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Fig. 5.1
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Root development in flowering
plants
1. Dicotyledon- have 2 “seed leaves”
- have taproots from which
secondary roots develop
- ex. Peas and carrots
2.Monocotyledon-have 1 “seed leaf”
- have fibrous roots
- ex. Corn and rice
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Root structure; 4 regions
1. Root cap- thimble-shaped mass of
parenchyma cells
- Protects the root from damage as
it pushes through the dirt
- Dictysomes produce slimy
substance to help root move
- Functions in the perception of
gravity
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4 root regions cont.
2. Region of cell division- composed of apical meristem in
the center of the root tip
- produces surrounding root cap
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3 types of meristem from apical
meristem
1. Protoderm- forms epidermis
2. Ground meristem- forms cortex
3. Procambium- forms primary
xylem and phloem
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4 root regions cont.
3. Regions of elongation- 1cm from root tip
- cells grow longer and wider
- no further increase in cell size
takes place above this region
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4 root regions cont.
4. Region of
maturation/differentiation
or “root hair zone”
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Fig. 5.2
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Cells and Tissues of Region of
Maturation
• Root hairs - absorb water &
minerals
- hold tightly to soil
- increase surface area
- not separate cells; extensions of
specialized epidermal cells
- up to 250,000/in2
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Fig. 5.4b
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Cells/Tissues cont.
• Cortex- parenchyma cells that lie
between epidermis and inner
tissues; stores food
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Cells/Tissues cont.
• Endodermis- separates cortex from
vascular tissue
- Consists of a single-layer
cylinder of cells
-Casparian strips- “mortar” of
suberin around endodermis cells
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Cells/Tissues cont.
• Endodermis cont.
• Casparian strips- force all water
and dissolved substances to pass
through the plasma membrane or
plasmodesmata of the endodermis
- this regulates types of minerals
absorbed
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Fig. 5.5
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Cells/Tissues cont.
• Vascular cylinder- inside area of
endodermis
- pericycle- parenchyma cells that
give rise to lateral vascular
cambium and lateral roots
- primary xylem- central core;
usually 4 arms
- branch roots grow opposite of
arms
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Cells/Tissues cont.
• Phloem- forms between xylem
arms
• Vascular cambium- develops
between xylem and phloem
• Cork cambium- woody plants only
- arises outside vascular cambium
- generally not found in monocots
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Fig. 5.6
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Tissue Development
Apical Meristem
Apical Meristem
Protoderm
Ground Meristem
Epidermis
Cortex
Pith
Procambium
Primary Primary Primary
Xylem Phloem Vascular
Cambium
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Specialized Roots
• Food storage roots- sweet potatoes,
carrots
• Large number of parenchyma cells
form in branch roots
• Store carbohydrates
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Specialized Roots
• Water-storage roots- pumpkin family roots
- plants store water in roots when
water supply in soil is low
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Specialized Roots
• Propagative roots- adventitious buds- buds
appearing in places other than the
stem
- buds develop into aerial stems
called suckers, with rootlets at
base
- found in cherry, apple, pear trees
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Specialized Roots
• Aerial roots- may support plant in
high wind; aid in climbing
• Contractile roots- pull plant deeper
into the soil
• Buttress roots- add stability
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Fig. 5.12
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Fig. 5.13
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Fig. 5.14
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Specialized Roots
• Parasitic roots- mistletoe
- haustoria-(sing. haustorium)
projections that develop along
stem in contact with host
- penetrate outer tissue and
connect with host’s vascular
tissue
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Mycorrhizae
• mycorrhizae- fungus roots
- fungus grows on roots; can’t
grow beyond Casparian strip
- absorb phosphorus, water and
minerals for the plant
- fungus lives on plant’s sugar
- ex. of mutualism
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Fig. 5.16ab
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Root Nodules
• root nodules- small swelling on
roots filled with N2 -fixing
bacteria
- found in Legumes
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Fig. 5.17
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Human Relevance of Roots
• food; carrots, sugar beets, turnips
rutabagas, parsnips,
horseradishes, sweet potatoes
• spices- sassafras, sarsaparilla,
licorice
• dyes
• drugs
• insecticides
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Soil
• several types of sand, rocks,
pebbles, clay, silt, small animals,
bacteria, fungi
• Humus- decomposed organic
matter
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Soil texture and mineral
composition
• sand and silt- small particles
• clay- colloid-suspension of
particles that are larger than
molecules, but do not settle out of
fluid
• loam-mixture of sand, clay organic
matter
- agricultural soil- 40% silt, 40%
sand, 20% clay
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Minerals in soil
• include- O2, H2, Si, Al, Fe, K, Ca,
Mg, Na
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Soil pH
• Some plants thrive in acidic
conditions, others basic
• liming- adding Ca or Mg to make
soil basic
• basic soil can be made more acidic
by adding S or N2
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Fig. 5.4a
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