Roots
Chapter 5
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Outline
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Root Function
Root Development
Root Structure
Specialized Roots
Mycorrhizae
Root Nodules
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Function
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There are several functions for a root
 Anchorage
 Water and mineral absorption/conduction
 Storage
- Starch
- Water
Extensive underground organ
 A single grass plant
6
3
- 15 x 10 roots = 400 miles of root / 2 ft
soil
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How Roots Develop
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When a seed
germinates, the
embryo’s radicle grows
out and develops into
the first root.
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Root development
Radicle
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Development: Tap root -- Fibrous root
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May develop into thick
taproot with branch roots.
 Dicotyledonous Plants
May develop a fibrous root
system.
 Monocotyledonous
Plants
Adventitious roots are
roots that develop from
non-root plant
partsBoth monocots
and dicots
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Adventitious Roots
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What are the advantages and disadvantages of
a fibrous and a taproot root system?
Advantages
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Fibrous
 Large surface area, increased access
to water and minerals
Taproot
 Good for a storage of nutrients, can
access water and nutrients at great
depths, good during drought periods
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Disadvantages
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Fibrous
 Does not reach water and nutrients deep in
soil profile - not good during drought
 Not good for storage of water or
carbohydrates
Taproot
 Not as efficient as fibrous at getting water &
minerals in upper soil profile.
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ROOTS
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Root Function
Root Development
Root Structure
Specialized Roots
Mycorrhizae
Root Nodules
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Root Structure
1.
2.
3.
4.
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Root Cap
Region of Cell Division
Region of Cell Elongation
Region of Cell Maturation
Not all regions well-defined at their
boundaries.
Development in roots unlike other organs
follows a linear vector
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Root Structure and Root Tissue Comparison
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Root Structure
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Root Cap - Thimbleshaped mass of
parenchyma cells covering
each root tip.
 Protects tissue from
damage.
 Function in gravity
perception.
 Cells secrete a slimy
substance and slough
off forming a slimy
lubricant that helps roots
move through soil
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Region of Cell Division
• Root apical meristem
• 12 to 36 h cell cycle
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Region of Elongation
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About 1 cm from root
tip
Cells become several
times their original
length.
 Vacuoles merge and
form 90% of the
mature cell
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Region of Maturation
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Sometimes called
“region of
differentiation” or “roothair zone”
Most cells differentiate
into various distinctive
cell types.
Vascularization and
development of root
hairs
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Region of
maturation
Root hairs
Tuberous
extensions of
specialized
epidermal cells
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Absorb water and minerals and adhere tightly to soil
particles.
Grass plant 2ft3 soil, 15 x 106 roots has 14 x 109 root hairs
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Root surface area ~ volleyball court
Root hair surface area ~ football field
Root hairs function for 2 days to 3 weeks
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Region of MaturationCortex cells
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Cortex cells mostly store food
Between epidermis and inner
tissues.
 Has endodermis as inner
boundary (single layer cells)
- Cell walls impregnated with
suberin bands; Casparian
Strips.
 Forces all water and
dissolved substances
entering and leaving the
central core to pass
through plasma
membranes of the
endodermal cells.
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Region of MaturationCortex cells
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Region of Maturation
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Vascular Cylinder lies at the inside of the
endodermis. (xylem/phloem)
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Development of Secondary Roots
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Pericycle lies directly against the inner boundary of
the endodermis.
 Lateral (branch) Roots develop
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What structure within the root is responsible
for forming secondary or lateral roots?
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A. Casparian strip
B. Vascular cambium
C. Cork cambium
D. Pericycle
E. Endodermis
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Specialized Roots
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Food Storage Roots
 Sweet Potatoes
 Yams
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Store starch,
carbohydrates
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Specialized Roots
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Carrots, beets, turnips, radishes are foodstorage tissues that are a combination of
root and stem
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Specialized Roots
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Manroot (Marah)
Water Storage Roots
 Pumpkin Family
Propagative Roots
 Adventitious Buds
develop into
suckers.
- Fruit Trees
Sisal
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Specialized Roots
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Pneumatophores
 Spongy roots that
extend above the
water’s surface and
enhance gas
exchange between
the atmosphere and
subsurface roots.
Mangroves
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Specialized Roots
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Aerial Roots-can
absorb water from the
air; has a thick
epidermis to reduce
water loss
 Orchids
 Banyan trees
www.hcs.ohio-state.edu/ hcs300/anat1.htm
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Specialized Roots
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Contractile Roots
 Pull plant deeper into the soil.
- Lilly Bulbs.
Buttress Roots
 Stability - Tropical Trees.
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Mycorrhizaefungi
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Mycorrhizae form a mutualistic association with
plant roots. (found in3/4 of all seed plants)
 Fungus is able to absorb and concentrate
phosphorus much better than it can be absorbed
by the root hairs.
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Mycorrhizaefungi
Two Hibiscus plants. Left
plant without Mycorrhizae,
right with Mycorrhizae.
Mucorrhizal
arbuscule inside a
plant cell
www.botany.hawaii.edu/faculty/ wong/BOT135/Lect26.htm
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Root Nodules
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Few species of bacteria (Rhizobium) produce
enzymes that can convert nitrogen into
nitrates and other nitrogenous substances
readily absorbed by roots.
 Legume Family (Fabaceae)
- Root nodules contain large numbers of
nitrogen-fixing bacteria.
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Root Nodules
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Review
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Root Development
Root Structure
Specialized Roots
Mycorrhizae
Root Nodules
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies