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Roots (Other types of roots and root modifications (Storage roots (In…
Roots
Other types of roots and root modifications
Storage roots
Long-term storage for carbohydrates
In biennial, roots=only permanent organs
Example: beets, carrots, and celery
Uses storage to produce new shoot in spring
Perennials store nutrients for winter
Prop roots
Some monocot roots grow in air
Example
Pandanus
3-4 meters long
After making contact with soil
Transport nutrients
Act as stabilizers
In eudicots genus
Ficus
Aerial roots of orchids
Grow along surface of tree bark
Contractile roots
In
Oxalis, Gladiolus, and Crinum
Roots undergo contraction
How the shoot is anchored in the soil
Mycorrhizae
Plants relationship with fungi
Two main types of relationships
Ectomycorrhizal relationship
Fungi penetrate cortex only
Endomycorrhizal association
Fungi penetrate to endodermis
Root nodules and nitrogen fixation
Chemical conversion of N2 is nitrogen fixation
Plants with symbiotic relationship
With nitrogen-fixing bacteria (
Rhizobium
)
Form tubelike invagination
In the plant cell wall
Tube=infection thread
Penetrates cells until cortex
Form root nodule
Where bacteria is released into cytoplasm
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Haustorial roots of parasitic flowering plants
Roots of parasitic plants=haustoria
Tristerix
is a diffuse root system
Roots of strangler figs
Kills host by "strangling" it
True in some, but mostly from lack of light
Several species in genus
Ficus
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Internal Structure of roots
Root cap
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Specific structure & growth pattern
Constantly regenerating
Cells grow through the edges
Starch is digested
ER is less conspicuous
Mucigel is secreted
Cells are small & meristematic
Develop dense starch grains
ER becomes displaced
Root apical meristem
Originate in the meristem
Extend into regions of mature root tissues
Quiescent center
Mitotically inactive region
Act as reserve of healthy cells
If root cap or apical meristem is damaged
It becomes active to form new meristem
Then a new quiescent center is formed
Zone of elongation
Just behind the root apical mertistem
Cells expand greatly
Begin differentiate into a pattern
None of the cells are mature
Outermost cells are protoderm
Differentiate into epidermis
Center cells are provascular
Develop into primary xylem & phloem
Ground tissue is between protoderm & provascular
Zone of maturation/root hair zone
Root hairs grow outward
Merges gradually from elongation
No distinct boundary exist
Transfer minerals
From epidermis to vascular tissue
Does not have free access
Due to endodermis
Lignin and suberin=waterproof wall
Bands of altered walls=Casparian strips
First layer of cells inside=pericycle
Where lateral roots are initiated
Done by apoplastic transport
Diffusion through walls & intercellular spaces
Symplastic transport
Absorption into cytoplasm
Then transfer cell to cell
Mature portions of the root
Absorption of water & minerals is reduced
Cells that have only Casparian strips
Passage cells
Once thought of as a passageway
Now known to just slowly develop
Zone of maturation causes root pressure
Powerful water pressure
Mature endodermis keeps water from leaking
Also becomes the root surface
Then bark is formed
Mostly in perennial roots
External structure of roots
Organization of root systems
Branch roots
Small lateral roots
May produce more lateral roots
Taproot
Develops from embryonic root
Called radicle
Dies during germination
Forming fibrous root system
Then more root primordia are initiated
Not radicles
Known as adventitious roots
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Don't arise on preexisting roots
In many monocots
Increase in size
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Some eudicots
Secondary growth=absorptive roots
Stoloniferous eudicots
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Main site of carbohydrate storage
Structure of individual roots
Fairly simple
No leaves or leaf scars
Root tip=where length occurs
Localized growth
Thick layer of cells
Root cap
Protects apical meristem
Contain dictyosomes
Secrete complex polysaccharide
Known as mucigel
Rich in carbohydrates and amino acids
Foster growth of soil bacteria
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Just beyond this
Zone of elongation
Cells undergo division & expansion
Beyond this
Root hair zone
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Origin and development of lateral roots
Lateral roots are initiated in pericycle
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Creates small root primordium
Becomes root apical meristem
Pushes itself outward
Destroys cortex and epidermis
Swells into cortex
May tear endodermis
Then forms covering over primordium
Called endogenous origin
Occurs on individual roots
AKA Zone of Maturation
Pericycle location in root
Same genus
