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Chapter 7: Roots (Other Types of Roots and Root Modifications (contractile…
Chapter 7: Roots
Origin and Development of Lateral Roots
lateral roots are initiated by cell division in pericycle
activity is localized to a few cells
creates a small root primordium
organizes root apical meristem
pushes outward
root primordium
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swells into cortex
endodermis may be crushed or undergo cell division
External Structure of Roots
organization
lateral/branch roots
present in most seed roots
develops from radicle
examples
with dozens of lateral roots
beets
turnips
carrots
can produce more lateral roots
fibrous root systems
present in most monocots
present in some eudicots
radicle dies during or after germination
form adventitious roots
structure of individual roots
fairly simple
no leaf axils or axillary buds
no leaves or leaf scars
only elongate at small meristematic regions
root cap
thick layer of cells
protects root apical meristem
constantly being worn away and renewed by cell multiplication
dictysomes of some of these cells secrete mucigel
zone of elongation
behind root cap and root apical meristem
only a few millimeters long
cells undergo division here
root hair zone
behind zone of elongation
epidermal cells extend as trichomes
hairs increase surface area
Internal Structure of Roots
zone of elongation
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behind root apical meristem
cells expand greatly
similar to shoot's subapical meristem region
cells differentiate into a visible pattern
none of the cells mature here
tissues are permeable here
zone of maturation/root hair zone
root hairs grow outwardly
merges gradually with zone of elongation
cortex cells
transfer minerals from epidermis to vascular tissue
continue to enlarge
endodermis
have tangential walls
have altered walls (Casparian strips)
impermeable
control mineral transport entering xylem
pericycle
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between vascular tissue and endodermis
lateral roots are initiated here
root apical meristem
more orderly than shoot
quiescent center
mitotically inactive
central region
resistant to harmful agents
mature portions of the root
passage cells
unchanged cells
continue maturation
cells with only Casparian strips
root pressue
result of continued endodermis maturation
functions to keep water in
endodermis
cortex and epidermis
may be superfluous
often die and are shed from the root
root cap
cells
small and meristematic when first formed
close to root meristem are meristematic
on the edges proliferate
Other Types of Roots and Root Modifications
contractile roots
undergo more contraction than prop roots
stem is pulled downward
caused by changes in the shape of cortex cells
many germinate at or near soil surface
mycorrhizae
symbiotic relationship with soil fungi
main types
ectomycorrhizal relationship
exists in fungal hyphae
penetrates between the outermost root cortex cells
never invade cortex cells
endomycorrhizal association
in herbaceous plants
penetrate root cortex
do not break plasma or vacuole membrane
aerial roots of orchids
many orchids are epiphytic
live in rainforests
velamen (root epidermis) acts as a waterproof barrier
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roots nodules and nitrogen fixation
few plants have a symbiotic relationship with nitrogen-fixing bacteria
infection thread
tube used by bacterium
occurs in plant cell wall
root nodule
formed by adjacent cortical cells undergoing mitosis
energy needed to form these is supplied by legume root cells
may be simple or complex
prop roots
transport nutrients and water to the stem
place tension on stem
can be found at the base of corn and many grasses
buttress roots
certain tropical trees
brace trunk against wind
haustoria roots of parasitic flowering plants
highly modified
roots of parasitic plants
little root structure
typical root organization is nonfunctional in haustoria
storage roots
long-term storage for carbohydrates
provides energy to produce new shoots
annual plants survive without much storage capacity
roots of strangler figs
grow as epiphytes perched on branch of host tree
grow rapidly downward from branch
penetrate soil rapidly and branch profusely
