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Ch. 7 Roots (Other Types of Roots & Modifications (Storage Roots (long…
Ch. 7 Roots
External Structures
Organization of Root Systems
must have enormous absorptive surface
prominent taproot
develops from embryonic root aka radicle
grows extensively & usually largest root in system
ex:) carrots, beets, & turnips
numerous small lateral or branch roots
monocots, some eudicots have a mass of many similarity sized roots
fibrous root system
arises because radicle dies during or after germination
root primordia @ base radicle grow out
first stages of fibrous root system formed
root primordia are initiated in stem tissue
adventious roots
not from existing roots
increase absorptive and transport capacities
stolons or rhizomes
horizontal shoots branch
produce adventitious roots
roots transport water directly to new portions of the shoot
unhindered by limited capacity of older portions of the shoot
Structure of Individual Roots
fairly simple
region of growth occurs in root tip
longitudinal growth
meristematic regions only
root cap
thick layer of cells
protection
constantly worn away
mucigel
lubricates passage of root through soil
causes soil to release nutrient ions
rich in cabs and amino acids
foster rapid growth of soil bacteria around root tip
zone of elongation
behind root cap & root apical meristem
few millimeters long
cells undergo division & expansion
root hair zone
epidermal cells extend out as narrow trichomes
root hairs
increases surface area
not elongating
enter any crevice
form carbonic acid
unicellular
no thick walls
extremely transitory
die/degenerate 4-5 days after forming
Internal Structures
Root Cap
provide effective protection
help remain in place
cells closest to root meristem are also meristematic
undergoing cell division w/transverse walls
forming flies of cell that are pushed forward
cells on edges grow toward side & proliferate
cells develop dense starch grains
ER becomes displaced
cells detect gravity
cell structure & metabolism chance dramatically
ER less conspicuous
starch grains digested
dictyostomes secrete copious amounts of mucigel
middle lameila breaks down & releases cells
crushed by expansion
Apical Meristem
produces root tissue
files of cells originate in meristem
cells extend into regions of mature root tissues
more orderly than shoot
no disruption
quiescent center
mitotically in active center
central cells not synthesizing DNA
more resistant to radiation & toxic chemicals
become active if damaged
form new apical meristem
reserve of healthy cells
Zone Elongation
just behind root apical meristem
region where cells expand greatly
cells mostly enlarging
cells differentiate into visible pattern
outermost cells differentiate into epidermis
cells develop into primary xylem & phloem
protoxylem & protophloem closest to meristem
form earliest
older larger cells develop into metaxylem & metaphloem
farther from root tip
older
ground tissue differentiates into root cortex
tissues quite permeable
zone short
little absorption occurs
Zone Maturation
root hairs grow outward
increase in absorption of water & minerals
merges gradually w/zone elongation
no distinct boundaries
cortex cells continue enlarging
transfer of minerals from epidermis to vascular tissue
diffusion through walls or intracellular spaces
apoplastic transport
arenchyma
absorption into cytoplasm of cortical cell
transfer cell to cell
endodermis
tangential walls
closest to vascular tissue or cortex
casparian strips
encrusted w/ ligin & suberin
waterproof
impermeable
controls minerals entering xylem
metaxylem & metaphloem fully differentiated & functional
xylem solid mass in center
surrounded by phloem strands
no pith present
pericycle
parenchyma cells
between vascular tissue and endodermis
lateral roots produced/initiated here
Mature Portions
continued endodermis maturation
layer suberin applies over all radial surfces
inner tangential face
sometimes even the outer tangential face
layer of ligin & more suberin
irregular process
some cells complete early
passage cells
represent passageways for absorption of minerals
formation of watertight sheath
around vascular tissue
root pressure
powerful absorption of water & water pressure
keeps from filling
prevent oxygen diffusion
endodermis becomes root surface till bark forms
Origin & Development of Lateral Roots
initiated by cell divisions in pericycle
some cells become more differentiated
smaller vacuoles
resume mitotic activity
localized to a few cells
small root primordium
organizes itself into root apical meristem
pushes outward
primordium swells into the cortex
endodermis torn/crushed
cell division
form thin covering
new lateral root destroys cells of cortex & epidermis
new root cap formed
protoxylem & protphloem begin differentiating
establish connection to vascular tissue of parent root
differs from bud formation
lateral roots initiated deep w/in root
endogenous origin
formed in mature regions of root
never develop flowers
Other Types of Roots & Modifications
Storage Roots
long-term storage for carbs
perennials
biennials
used to produce a new shoot in spring
roots only permanent organs
annual plants can survive w/out such storage capacity
subterran
roots less available as food
highly nutritious, easily visible stems
subject to less change
Prop Roots
monocot stem becomes wider
more vascular bundles
capable of extensive growth through air
in many palms
20-50 cm long
screwpine
3-4 m long
grow through air for months
contact w/soil
transport additional nutrients & water
contract slightly & tension on stem
stabilizer
secondary growth
woody
ex:) banyan trees
ex:) mangrooves
ex:) buttress tree
upper side grows more rapidly
brace trunk
plate-like
Aerial Roots of Orchids
epiphytic
living attached to branches of trees
roots spread along surface of bark
dangle freely in air
adapted to drought conditions
root epidermis
velamen
composed several layers of large dead cells
white in appearance
waterproof barrier
Contractile Roots
even more contraction than prop roots
uppermost portions slowly contract
root firmly fixed to soil
stem pulled downward
base kept @ soil level
contraction changes cortex cell shape
shorten
expand
lose 2/3 height
vascular tissues buckle
ex:) Oxalis, Crinum, Gladiolus
Mycorrhizae
symbiotic relationship w/ soil fungi
two main types
ectomycorhizal
all woody plants
fungal hyphae penetrate between root cortex
cells never invade themselves
endomycorizal
herbaceous plants
hyphae penetrate root cortex
as far as endodermis
pass through cortex cells
can't go through casparian strips
invade cell, do not break host membrance
form arbuscule
fills w/granules
fungus unable to live w/out sugars from plant
many cases plant severely stunted
fungus killed
Haustorial Roots of Parasitic Flowering Plants
some angiosperms parasites
substrate is the body of another plant
haustoria
highly modified roots
must adhere firmly to their host
secrete adhesive
grow around small branch or root
penetration occurs by forcing shaft cells through host dermal system
after penetration parasite cells make contact w/host xylem
continuous vessel from host to parasite
typical organization of root nonfunctional in haustoria
inadequate for growth in soil
Roots of Strangler Figs
young plants grow as epiphytes perched on braches of host
birds deposit seeds
germinate
roots cling to bark
grow rapidly downward
reach soil & penetrate profusely
strangle tree
prevent from going
lack photosynthesis
