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Roots (Other Types of Roots and Root Modifications (roots of strangler…
Roots
Other Types of Roots and Root Modifications
storage roots
storage of accumulated carbs from the summer
roots have more of a stable environment
prop roots
can produce adventitious roots for more water absorption
can grow through the air, not touching the ground for months
once touching the soil they transport nutrients and water to the stem
adventitious roots also stabilize the stem
mangrove roots provide stability against the strong current
aerial roots of orchids
epiphytic=living attached to the branches of trees
roots dangle in the air and spread along the bark
velamen on the roots helps retain water loss
contractile roots
many plants that have bulbs are contractile
after going through soil and anchoring, the uppermost portions contract
the stem gets pulled downward so the base of the shoot is kept at soil level or buried deeper
cortex cells simultaneously shorten and expand radially=losing half or 2/3 their height
mycorrhizae :silhouettes:
mycorrhizae=roots of plants having a symbiotic relationship with soil fungi where both benefit
ectomycorrhizal relationship
=woody forest plants=fungal hyphae pemetrate cortex cells but do not invade
endomycorrhizal association
=herbaceous plants=hyphae penetrate root cortex as far as endodermis= pass through walls of cortex= can not pass Casparian strip
hyphae invades cell but do not break plasma membrane
hyphae branch repeatedly, forming an arbuscule
root nodules and nitrogen fixation
scarcity of nitrogen in soil is main growth limiting factor
nitrogen fixation=chemical conversion of atmospheric nitrogen into usable compounds
legumes have symbiotic relation with
rhizobium
bactieria*
root nodule=cortical cells undergoing mitosis form it
plant produces
leghemoglobin
that binds to oxygen to protect bacteria
haustorial roots of parasitic flowering plants :black_flag:
haustoria
=roots of parasitic plants
secrete an adhesive or grow around a small branch or root of host plant
crack the hosts' epidermis or go through the hosts' dermal system
cells of the parasite make contact with the host xylem
parasite plant does not draw on sugars from the host
roots of strangler figs
grow as epiphytes perched on a branch of a host tree
birds eat fruits of strangler figs and deposit seeds on the branches of other trees
when the seeds germinate they cling to the bark of the host tree branch and grow rapidly downward
have no roots touching soil for a long time, absorbing nutrients from rainwater
host tree becomes entangled in strangler figs' root system
host tree dies from a lack of photosynthesis due to fig tree's leaves shading it
External Structure of Roots
taproot=plant's main site of carb storage during the winter
numerous small lateral roots/branch roots; get removed before shipped out
radicle=embryonic root=what taproot develops from=usually the largest root
fibrous root system=arises if radicle dies during or after germination
adventitious roots= increase absorptive and transport capacities of root system
Structure of Individual Roots
no leaves
no leaf scars
no leaf axils or axillary buds
growth occurs at root tip :recycle:
localized growth at meristematic regions
shoot apical meristem is protected by bud scales or leaves
the root apical meristem is protected by the root cap=thick layer of cells
root cap must be renewed by cell multiplication
dictyosomes of root cap cells secrete mucigel=lubricates passage of root through soil
mucigel from dictyosomes causes soil to release its nutrient ions and permits a rapid diffusion of those ions
zone of elongation=just behind root cap & root apical meristem (only a few mm long)=cells undergo division & expansion
behind zone of elongation is the root hair zone=region where many epidermal cells extend out as trichomes
root hairs=form only in a part of root that IS NOT elongations; they would be shorn off otherwise
root hairs increase surface area
root hairs are smaller than roots so they reach hard to get water & minerals
behind root hair zone=where new lateral roots emerge
Internal Structure of Roots
root cap
cells are small & meristematic
as they develop they become dense starch grains and endoplasmic reticulum displaces to the front of the cell
cells detect gravity because their dense starch grains settle to the lower side
the cell's dictyosomes secrete lots of mucigel by exocytosis
the middle lamina breaks down and releases cells
cells live for about four to five days
root apical meristem
more orderly than the shoot
central cells do not synthesize DNA=region is called quiescent center=more resistant cells=reserve of healthy cells
if damaged, the quiescent center becomes active to form a new apical meristem
after a new apical meristem is established, a new quiescent center is formed
zone of elongation
just behind root apical meristem
area of cell expansion=cells enlarging
none of the cells are mature
outermost cells=protoderm and differentiate into epidermis
center=provascular tissue=cells that develop into primary xylem & primary phloem
protoxylem & protophloem= form earliest=closest to meristem
farther from root=older larger cells=develop into metaxylem & metaphloem
between provascular tissue and protoderm=ground tissue=uniform parenchyma that differentiate into root cortex
tissues are quite permeable
zone of maturation/root hair zone
root hairs grow outward
zone of elongation gradually merges with zone of maturation
activity=transfer of minerals from epidermis to vascular tissue
transfer of minerals by way of diffusion(apoplastic transport)
transport of minerals by way of absorption into cytoplasm of cortical cell and then cell to cell(symplastic transport)
minerals don't have free access to vascular tissues due to the endodermis
endodermis cells=tangential walls (closest to vascular tissue or cortex)
endodermis's radial walls (the top,bottom, & side walls) are encrusted with
liging & suberin
which allow waterproofing
casparian strips=bands of altered walls involved in controlling the types of minerals that enter xylem water stream
cortex cells cannot exert any control over movement of minerals within intercellular spaces
without an endodermis, minerals of any type could move from soil to the spaces then into xylem and then to all parts of the plant
casparian strips are impermeable
minerals can cross the endodermis if endodermal protoplasts absorb them from intercellular spaces or from cortical cells
endodermis protects against harmful minerals
minerals have free access to protoxylem, but is a small amount
pericycle=between vascular tissue and endodermis=parenchyma cells in an irregular region=where lateral roots initiate
mature portions of the root
root hairs function a few days only
passage cells=in fairly mature parts of root where some cells have only casparian strips
passage cells are slow to develop
root pressure=watertight sheath around vascular tissues to keep water in and absorption in root hair zone causes powerful absorption of water and water pressure
the mature endodermis allows water to not be leaked out and instead moves up to the shoot
Origin and Development of Lateral Roots
lateral roots initiated by cell divisions in the pericycle
as the primordium of the root pushes outward, the new lateral root destroys cells of the cortex & epidermis, breaking the endodermis
when lateral root emerges, it's formed a root cap, its first protoxylem & protophloem begin to differentiate=establishes a connection to vascular tissues of parent root
lateral roots are initiated deep within the root (endogenous origin)
superficial origin=initiation at the surface (axillary buds)
Root Functions
anchoring the plant to a substrate
absorbing water & minerals
producing hormones
Side notes
several types of roots can occur in one plant
