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Roots (Other types of roots and root modifications (prop roots image…
Roots
Other types of roots and root modifications
Contractile roots
since root firmly fixed in soil, stem pulled downwards so base of shoot is kept either at soil level of buried deeper (bulbs) :pick:
contraction caused by change in shape of cortex cells
after extending through soil and becoming anchored, uppermost portions slowly contract
simultaneously shorten and expand radially, losing form a half to 2/3 height
roots undergo more contraction than prop roots do
may be more common than is generally appreciated
many seeds germinate at or near the soil surface :seedling:
may be reason that shoot becomes anchored in soil
in bulbs, corms, rhizomes, and other subterranean stems, contractile roots can be important in keeping stem at proper depth :bulb:
mycorrhizae
known as mycorrhizae
2 types of relationships known
in nearly all woody plant an ectomycorrhizal relationship exists in which fungal hyphae penetrate outermost root but never invade cells
herbaceous plants have an endomycorrhizal association which hypae penetrate root cortex to endodermis but not past casparian strip
invade but do not break host plasma membrane or vacuole membran
inside cell they branch repeatedly forming small structures called arbuscle
this fills with granules of phosphorous that disappear once absorbed by plant
other hypae fill with membranous vesicles
plant cell lacks starch grains as they are transferred to fungus
fungus unable to live without sugars form plant :skull_and_crossbones:
plant stunted by fungus death as it is important means of phosphorus absorption
roots of most species of seed plant have relationship with soil fungus which both benefit :earth_africa:
has similar structures as
Aerial Roots of Orchids
roots spread along surface of bark and dangle freely in air
though found in rain forests, are adapted for arid conditions
when rain stops, bark dries and pulls water out of orchids roots if no H2O conservation mechanism was available :potable_water:
many are epiphytic, living attached to branches
root epidermis called velamen is composed of several layers of large dead cells that are white in appearance.
velamen acts as waterproof barrier, not permitting water to leave the sides of the root :non-potable_water:
Root nodules and Nitrogen Fixation
plants have no enzyme that can use atmospheric nitrogen
only some prokaryotes can incorperate them in their bodies as amino acids and nucleotides
once dead, nitrogenous compounds released in soil for plants
this is called nitrogen fixation
small number of plants (legumes) have relationship with nitrogen fixation bacteria :world_map:
bacteria secrete substance that curls root hairs sharply
then attach to side of hair and push into cell by tubelike invagination
tube is called infection thread and bacterium sits here, penetrating cell by cell until it reaches inner cortex :face_with_head_bandage:
here is where cortical cell undergo mitosis and form ROOT NODULE
can be complex with vascular tissue, meristemic region and endodermis
nodule functions for extended lengths of time
nodule can be simple
example of a sophisticated symbiosis
not by choice but by natural selection, caused by genetic mutation to allow bacteria in
both parties benefit
bacterium released form infection thread enter plant cell cytoplasm and proliferates rapidly
fills plant with bacteria capable of converting C2 into nitrogenous compounds for cell to use
bacteria very sensitive to oxygen :skull_and_crossbones:
plant produces leghemoglobin that binds to oxygen and protect bacteria
nitrogen scarcity in soil main growth limiting factor
prop roots
can be capable of growth through the air :wind_blowing_face:
examples include palm trees and screwpines and at the base of corn :palm_tree:
stem on monocot can become wider with more vascular bundles if it can produce adventitious roots that extend to the ground
once prop root makes contact with soil, it can transport additional nutrients and water to stem
they also act as stabilizers for the stem and create tension
if root undergoes secondary growth, they become woody and extremely strong
permit further extension of branches and reduce breaking and sagging of branches :star:
in banyan trees, prop roots and branches can produce massive trees many meters in diameter :crocodile:
can create buttress root which brace the trunk agaisnt wind
mangroves have prop roots but face different challenges
are primarily found in marshes and swamps
must brace more against water currents than wind
prop root system creates stability needed rather than taproot system :check:
mangroves also contain numerous air pockets in aerial portion
also contains a cortex that is a wide aerenchyma
root grows in stagnant muck with little O2
with these adaptations, air is diffused down to roots, otherwise respiration would be very difficult :checkered_flag:
Haustorial Roots of Parasitic Flowering plants
parasitic roots have become highly modified and are known as hausroia
very little root structure remains :frowning_face:
number of angiosperms are parasites on other plants as
their substrate is the body of another plant, a normal root system would not penetrate the host or absorb materials form it effectively :<3:
must adhere firmly to host by either secreting an adhesive of growing around a small branch or root
penetration occurs through host's dermal system or by expanding the haustorium radially, cracking host epidermis
after penetration, cells of parasite make contact with host xylem
both host and parasite cells divide and proliferate into irregular mass of parenchyma
creates continuous vessel from host to parasite constructed by both cells :fist:
then column of cells differentiate into series of vessel elements
parasite usually only attacks xylem, do not take sugars but perform own photosynthesis
those that attack both xylem and phloem carry out very little or no photosynthesis
parasitism as evolved several times, structures termed haustoria are not all related and generalizations are difficult :star:
Parasitic plant names
Tristerix
spends most of life as diffuse root system called a HAUSTORIUM
grows inside of host body :alien:
body is just parenchyma cells with no stems, leaves, cortex, epidermis, or vascular tissue unless
Tristerix
produces flowers :check:
typical root organization would be nonfunctional in haustoria
root cap, root apical meristem, and cortex would prevent vascular tissues to make contact with host :red_cross:
haustoria are completely inept at ground growth no matter how well or bad soil and microhabitat is :skull:
storage roots
in biennial species such as carrots and many perennials
roots are the only permanent organs
as plant dies in autumn, stem dies but survives by its roots
carbohydrates stored in root are used to produce new shoot in the spring :recycle:
more costly to winterproof stem than to replace it with nutrients in roots :moneybag:
annual plants can survive without said storage
perennial plants whose shoots do not die also store significant amounts of nutrients within themselves during winter
root however
less available for food than the stem
have a much more stable environment
provide long term storage for carbohydrates that accumulate over the summer
Roots of Strangler figs
birds eat strangler figs and deposit seeds of other branches :bird:
seed germinates and sticks roots into bark of host tree and grow rapidly downward
young strangler figs grow as epiphytes perched on a branch of a host tree
can go months to years without soil contact, must absorb nutrients from rainwater that runs down trunk :timer_clock:
believed that strangler figs would prevent growth of tree thus killing it, but may be do to overshading and killing host tree with lack of light :skull_and_crossbones: :bulb:
once strangler fig reaches soil, it penetrates and branches rapidly and profusely
upper portions enlarge and become woody :deciduous_tree:
can grow at various angles, encircling hosts tree trunk and root fuse to each other when ever they meet
before long entire host tree encased in strangler figs :warning:
tree will finally die and rot away, leaving fig as a self supporting tree with root shoot junction high in the air :check:
Internal Structure of Roots
mature portion of roots
root hairs only function for several days
they die and then regenerate :skull_and_crossbones:
absorption of water and minerals are greatly reduced on this region
endodermis remains unchanged
however, continued maturation occurs, with layer of suberin applied to
all radial surfaces
inner tangential face
sometimes outer tangential face
can be followed by layer of lignin and then more suberin
is an irregular process and occurs at different rates for various cells
passage cells are cells that only contain Casparian strips
continued endodermis maturation produces a watertight sheath around vascular tissues :non-potable_water:
without, root pressure would flood plant and stop growth, and suffocate plant
important events in the endodermis include
can be form 1-4 weeks in some plants
endodermis becomes root surface until bark can form when cortex and epidermis die
water pressure created by hair roots is called root :explode: pressure
zone of elongation
similar to shoot's subapical meristem region
begin to differentiate into visible patterns though none are mature
cells expand rapidly and some meristematic activity continues. mostly enlarging of cells :scales:
outermost cells are protoderm, differentiate into epidermis
tissues are quite permeable here
very small, little absorption occurs here :non-potable_water:
minerals penetrate deep through apoplast by diffusing along thin, fully hydrated young walls and intercellular space :<3:
when absorption occurs, most likely used towards root growth
center is vascular tissue, becomes primary xylem and phloem
protophloem and protoxylem form earliest and closest to meristem
farther from root tip, older, larger cells develop into metaxylem and metaphloem
between provascular tissue and protderm is ground tissue. Uniform parenchyma that differentiate into root cortex
Root Apical Meristem
regular files of cells can be seen originated in meristem and extend to regions of mature root tissues
root more orderly than shoot as it has no interruptions from leaf primordia, leaf traces, or axillary buds
:alarm_clock:
examined by relationship to the root tissues it produces
contains a inactive central region known as the quiescent center
contain cells that extend to center of meristem but do not participate in cell division
cells are more resistant to harmful agents both toxic and radioactive :radioactive_sign:
act as primary reserve for healthy cells
becomes active when root apical meristem or root cap is damaged
can form new meristem, becomes inactive, and forms new quiescent center
very important as root apex is constantly damages by various agents :skull:
zone of maturation/root hair zone
minerals do not have free access to vascular tissues as innermost layer of cortical cells differentiates into cylinder called endodermis
cells of endodermis have tangential walls, those closest to vascular issue of cortex
have ordinary thin primary walls
the bands of alternating walls are called Casparian strips
involved in controlling types of minerals that enter xylem water stream :potable_water:
also appear in glands and secretory cavities and prevents seeping into surrounding tissues :warning:
radical walls (top, bottom, sides) are encrusted with lignin and suberin, causing waterproofing :no_entry:
cortex cells can not exert control over movement of minerals within intercellular space :forbidden:
without endodermis, minerals of any type could move from the soil to the spaces
then to xylem and into parts of plants
because Casparisan strips are impermeable, minerals can cross endodermis only if endodermal protoplasts absorb them
from intercellular spaces of cortex apoplast or corical cells the secrete the into vascular tissues
many harmful minerals can be excluded by endodermis
not ideal against apoplastic movements and minerals have free access to protoxylem as endodermis is not mature in ZoE :red_cross:
only makes up a small amount of movement
many important thing occur simultaneously :explode:
thin cuticle appears (could be layer of fat)
zone of elongation merges gradually with zone of maturation
no distinct border will exist as gradual cell differentiation occurs
root hairs grow outward
increases absorption of water and minerals
cortex cells continue to enlarge
mainly transport minerals from epidermis to vascular tissues
done by diffusion through walls/intercellular space
or done by absorption into cytoplasm of cortical cell the transfered cell to cell
cortical intercellular spaces improtant as an aerenchyma allow O2 to diffuse throughout root form stem or soil :unlock:
with vascular tissue, cells of the metaxylem and metaphloem differentiate and functional in zone of maturation
xylems of almost all plants (except monocots) from solid mass in center surrounded by strands of phloem. no pith :black_medium_square:
in roots of monocots strands of xylem and phloem are distributed in ground tissue : :!:
differs form that of stems
within xylem
inner wide cells are metaxylem and outer narrow ones are protoxylem
2-4+ groups of protoxylem may be present, number depends on species with larger roots :silhouettes:
number of phloem = number of protoxylem
within phloem strand
protophloem occurs on outer side, metaphloem on inside
other than arrangement, vascular tissue are similar to leaf and stem
formed first are narrowest and extensible, often being torn by continued use
those formed after adjacent cells have stopped expanding
between vascular tissue and endodermis are parenchyma cells making up region called pericycle
root cap
cells in layer closest to root meristem are also meristematic
they undergo cell division with transverse walls, forming files of cells that push forward
cells are small and meristematic whehn first formed at base of root cap
as they are pushed forward, develop dense starch grains and endoplasmic reticulum :sweet_potato:
cells can detect gravity as dense starch cells settle on lower side of cell :new_moon:
on the edge of root cap, cells grow toward the sides and proliferate
as cells are pushed towards edge of cap, structure and metabolism changes dramatically :warning:
cells dictysomes secrete copious amounts of mucigel by exocytosis
middle lamina breaks down and releases cells, and usually crushed by expansion of the root
starch grains are digested
usually takes 4-5 days :hourglass_flowing_sand:
Endoplasmic reticulum becomes less conspicuous
root cap constantly replacing itself
dynamic equilibrium must be maintained between the two processes
growth actually occurs through the edges of the root cap :flower_playing_cards:
must remain in place and provide effective protection for root apical meristem
warrants a specific structure and growth pattern
External Structure of Root
organization of root system
root must have enormous absorptive surface
roots developed highly branched systems to help increase absorption area
most plants have single branched taproot with many lateral/branch roots attached
taproot develops from embryonic root to radicle. Usually becomes largest root
carrots, beets and turnips develop from these roots :carrot:
lateral roots can also produce more lateral roots :<3:
can become swollen like taproots in sweet potatoes
if perennial and woody, roots can undergo secondary growth, producing wood and bark :evergreen_tree:
if unbranched, root would be hundreds of meters long :straight_ruler:
most monocots and some eudicots have mass of similar sized roots called fibrous root system
root primordia at base of radicle grows out and forms first stage of fibrous root system :first_place_medal:
as plant ages more root primordia are initiated in stem tissue :timer_clock:
arises because radicle dies during of immediately after germination
known as adventitious roots
increase absorptive and transport capabilities of the root system
ability not limited to monocots, rhizomatous and stoloniferous eudicots can grow this way naturally
many eudicots do not produce adventitious roots in nature
must be cut, and cutting is important process of asexual propagation :scissors:
functional significance between taproot and fibrous roots
many eudicots can perform secondary growth
results in increased quality of healthy, functional wood in trunks and roots
increase in leaves and absorptive roots.
monocots cannot undergo secondary growth, limiting their conducting capabilities
monocots form (after stem)
tracheary elements
sieve tubes
vascular bundles
extra leaves cannot be supplied properly :red_cross:
some monocots can increase size by means of stolons and rhizomes
horizontal roots branch, then form adventitious roots :star:
can transport water to new tissue of root
allows plant to branch out and become larger
only able to do so if they remain close to substrate to produce new adventitious roots :check:
structure of individual roots
root tip, where growth in length occurs in the root
longitudinal growth occurs by discrete apical meristems :silhouette:
growth only happens at small meristematic regions (localized growth). :small_blue_diamond:
root apical protected by thick layer of cells, called root cap :dark_sunglasses:
soil contains lots of sand and crushed crystals which would damage apical meristem
means thick layer of cells is constantly worn down as root grows :tired_face:
dictyosomes of root cap cells secret complec polysaccharide called mucigel
lubricated passage of root through soil
causes soil to release nutrients ions, making ions diffuse faster toward root
mucigel rich in carbohydrates and amino acids
fosters rapid growth of soil bacteria which is believed to help release soil nutrients :explode:
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behind root cap and root apical meristem is zone of elongation :straight_ruler:
cell division and expansion occurs here
behind zone elongation is the root hair zone
root hairs form in part of root that is not elongating
would be shorn off otherwise
root hairs greatly increase root surface area :check:
can almost double total surface area in certain plants :fire:
can enter any crevice and extract water/minerals from area
root hairs give of CO2 which forms carbonic acid, aiding in release of ions
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behind root hair zone is where new lateral roots emerge
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region hwere many epidermal cells extend out as narrow trichomes
only a few mm long
Root is embedded in solid matrix, prohibiting growth to extend at once
apical growth allows only extreme tip to push through soil :star:
considered fairly simple compared to other parts of the plant as it does not contain multiple, complicated parts
Origin and Development of Lateral roots
lateral roots initiated by cell divisions on the pericycle
some cells become more densely cytoplasmic with smaller vacuoles and resume mitotic :zap: activity
forms small root primodium
organizes itself into a root apical meristem
pushes outward
new lateral root destroys cells of cortex and epidermis that lie in path
ultimately breaks into endodermis :fire:
by the time lateral root emerges, contains root cap and first protoxylem and protophloem elements
and has begun establishing vascular tissue connections
as root primordium swells into the cortex, the endodermis may be torn or crushed or undergo cell division
activity is localized to a few cells
form different than bud formation in shoots
formed in mature regions of the roo not at base of apical meristem
never develop into flowers
lateral root initiated deep within root not at surface
Concepts
most roots have three functions
absorbing water and minerals
root shape and size play large role in water absorbtion and retention
producing hormones
integral in grow of plant
anchoring plant into substrate
necessary for vertical plants
can also provide storage as in taproots
