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Roots (Zone of Maturation/Root Hair Zone (several important processes…
Roots
Zone of Maturation/Root Hair Zone
several important processes happen simultaneously
root hairs grow outwards
greatly increasing absorption of water and minerals
zone of elongation merges gradually with the zone of maturation
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no distinct boundary exists between the two
cortex cells continue to enlarge
most significant activity is the transfer of minerals from epidermis to vascular tissue
done by either diffusion through the walls and intercellular spaces
or by absorption into the cytoplasm of a cortical cell and then transferal from cell to cell
cortical intercellular spaces-important- allowing oxygen to diffuse through out the root from soil or stem
in the zone minerals do not have free access to vascular tissue
because the innermost layer of cortical cells differentiates into endodermis
cells of endodermis have tangential walls, which are thin ordinary primary walls
their radial walls -top,bottom,sides- are encrusted with lignin and Suberin
the bands of casporian strips are involved in controlling types of minerals that enter the xylem water stream
cortex cells cannot exert any control without endodermis
because they are impermeable, minerals can cross the endodermis
however only if the endodermal protoplasts absorb them
within the vascular tissues
many of the much larger cells of the metaxylem and metaholem become fully differentiated and functional
the arrangement of these differs from the stem
the xylem of almost all plants except monocots, forms a solid mass in the center
surrounding it is strands of phloem, no pith is present
roots of many monocots have strands of xylem and phloem that are distributed in ground tissue
Other types of Roots and Root Midifications
Storage Roots and Prop Roots
roots frequently provide storage for carbohydrates which accumulate during summer
in biennial species, roots are the only permanent organs
in autumn, most of the stem dies back to a few nodes in the ground
carbs stored in the root are used to produce a new shoot in the spring
its less economical to winter proof the shoot than replace is using the nutrients stored in the root
Prop roots make contact with the soil
they transport additional nutrients and water to the stem
they also contract slightly and place some tension on the stem
which acts as a stabilizer
if they undergo a secondary growth and become woody they can be very strong supports
prop roots and branches can spread and produce massive trees many meters in diameter
Aerial Roots of Orchids and Contractile Roots
many orchids are epiphytic
their roots spread along the surface of bark
and often dangle freely in the air
although they live in rainforests, they are adapted to drought conditions
the root epidermis called velamen is composed of several layers of dead cells
the velamen acts as a waterproof barrier
in contractile roots, the uppermost portions slowly contract, making in firmly anchored
because the root is firmly fixed, the stem is pulled downward
this makes the base of the shoot kept at soil level
in the case of bulbs it gets buried deeper
the contraction is caused by changes in the shapes of the cortex cells
they simultaneously shorten and expand rapidly
the vascular tissues buckle and become undulate
Mycorrhizae and Root Nodules and Nitrogen Fixation
mycorrhiza has two main relationships
ecotomycorrhizal relationships happens in woody forest plants
fungal hyphae penetrate between the outermost root cortex cells
but never invade the cells themselves
endomycorrhizal association
herbaceous plants have a hyphae that penetrates the root cortex
cannot pass the casporian strips
they invade the cells but do not break the host plasma membrane
the chemical conversion of atmospheric nitrogen into usable compounds is nitrogen fixation
sum plants have a relationship with anitrogen-fixing bacteria
bacteria free in the soil secretes a substance which causes root hair to curl sharply
bacterium then attaches to the convex and pushes the infection thread
infection thread penetrates once cell after the other all the way into inner cortex
causing adjacent cortical cells to undergo mitosis and form root nodule
Roots of Parasitic Flowering Plants and Roots of Strangler Figs
haustoria, roots of parasitic plants
in most cases very little root-like structure remains
parasitism has evolved several times
however, haustoria typically remains firmly to their host
either by adhesive or growing around a small branch or root
penetration either occurs by forcing a shaft of cells through the dermal system
or by expanding the haustorium radically, cracking hosts epidermis
surprisingly most parasites attack only the xylem
young strangler figs grow as epiphytes perched on a branch of a host tree
birds eat the fruit of the figs and deposit the seeds onto different branches of other trees
when the seed germinates , its roots cling to the bark of the host branch
which then grows rapidly and wraps around the tree extending downwards
for most and even years the roots don't touch the soil , so they must get nutrients from rain water on the tree
when the roots finally reach the soil it penetrates and rapidly branches
before to long the trunk of the host in encased in numerous strangler fig roots
eventually the host tree dies, leaving the strangler fig as a self supporting tree
External Structure of Roots
Organization of root systems
have enormous absorptive surface, which is also a highly branched root system
most plants have one taproot which is longer, then many small lateral roots or branch roots
the tap root develops from the radicle, after germination it grows extensively becoming the largest root
lateral roots can make more lateral roots which results in a highly ramified set of roots
can become prominently swollen like a taproot
if the plant s perennial and woody the roots undergo secondary growth, producing wood and bark
most monocots and some eudicots have a fibrous root system
this happens because the radicle dies
as a result root primordial grows outwards and forms the first stage of fibrous root system
as the plant ages more toor primordial are initiated in the stem tissue, resulting in adventitious roots
these roots increase the absorptive and transport capacity of the root system
functional significance of taproots vs. fibrous roots
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many eudicots are perennial and undergo secondary growth
results in an increased quality of healthy functional wood in both trunk and root
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this enlarging permits an increase in leaves and fine absorptive roots
most monocots cannot undergo secondary growth
after stem is formed, the number of vascular bundles, tracheary elements and sieve tubes are set
conducting capacity and leaves can not be increased
some do increase their size by means of stolons or rhizomes
their horizontal shoots branch and then produce adventitious roots
not just limited to monocots, many eudicots also grow this way naturally
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Structure of Individual Roots
fairly simple, no leaves, leaf scars, leaf axils nor axially buds
the root tip is the region that growth in length occurs
growth by discrete apical meristem is the only feasible type for longitudinal growth
grows very slowly because it is embedded in a solid matrix
with apical growth only the extreme tip pushes through the soil
the root apical meristem is protected by a thick layer of cells called root cap
just behind root cap and meristem is a zone of elongation
behind that is the root hair zone, many of the epidermal cells extend out
root hair only form in a part of the root that is not elongating
because it pushes through the soil, its constantly damaged and must be renews by cell multiplication
dictyosomes of root cap secrete mucigel, which lubricates the passage of the root through the soil
also causes soil to release its nutrient ions and permits ions to diffuse toward the root
Internal Structure of Roots
Root Cap
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must have a specific structure and growth pattern to provide maximum protection to meristem
cells closest to root meristem are also meristematic
undergoing cells division with transverse walls
cells on the edge grow towards the sides of the root
although cells appear to extend through the sides , the root is actually growing through the cap
as cells are pushed forward, they develop dense starch grains and their endoplasmic reticulum
as they are pushed closer to the cap their structure metabolizes and metabolism changes drastically
endoplasmic reticulum becomes less conspicuous, starch grains are digested
cell's dictyosomes secrete copious amounts of mucigel
and the middle lamina breaks down and releases cells
Root Apical Meristem and Zone of elongation
root apical meristem is examined in relationship to the root tissue
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regular files of cells can be seen to originate the meristem
root is more orderly then shoot because it experiences no disruptions
central cells do not synthesize DNA
this region is known as the quiescent center
these cells are more resistant to various types of harmful agents
believed to act primarily as a reverse of healthy cells
if cap or meristem are damaged this center creates a new one
very important because damage happens frequently
zone of elongation is just behind the root apical meristem
similar to the shoots sub apical meristem region
cells begin to differentiate
outermost cells are protoderm and differentiate into epidermis
center is provovascular tissue cells that develop into primary xylem and phloem
in the zone tissues are quite permeable
