Download Plant Roots

ROOT ORIGINS
THE FIRST VASCULAR PLANTS HAD RHIZOIDS
THAT ABSORBED WATER & NUTRIENTS.
RHIZOIDS ARE DELICATE UNI- OR MULTICELLULAR ROOT- AND HAIR-LIKE
STRUCTURES THAT FUNCTION IN ANCHORAGE
& ABSORPTION BUT LACK XYLEM & PHLOEM.
ROOTS EVOLVED IN RESPONSE TO THE
PRESSURES OF A LAND ENVIRONMENT &
INCREASING PLANT SIZE. ROOTS HAVE
CHANGED RELATIVELY LITTLE OVER TIME,
LIKELY BECAUSE OF THEIR PROTECTIVE
SUBTERRANEAN ENVIRONMENT THEY WERE
NOT EXPOSED TO THE SAME SELECTIVE
PRESSURES AS THE STEM.
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SEED PLANT ROOTS MAY BE CONSIDERED TO
BE 1.) A MODIFIED STEM OR 2.) AN ENTIRELY
NEW ORGAN THAT EVOLVED INDEPENDENTLY
OF THE STEM. THE FIRST VIEW IS ACCEPTED
NOW BECAUSE THE ROOTS OF EXTANT PLANTS
IS VERY SIMILAR TO STEMS OF ANCESTRAL
FOSSIL PLANTS. ROOTS OF MOST PLANTS ARE
PROTOSTELIC (LIKE THE STEMS OF VERY
PRIMITIVE PLANTS); ROOTS WITH A CENTRAL
PITH HAVE AN ALTERNATE ARRANGEMENT OF
XYLEM & PHLOEM THAT MAY REFLECT A
PROTOSTELIC ORIGIN.
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ROOT FUNCTIONS
1. ROOTS ANCHOR PLANTS TO SOIL; PROVIDE
STABILITY TO ABOVE GROUND PARTS OF THE
PLANT.
2. ROOTS ABSORB WATER & DISSOLVED
MINERAL NUTRIENTS FROM SOIL AND
TRANSPORT THEM TO THE STEM.
3. ROOTS CAN BE USED FOR STORAGE OF FOOD
RESERVES.
4. ROOTS CAN DETERMINE THE LONGEVITY OF
THE PLANT – ANNUAL, BIENNIAL OR
PERENNIAL. IN BIENNIAL PLANTS LARGE FOOD
RESERVES ACCUMULATE DURING THE FIRST
YEAR OF VEGETATIVE GROWTH THEN ARE
USED TO PRODUCE FLOWERS, FRUITS & SEEDS
THE SECOND YEAR. IN PERENNIAL PLANTS THE
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FOOD RESERVES ALLOW THE ROOT TO
SURVIVE ADVERSE ENVIRONMENTAL
CONDITIONS , THEN PRODUCE NEW
VEGETATIVE GROWTH IN THE SPRING. DO
ROOTS OF ANNUALS STORE A LOT OF FOOD?
5. ROOTS ARE SITES OF HORMONE
PRODUCTION – ESPECIALLY CYTOKININS AND
GIBBERELLINS, WHICH ARE THEN
TRANSPORTED UPWARD IN XYLEM TO STEMS
AND LEAVES.
ROOT TYPES
TAPROOTS – THE PRIMARY ROOT OF A PLANT
FORMED FROM THE RADICLE OF THE
EMBRYO; A STOUT TAPERING MAIN ROOT
FROM WHICH ARISE SMALLER LATERAL
ROOTS OR BRANCHES; IF THE RADICLE OR
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TAPROOT IS DAMAGED A LATERAL ROOT
EMERGES AND ASSUMES THE DOMINATING
ROLE; GROWTH OF THE TAPROOT CONTINUES
THROUGHOUT THE LIFE OF THE PLANT.
COMMON IN GYMNOSPERMS AND DICOTS.
LATERAL OR BRANCH ROOTS – A ROOT THAT
ARISES FROM ANOTHER OLDER ROOT, ARISES
FROM THE PERICYCLE.
FIBROUS ROOTS – A ROOT SYSTEM IN WHICH
NO ONE ROOT IS MORE PROMINENT THAN THE
OTHERS – ALL ROOTS ARE MORE OR LESS THE
SAME SIZE (AT LEAST IN DIAMETER – LENGTH
MAY VARY), TYPICALLY THE PLANT HAS A
MASS OF THESE; COMMON IN MONOCOTS;
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ARISE BECAUSE THE RADICLE DIES DURING OR
SOON AFTER GERMINATION; USUALLY
ADVENTITIOUS IN ORIGIN; PLANTS MAY BE
PERENNIAL OR ANNUAL; WHILE INDIVIDUAL
ROOTS ARE SMALL THE MASS OF THEM HAVE
ENOUGH STORED FOOD RESERVES TO ALLOW
THE PLANT TO SURVIVE MORE THAN ONE
GROWING SEASON.
ADVENTITIOUS ROOTS – ROOTS THAT
DEVELOP FROM ORGANS OTHER THAN PREEXISTING ROOTS (LEAVES, STEMS); THE ROOT
SYSTEM OF GRASSES IS ALL OR MOSTLY
ADVENTITIOUS; FOUND IN FERNS,
CLUBMOSSES & HORSETAILS.
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CONTRACTILE ROOTS – ROOTS THAT
CONTRACT TO PULL THE SHOOT, CORM, BULB
DOWN DEEPER INTO THE SOIL. ROOTS EXTEND
THROUGH THE SOIL & BECOME FIRMLY
ANCHORED, THE UPPERMOST PARTS BEGIN TO
CONTRACT AND THE STEM, ETC. IS PULLED
DOWNWARD SO IT BURIES DEEPER; CAUSED BY
CHANGES IN THE SHAPE OFCORTICAL CELLS –
EXPAND RADIALLY AND SHORTEN, LOSING ½2/3 OF THEIR HEIGHT; VASCULAR TISSUE
BUCKLES BUT DOES NOT LOSE ITS FUNCTION.
STORAGE ROOTS – MOST ROOTS STORE FOOD
RESERVES, SOME PLANTS HAVE ROOTS
SPECIALIZED FOR THIS FUNCTION; USUALLY
FLESHY BECAUSE OF ABUNDANCE OF
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STORAGE PARENCHYMA PERMEATED BY
VASCULAR TISSUE; THE UPPER PORTION OF
MOST FLESHY ROOTS ACTUALLY DEVELOP
FROM THE HYPOCOTYL; SWEET POTATO,
SUGAR BEET; ROOTS OF SOME DESERT PLANTS
CAN STORE MORE THAN 70 Kg OF WATER.
AERIAL ROOTS – ARE ADVENTITIOUS ROOTS
PRODUCED FROM ABOVE GROUND
STRUCTURES; IN IVY (HEDERA) ROOTS CLING
TO THE SURFACE OF OBJECTS (WALLS) TO
SUPPORT THE CLIMBING STEM; AERIAL ROOTS
OF SOME EPIPHYTIC ORCHIDS MAY BE
PHOTOSYNTHETIC AND IN SOME CASES MAY
BE THE ONLY PHOTOSYNTHETIC ORGAN.
THERE ARE MANY KINDS OF AERIAL ROOTS.
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1. VELAMEN – A MULTIPLE EPIDERMIS THAT
SURROUNDS THE AERIAL ROOTS OF SOME
EPIPHYTIC ORCHIDS;THE VELAMEN PROVIDES
MECHANICAL PROTECTION FOR THE CORTEX
AND REDUCES WATER LOSS.
2. PROP ROOTS – ROOTS THAT SERVE FOR
SUPPORT, AS IN CORN; CAN BRANCH DOWN
FROM LOWER NODES OF THE STEM OR DROP
DOWN FROM BRANCHES AS IN SOME TREES,
THEY CAN ENTER THE SOIL AND ABSORB
WATER & NUTRIENTS; MANGROVES, BANYAN
TREE, PALMS.
3. PNEUMATOPHORES – ROOTS THAT GROW
UPWARD FROM SUBMERGED ROOTS IN
MUD/WATER; FOUND IN TREES THAT GROW IN
SWAMPS; DO HELP TO ANCHOR THE PLANT
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BUT MAIN FUNCTION IS TO AERATE THE
PLANT, ALLOWS AIR TO TRAVEL THROUGH
THE ROOT SYSTEM AND PROVIDE O2 FOR
RESPIRATION; ROOTS HAVE AERENCHYMA
AND LENTICELS.
4. BUTTRESS ROOTS – IN SHALLOWLY ROOTED
TREES IN TROPICS AT THE BASE OF THE
TRUNKS ARE BUTTRESS ROOTS – MAY BE TO 4
M TALL; SPECIALIZED FOR SUPPORT WITH
LARGE AMOUNT OF FIBERS AND SMALL
AMOUNTS OF XYLEM.
A UNIQUE AERIAL ROOT IS FOUND IN THE
EPIPHYTE DISCHIDIA RAFFLESIANA
(ASCLEPIADACEAE) – THE FLOWER POT PLANT.
SOME LEAVES FORM HOLLOW CONTAINERS
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THAT COLLECT DEBRIS AND RAINWATER, ANT
COLONIES LIVE IN THE “POTS” AND ADD N2.
ROOTS FORM AT THE NODE ABOVE THE LEAF
AND GROW DOWNWARD INTO THE POT –
ABSORB WATER & NUTRIENTS.
HAUSTORIA (HAUSTORIUM)
THESE ARE SPECIALIZED ROOTS ON PLANTS
THAT ARE PARASITIC; THEY ARE MODIFIED
STRUCTURES THAT PENETRATE THE HOST
TISSUE TO TRANSFER NUTRIENTS FROM THE
HOST TO THE PARASITE; A PRIMARY
HAUSTORIUM IS A DIRECT OUTGROWTH OF
THE RADICLE; A SECONDARY HAUSTORIUM IS
A LATERAL ORGAN THAT DEVELOPS FROM A
MODIFIED ADVENTITIOUS ROOT OR FROM
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OUTGROWTHS OF ROOTS OR STEMS; FOUND IN
MISTLETOE (VISCUM), DODDER (CUSCUTA),
BROOMRAPES (OROBANCHE); IN SOME SPECIES
THE HAUSTORIUM PENETRATES THE HOST
TISSUE TO THE XYLEM AND FORMS A
CONTINUOUS XYLEM BRIDGE; IN OTHERS THE
HAUSTORIUM FORMS A MANTLE OF
PARENCHYMATOUS TISSUE AROUND THE HOST
ORGAN,THIS INDUCES THE HOST TO FORM A
PLACENTA-LIKE OUTGROWTH OF VASCULAR
TISSUE TO SUPPLEY NUTRIENTS TO THE
PARASITE.
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NITROGEN FIXATION
ROOT NODULES ARE ENLARGEMENTS FOUND
ON THE ROOTS OF MANY PLANTS (ESPECIALLY
THE FABACEAE) INHABITATED BY NITROGEN
FIXING BACTERIA. PLANTS CANNOT USE
ATMOSPHERIC N2 BUT SOME BACTERIA CAN
CONVERT IT TO A FORM PLANTS CAN USE. A
SYMBIOTIC RELATIONSHIP HAS EVOLVED IN
SOME PLANTS BETWEEN ROOTS AND SOIL
BACTERIA – NITROGEN FIXATION OCCURS IN
THE NODULES. INFESTATION OF THE HOST
ROOT OCCURS THROUGH ROOT HAIRS.
BACTERIAL AGGREGATE WITHIN CELLS IN
THE CENTER OF THE NODULE, THIS IS
SURROUNDED BY PARENCHYMA TISSUE WITH
VASCULAR BUNDLES AND AN ENDODERMIS.
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THE ENDODERMIS RETARDS ENTRY OF O2 TO
THE NODULE, WHICH WOULD ADVERSELY
AFFECT N2 FIXATION. PLASMODESMATA
CONNECT THE BACTERIA CONTAINING CELLS
WITH SURROUNDING PARENCHYMA CELLS.
THE BACTERIA PROVIDE THE ENZYMES
NEEDED FOR NITROGEN FIXATION;THE HOST
PLANT PROVIDES THE SUGAR TO RUN THE
OPERATION.
IN THE FABACEAE (PEA FAMILY) THE
BACTERIA IS USUALLY IN THE GENUS
RHIZOBIUM; IN SOME WOODY DICOTS ALNUS
(ALDER), CEANOTHUS (TEA BUSH), MYRICA
(SWEET GALE) AND ELAEAGNUS (OLEASTER)
ARE USUALLY INFECTED BY AN
ACTINOMYCETE FRANKIA.
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CYCADS HAVE ROOT NODULES CONTAINING
CYANOBACTERIA – NOSTOC OR ANABAENA;
THE WATER FERN AZOLLA MAY ALSO BE
INFECTED WITH ANABAENA.
MYCORRHIZAE
THE ROOT TIP AND ROOT HAIRS ARE THE
REGION OF WATER & MINERAL ABSORPTION.
THE SURFACE AREA OF THESE REGIONS IS
RELATIVELY SMALL. IN TIMES OF DRAUGHT
THE VOLUME OF SOIL THAT PROVIDES WATER
& MINERALS CAN BE DEPLETED.
MYCORRHIZAE GREATLY ENHANCE THE
ROOTS ABSORPTIVE CAPACITY.
THIS IS A SYMBIOTIC RELATIONSHIP BETWEEN
A PLANT AND A FUNGUS. THE FUNGUS CAN BE
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EITHER A ASCOMYCETE, BASIDIOMYCETE OR
ZYGOMYCETE. 90% OF ALL TERRESTRIAL
PLANTS ARE MYCORRHIZAL. MYCORRHIZAE
HAVE NOT BEEN FOUND IN THE
BRASSICACEAE, CYPERACEAE, AND
CUCURBITACEAE.
ECTOMYCORRHIZAE – THE FUNGUS FORMS A
DENSE SHEATH OF HYPHAE AROUND THE
ROOT TIP – HYPHAE EXTEND FROM THE
SHEATH INTO THE SOIL AND ALSO PENETRATE
THE EPIDERMIS AND CORTEX OF THE ROOT.
THE HARTIG NET IS A MUCH BRANCHED
NETWORK OF HYPHAE IN THE
INTERCELLULAR CHANNELS OF THE
PARENCHYMA CELLS, WHICH FUNCTION AS
TRANSFER CELLS – TRANSPORTING
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PHOTOSYNTHATE TO THE FUNGUS AND WATER
& MINERALS FROM THE FUNGUS TO THE
PLANT.
ENDOMYCORRHIZAE – THE MOST COMMON
TYPE (80% OF VASCULAR PLANTS) – THE
FUNGAL HYPHAE DO NOT FORM A SHEATH
AROUND THE ROOT; THE HYPHAE
PENETRATES THE ROOTS AND GROWS WITHIN
THE CORTEX, HYPHAE PENETRATE CELLS
WALLS BUT NOT THE PLASMA MEMBRANES;
COMPLEX MUCH BRANCHED STRUCTURES
CALLED ARBUSCULES ARE FORMED BY THE
HYPHAE THAT ARE ENCLOSED BY PLASMA
MEMBRANES (BUT NOT WITHIN THE
PROTOPLAST); THIS IS AN EFFIECIENT REGION
OF TRANSFER OF PHOTOSYNTHATE TO THE
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FUNGUS AND WATER & MINERALS TO THE
PLANT. THIS IS ALSO CALLED VESICULARARBUSCULAR MYCORRHIZE.
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