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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. 1 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. 2 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 3 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 4 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; 5 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. 6 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 7 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. 8 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 9 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 10 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 11 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. 12 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. 13 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. 14 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 15 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 16 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 17 FUNGUS AND WATER & MINERALS TO THE PLANT. THIS IS ALSO CALLED VESICULARARBUSCULAR MYCORRHIZE. 18