Download STEMS PLANT BODIES ARE COMPOSED OF CELLS, TISSUES

STEMS
PLANT BODIES ARE COMPOSED OF CELLS,
TISSUES AND ORGANS. A ORGAN IS A
STRUCTURE COMPOSED OF A VARIETY OF
TISSUES. SEED PLANTS HAVE THREE BASIC
VEGETATIVE ORGANS: STEMS, ROOTS AND
LEAVES. REPRODUCTIVE ORGANS INCLUDE
FLOWERS, FRUITS, AND SEEDS.
PLANT GROWTH IS THE RESULT OF TWO BASIC
PROCESSES: 1. PRODUCTION OF NEW
CELLS/TISSUES BY MITOSIS IN THE MERISTEM.
2. CELL ENLARGEMENT.
PRIMARY GROWTH IS RESPONSIBLE FOR AN
INCREASE IN HEIGHT OR LENGTH OF A PLANT
PART. MERISTEMS ARE REGIONS OF
UNDIFFERENTIATED TISSUE, WHICH BY CELL
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DIVISION, PRODUCE NEW CELLS. THESE CELLS
CAN DIFFERENTIATE INTO THE VARIOUS
TISSUES FOUND IN A PLANT. PRIMARY APICAL
MERISTEMS ARE FOUND AT THE APEX OF
ROOTS AND SHOOTS AND PRODUCE THE
PRECURSORS OF THE PRIMARY TISSUES OF
THE ROOT AND SHOOT. THE GROWTH MAY BE
VEGETATIVE – INITIATING VEGETATIVE
TISSUE & ORGANS – OR REPRODUCTIVE –
INITIATING REPRODUCTIVE TISSUE & ORGANS.
THE REGION JUST BELOW THE APICAL
MERISTEM IS THE SUB-APICAL MERISTEM – A
REGION WHERE VISIBLE DIFFERENTIATION OF
CELLS BEGINS. IN THE VASCULAR SYSTEM THE
FIRST XYLEM TO APPEAR IS THE
PROTOXYLEM; THE CELLS JUST EXTERIOR TO
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THESE GROW AND EXPAND AND
DIFFERENTIATE AND DEVELOP INTO
TRACHEARY ELEMENTS EVEN LARGER THAN
THE FIRST – THIS FORMS THE METAXYLEM –
THE LARGEST TRACHEARY ELEMENTS OF THE
PRIMARY XYLEM.
A SIMILAR PROCESS OCCURS WITH PHLOEM.
THE EXTERIOR CELLS MATURE AS
PROTOPHLOEM AND THE CELLS CLOSEST TO
THE METAXYLEM BECOME METAPHLOEM.
PROTOPHLOEM IS FORMED WHILE THE STEM
IS ACTIVELY GROWING; THE CELLS ARE VERY
SENSITIVE TO THIS AND DIE WHEN STRESSED
TOO MUCH. AS A RESULT PROTOPHLOEM
CELLS ARE VERY SHORT-LIVED; OFTEN
FUNCTIONING ONLY ONE DAY. THEY DO NOT
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BECOME WELL DIFFERENTIATED AND MAY
LACK COMPANION CELLS. METAPHLOEM
DIFFERENTIATES LATER WHEN SURROUNDING
CELLS HAVE STOPPED EXPANDING AND FORM
TYPICAL SIEVE ELEMENTS.
SECONDARY GROWTH IS CHARACTERIZED BY
AN INCREASE IN THICKNESS OF THE STEM &
ROOT. THIS RESULTS FROM THE FORMATION
OF SECONDARY TISSUES BY A LATERAL
MERISTEM – A MERISTEM LOCATED PARALLEL
TO THE SURFACE OF AN AXIS. THE VASCULAR
CAMBIUM IS A LATERAL MERISTEM FROM
WHICH SECONDARY XYLEM & PHLOEM ARE
PRODUCED IN THE STEM & ROOT.
STEMS MAY BE CONSIDERED TO BE EITHER: 1.
HERBACEOUS – NON-WOODY – GROWTH IS
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MOSTLY PRIMARY, BUT SOME SECONDARY
GROWTH IS POSSIBLE. 2. WOODY – PLANTS
PRODUCE LARGE AMOUNTS OF WOOD –
SECONDARY XYLEM RICH IN LIGNINS.
STEMS ARE TYPICALLY ABOVE GROUND
STRUCTURES THAT:
1. DETERMINE THE DISTRIBUTION OF LEAVES
IN THE AIR.
2. PROVIDE THE VASCULAR CONNECTIONS
BETWEEN THE ROOTS AND THE LEAVES
3. MAY BE AN IMPORTANT STORAGE ORGAN
4. MAY BE GREEN & PHOTOSYNTHETIC ,
PRODUCING CARBOHYDRATES
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DURING THE EVOLUTION OF VASCULAR
PLANTS STEMS APPEARED BEFORE LEAVES
AND ROOTS. EARLIEST FOSSILS OF VASCULAR
PLANTS HAVE STEMS BUT NO ROOTS OR
LEAVES. THE FOSSIL RECORD SUGGESTS THAT
ROOTS & LEAVES EVOLVED FROM ANCESTRAL
STEMS. ALL OTHER ORGANS – ROOTS, LEAVES,
BUDS, FLOWERS, FRUITS, CONES – ARE
ATTACHED TO STEMS.
A STEM USUALLY CONSISTS OF NODES AND
INTERNODES. NODES ARE THE PLACES ON THE
STEM WHERE LEAVES ARE ATTACHED.
PORTIONS OF THE STEM BETWEEN THE NODES
ARE INTERNODES. ROSETTE PLANTS HAVE
STEMS THAT DO NOT ELONGATE – THE
INTERNODES ARE VERY SHORT, THE LEAVES
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ARISING FROM THE NODES ARE OVERLAPPING
AND SEEM TO ORIGINATE AT ALMOST THE
SAME SPOT. LEAVES CAN BE CALLED A BASAL
ROSETTE.
PLANTS ARE EITHER CAULESCENT OR
ACAULESCENT.
STEMS ARE MADE UP OF EPIDERMAL TISSUE,
VASCULAR TISSUE AND GROUND TISSUE.
THREE BASIC TYPES OF ORGANIZATION IN THE
INTERNAL STRUCTURE OF PRIMARY STEMS
EXIST:
1. THE VASCULAR TISSUE APPEARS AS A
MORE OR LESS CONTINUOUS HOLLOW
CYLINDER WITHIN THE GROUND TISSUE.
THE OUTER REGION OF GROUND TISSUE IS
THE CORTEX – THE REGION BETWEEN THE
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EPIDERMIS AND THE VASCULAR TISSUE.
THE INNER REGION IS THE PITH – GROUND
TISSUE IN THE CENTER OF THE STEM OR
ROOT, WITHIN THE VASCULAR CYLINDER;
USUALLY PARENCHYMA. SOMETIMES THE
PITH MAY DISINTEGRATE TO FORM A
CAVITY (LUMEN). MATURE GRASS
INTERNODES ARE OFTEN HOLLOW (FILLED
WITH PITH WHEN YOUNG). THIS
CONDITION IS FOUND IN CONIFERS AND
DICOTS.
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2. THE PRIMARY VASCULAR TISSUES
DEVELOP AS A CYLINDER OF DISCRETE
STRANDS SEPARATED FROM ONE ANOTHER
BY GROUND TISSUE. GROUND TISSUE IS
CONTINUOUS WITH THE PITH AND CORTEX
AND FORMS THE INTERFASCICULAR
PARENCHYMA (=PITH RAYS). FOUND IN
CONIFERS AND DICOTS.
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3. THE VASCULAR TISSUE APPEARS TO BE
SCATTERED THROUGHOUT THE GROUND
TISSUE. IN THIS CASE THE GROUND TISSUE
OFTEN CANNOT BE DISTINGUISHED AS
CORTEX OR PITH. FOUND IN MONOCOTS AND
SOME DICOTS.
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VASCULAR CAMBIUM CAN BE FOUND IN TWO
REGIONS: FASCICULAR CAMBIUM THAT
DEVELOPS WITHIN A VASCULAR BUNDLE;
INTERFASCICULAR CAMBIUM THAT
DEVELOPS IN THE INTERFASCICULAR
PARENCHYMA, BETWEEN VASCULAR
BUNDLES.
THE CORTEX MAY BE SIMPLE AND
HOMOGENOUS, COMPOSED OF
PARENCHYMA, SOMETIMES COLLENCHYMA.
IN SOME SPECIES IT MAY BE COMPLEX,
CONTAINING SPECIALIZED CELLS THAT
SECRETE LATEX, MUCILAGE OR RESIN;
SOME CELLS MAY CONTAIN CRYSTALS OF
CALCIUM OXALATE OR DEPOSITS OF SILICA.
SOMETIMES A LAYER OF CORTICAL CELLS
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IMMEDIATELY ADJACENT TO THE
VASCULAR TISSUE IS PACKED WITH STARCH
GRAINS – FORMS A STARCH SHEATH DISTINCT FROM THE REST OF THE CORTEX.
CONNECTIONS BETWEEN THE STEM AND THE
LEAF
THE VASCULAR SYSTEMS OF STEMS AND
LEAVES MUST BE CONNECTED. THS IS DONE
THROUGH A LEAF TRACE – A VASCULAR
BUNDLE WHICH EXTENDS INTO A LEAF
FROM ITS CONNECTION WITH ANOTHER
VASCULAR BUNDLE IN THE PRIMARY
VASCULAR SYSTEM OF THE PLANT.
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LEAF TRACES MAY DIVERGE FROM THE
STEM VASCULAR SYSTEM SOME DISTANCE
BELOW, OR VERY NEAR, THE NODE. SO
TRACES VARY IN LENGTH. BESIDES XYLEM
AND PHLOEM LEAF TRACES CONTAIN
TRANSFER CELLS – A PARENCHYMA CELL
WITH WALL INGROWTHS THAT INCREASES
THE SURFACE AREA OF THE PLASMALEMMA
THAT LINES THE CELL WALL SURFACE;
SPECIALIZED FOR SHORT DISTANCE
APOPLASTIC TRANSFER OF SOLUTES.
THE NUMBER OF LEAF TRACES GOING TO A
SINGLE LEAF VARIES FROM 1-2-3-5-7-MANY.
THE DIVERGENCE OF THE VASCULAR
BUNDLE TOWARDS THE LEAF LEAVES A
REGION OF GROUND TISSUE IN THE
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VASCULAR CYLINDER CALLED THE LEAF
TRACE LACUNAE (=LEAF GAP).
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THE STELE
THE STELE IS A MORPHOLOGIC UNIT OF THE
PLANT AXIS (STEMS & ROOTS) COMPRISING
THE PRIMARY VASCULAR SYSTEM &
ASSOCIATED GROUND TISSUE. THE STELE,
MEANING A COLUMN, IS THE CORE OF THE
PLANT AXIS, INCLUDES THE VASCULAR
SYSTEM WITH ALL ITS INTERFASCICULAR
REGIONS, GAPS AND PITH.
THE SIMPLEST TYPE OF STELE, AND THE
MOST PRIMITIVE, CONTAINS A SOLID
COLUMN OF VASCULAR TISSUE WITH NO
PITH. THIS IS THE PROTOSTELE. XYLEM IS AT
THE CORE AND THE PHLOEM SURROUNDS IT.
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THE PROTOSTELE IS FOUND IN THE FOSSILS
OF THE EARLIEST VASCULAR PLANTS; IN
LIVING PLANTS IT IS FOUND IN SOME FERNS,
THE LYCOPHYTA, AND IT IS THE TYPE OF
STELE FOUND IN MOST ROOTS.
IN PLANTS WITH PROTOSTELES LEAF
TRACES SIMPLY DIVERGE FROM THE
SURFACE OF THE CENTRAL VASCULAR
COLUMN NEAR THE LEVEL OF THE LEAF AND
ENTERS THE LEAF BASE. USUALLY JUST ONE
LEAF TRACE IS PRESENT. THERE IS NO LEAF
GAP.
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THE SECOND TYPE OF STELE IS THE
SIPHONSTELE – A STELE IN THE VASCULAR
SYSTEM IS IN THE FORM OF A CYLINDER
ENCLOSING THE PITH. THIS IS COMMON IN
MOST FERNS, SOME GYMNOSPERMS AND A
FEW ANGIOSPERMS.
THE SIMPLEST FORM OF SIPHONSTELE HAS
NO LEAF GAPS – VASCULAR TISSUE SIMPLY
BRANCHES OFF AS IN THE PROTOSTELE.
IN OTHER SIPHONSTELES THE CONTINUITY
OF THE CYLINDER IS BROKEN IMMEDIATELY
ABOVE THE OUTWARD DIVERGENCE OF THE
LEAF TRACE, FORMING A DISCONTINUITY
CALLED A LEAF GAP. A LEAF GAP IS A
REGION IN WHICH THE PARENCHYMA OF
THE CORTEX AND PITH ARE CONTINUOUS.
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IN SOME FERNS WHEN THE LEAVES ARE
CLOSELY SPACED (SMALL INTERNODES) THE
CYLINDER WILL BE DISSECTED BY MANY
LEAF GAPS, SO THAT IN CROSS SECTION THE
GAPS WILL BE VISIBLE BETWEEN REGIONS
OF VASCULAR TISSUE GIVING THE FALSE
IMPRESSION OF A SYSTEM MADE UP OF
SEPARATE VASCULAR BUNDLES – THIS IS A
DICTYOSTELE. IN THIS THE LARGE
OVERLAPPING LEAF GAPS DISSECT THE
PRIMARY VASCULAR CYLINDER INTO A NETLIKE APPEARANCE. IN THIS CASE THE
PHLOEM SURROUNDS THE XYLEM.
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THE EUSTELE HAS THE PRIMARY VASCULAR
TISSUE IN AXIAL VASCULAR BUNDLES
(=AXIAL BUNDLES) AND LEAF TRACES
ARRANGED AROUND A PITH. THERE ARE NO
LEAF GAPS IN A EUSTELE. THIS TYPE OF
STELE IS CHARACTERISTIC OF
GYMNOSPERMS AND ANGIOSPERMS.
THE STEM VASCULAR BUNDLES AND
ASSOCIATED LEAF TRACES COMPRISE
SYMPODIA. THE SYMPODIA ARE
CONSIDERED TO BE DISCRETE ENTITIES,
ALTHOUGH THERE MAY BE VASCULAR
CONNECTIONS BETWEEN THEM. THE
EUSTELE MOST LIKELY EVOLVED FROM A
PROTOSTELE.
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THE NUMBER OF SYMPODIA LAID DOWN BY
THE APICAL MERISTEM CAN VARY AMONG
PLANTS. WHEN EXAMINING A CROSS
SECTION YOU ARE LOOKING AT A
COMBINATION OF SYMPODIA AND LEAF
TRACES.
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TRACES MAY BE QUITE LONG SO THAT
WHEN LOOKING AT THE CROSS SECTION
YOU SEE BOTH SYMPODIA AND TRACES. THE
ONLY WAY TO TELL WHICH IS WHICH IS BY
TRACING A STRAND BACKWARD, OR IF YOU
JUST HAPPEN TO GET A CROSS SECTION IN
THE RIGHT PLACE WHERE THE TRACE
DIVERGES.
IN ANGIOSPERMS THIS CAN GET VERY
COMPLEX IN THAT THE LEAVES MAY BE
SUPPLIED BY 3 OR 5 OR 7 LEAF TRACES.
OFTEN A CENTRAL TRACE IS LARGER THAN
THE OTHERS – FLANKED BY SMALLER
LATERAL TRACES – ALL OF WHICH GO INTO
THE SAME LEAF. NOT ALL TRACES GOING
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INTO A LEAF COME FROM THE SAME
SYMPODIA.
THE PRIMARY VASCULAR SYSTEM OF
MONOCOTS IS AN ATACTOSTELE (GREEK –
ATACTOS – WITHOUT ORDER). THIS TYPE OF
STELE MAY BE FUNDAMENTALLY
DIFFERENT FROM THOSE OF DICOTS OR MAY
JUST BE A HIGHLY MODIFIED EUSTELE. THE
VASCULAR SYSTEM MAY BE RELATED TO
THE NATURE OF THE LARGE LEAVES WITH
BROAD BASES THAT MAY OVERLAP AND
ENCIRCLE THE STEM – THE LEAVES BEING
SUPPLIED WITH A VERY LARGE NUMBER OF
LEAF TRACES.
IN MONOCOTS AS THE STEM GROWS THE
AXIAL BUNDLES GRADUALLY INCREASE IN
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NUMBER BY BRANCHING TANGENTIALLY.
THIS PRODUCES TWO TYPES OF AXIAL
BUNDLES: MAJOR BUNDLES – WHICH
TRAVERSE, LONGITUDINALLY, RELATIVELY
GREAT DISTANCES BETWEEN LEVELS OF
DIVERGENCE OF CONSECUTIVE LEAF
TRACES AND MINOR BUNDLES – WHICH
TRAVERSE MUCH SHORTER DISTANCES
BETWEEN LEVELS OF LEAF DIVERGENCE.
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BRIDGE BUNDLES CONNECT ADJACENT
AXIAL BUNDLES TO FACILITATE LATERAL
TRANSPORT.
IN BOTH MONOCOTS AND DICOTS BESIDES
LEAF TRACES, THERE ARE BRANCH TRACES –
VASCULAR BUNDLES CONNECTING THE
PRIMARY VASCULAR SYSTEM OF THE
BRANCH AND MAIN STEM; AND BRANCH
GAPS – A REGION OF PARENCHYMA IN THE
PRIMARY VASCULAR CYLINDER THROUGH
WHICH BRANCH TRACES EXTEND TOWARD A
LATERAL BRANCH; IT MAY BE CONFLUENT
WITH A SUBTENDING LEAF TRACE LACUNAE.
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IT IS THOUGHT THAT THE VARIOUS STELAR
TYPES EVOLVED IN RELATION TO THE
INCREASING SIZE OF THE PLANT AND IN
RELATION TO THE SIZE AND COMPLEXITY
OF LATERAL BRANCH SYSTEMS AND LEAVES.
ALSO INVOLVED IS THE INFLUENCE OF THE
HORMONE AUXIN. AUXIN DIRECTLY
CONTROLS THE DIFFERENTIATION OF
PRIMARY XYLEM AND PHLOEM FROM
PROVASCULAR TISSUE; PRIMARY XYLEM
DIFFERENTIATES IN HIGH CONCENTRATIONS
OF AUXIN AND PRIMARY PHLOEM IN LOW
CONCENTRATIONS. IN ANGIOSPERMS
DEVELOPING LEAF PRIMORDIA ARE THE
PRIMARY SOURCE OF THE AUXIN. EXACTLY
HOW ALL THIS WORKS IS UNCERTAIN.
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