Download LEAVES PRIMITIVE VASCULAR PLANTS CONSISTED OF

LEAVES
PRIMITIVE VASCULAR PLANTS CONSISTED OF
RADIALLY SYMMETRICAL BRANCHING
PHOTOSYNTHETIC AXES. WITH THE
EVOLUTION OF LARGER PLANTS AN
ADAPTATION THAT FACILITATED THE
PROCESS OF PHOTOSYNTHESIS WAS
NECESSARY. THE EVOLUTION OF LEAVES
INCREASED THE SURFACE/VOLUME RATIO OF
PHOTOSYNTHETIC TISSUE. LEAF TISSUE HAS
THIN WALLED ELONGATE CELLS ORIENTED AT
RIGHT ANGLES TO THE UPPER SURFACE.
THESE CHANNEL LIGHT AT RELATIVELY HIGH
INTENSITY INTO THE LEAVES. A COMPLEX
SYSTEM OF INTERCELLULAR CHANNELS WITH
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EXTENSIVE WET SURFACE AREA INCREASES
THE ABSORPTION OF CO2 AND O2.
IT IS THOUGHT THAT LEAVES EVOLVED IN AT
LEAST TWO WAYS. THE LEAVES OF
LYCOPHYTES ARE ENATIONS – SIMPLE OUT
GROWTHS FROM THE STEM. THESE ARE ALSO
CALLED MICROPHYLLS. THEY ARE
GENERALLY SMALL AND VASCULARIZED BY
ONLY A SINGLE VEIN. IN SEED PLANTS & FERNS
LEAVES ARE THOUGHT TO REPRESENT A
EVOLUTIONARILY MODIFIED LATERAL
BRANCH SYSTEM – OVER TIME THESE 3DIMENSIONAL BRANCH SYSTEMS BECAME
FLATTENED AND THEN LAMINATE. LEAVES OF
SEED PLANTS ARE CALLED MEGAPHYLLS.
THEY ARE MUCH MORE COMPLEX THAN
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MICROPHYLLS BOTH IN GROSS MORPHOLOGY
AND INTERNAL STRUCTURES.
LEAVES CAN BE CLASSIFIED INTO SEVERAL
GROUPS: FOLIAGE LEAVES FUNCTION IN
PHOTOSYNTHESIS; CATAPHYLLS ARE BUD
SCALES AND SCALES ON UNDERGROUND
STEMS AND FUNCTION IN PROTECTON AND/OR
STORAGE; HYPSOPHYLLS ARE FLORAL
BRACTS WHICH HAVE A PROTECTIVE
FUNCTION AND COTYLEDONS.
LEAVES DIFFER FROM STEMS AND ROOTS IN
THAT THEY ARE DETERMINATE IN GROWTH.
THEIR MERISTEMS CEASE TO FUNCTION
AFTER A GENETICALLY PREDETERMINED
PERIOD OF GROWTH; THEIR GROWTH IS
THEREBY RESTRICTED. THERE ARE SOME
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EXCEPTIONS TO THIS; SOME FERNS AND
DICOTS HAVE LEAVES THAT MAY GROW FOR A
NUMBER OF YEARS. LEAVES OF DECIDUOUS
PLANTS LIVE FOR ONLY ONE GROWING
SEASON; LEAVES OF EVERGREEN PLANTS
REMAIN FOR TWO OR MORE YEARS.
BASIC LEAF STRUCTURE
MOST LEAVES CONSIST OF A STALK-LIKE
PETIOLE AND A BROAD LAMINATE BLADE. THE
BLADE IS COVERED ON BOTH UPPER AND
LOWER SURFACES BY AN EPIDERMIS. THE
EPIDERMIS GENERALLY HAS A CUTICLE AND
STOMATA. TRICHOMES MAY ALSO BE
PRESENT. THE INTERIOR TISSUE OF THE LEAF
CONSISTS OF VASCULAR BUNDLES AND
MESOPHYLL – THE PHOTOSYNTHETIC TISSUE.
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THE MESOPHYLL USUALLY IS COMPOSED OF
TWO LAYERS: 1. THE UPPER PALISADE
PARENCHYMA (=PALISADE MESOPHYLL) WITH
CELLS USUALLY TUBULAR IN FORM WITH
CHLOROPLASTS AND 2. THE LOWER SPONGY
PARENCHYMA (=SPONGY MESOPHYLL) WITH
CONSPICUOUS INTERCELLULAR AIR SPACES,
WITH CELLS CONTAINING CHLOROPLASTS. IN
THE LEAVES OF SOME PLANTS THE
MESOPHYLL IS NOT DIFFERENTIATED INTO
LAYERS BUT IS HOMOGENEOUS (MONOCOTS
AND GYMNOSPERMS) WHILE IN DICOTS
TYPICALLY DO HAVE BOTH LAYERS. THERE
ARE EXCEPTIONS HOWEVER. IN A FEW CASES
(PARTICULARLY XEROPHYTIC GYMNOSPERMS
[ARAUCARIA & PODOCARPUS] AND
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ANGIOSPERMS [ATRIPLEX & ACACIA] THERE IS
A PALISADE MESOPHYLL IN BOTH THE UPPER
AND LOWER PARTS OF THE LEAF. SUCH CASES
ARE CALLED ISOBILATERAL. THE PALISADE
MESOPHYLL MAY FUNCTION IN THE
TRANSMISSION OF LIGHT TO THE INNER PARTS
OF THE LEAF.
LEAVES HAVE AN UPPER AND LOWER
SURFACE. THIS CAN BE DETERMINED BY THE
INTERNAL PLACEMENT OF THE XYLEM AND
PHLOEM. VASCULAR BUNDLES ARE
COMMONLY COLLATERAL – AS THE LEAF
TRACES DIVERGE INTO THE PETIOLE & LEAF
THE PRIMARY XYLEM WILL BE ORIENTATED
TOWARDS THE UPPER SURFACE OF THE LEAF
AND THE PRIMARY PHLOEM TOWARDS THE
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LOWER SURFACE. THIS ORIENTATION DOES
NOT WORK ALL THE TIME (TUBULAR LEAVES
AND SOME MONOCOTS).
THE VASCULAR BUNDLES IN THE LEAF ARE
CALLED VEINS – THE PATTERN FORMED BY
THEM IS VENATION. IN MOST DICOTS THE
VASCULAR TISSUE FORMS A RETICULATE
VENATION SYSTEM – THE VEINS, HIGHLY
BRANCHED, FORM AN ANASTOMOSING SYSTEM
RESEMBLING A NET. OFTEN THERE IS A LARGE
MIDRIB (=MIDVEIN) FROM WHICH EMERGE
LATERAL VEINS THAT GIVE RISE TO MINOR
VEINS. THIS BRANCHING PATTERN HAS
SUCCESSIVELY THINNER VEINS DIVERGING AS
BRANCHES FROM THICKER VEINS.
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IN MOST MONOCOTS PARALLEL VENATION
OCCURS WITH MAIN VEINS APPROXIMATELY
PARALLEL TO EACH OTHER, BUT THEY DO
CONVERGE AT THE BASE AND APEX OF THE
LEAF. THE VEINS ARE OF RELATIVELY
UNIFORM SIZE ORIENTATED LONGITUDINALLY
SIDE BY SIDE. THESE VEINS ARE
INTERCONNECTED BY MUCH SMALLER VEINS
WHICH MAY APPEAR AS SIMPLE CROSS
CONNECTIONS OR EVEN AS A COMPLEX
NETWORK.
VASCULAR BUNDLES ARE ENCLOSED BY
BUNDLE SHEATHS CONSISTING OF
PARENCHYMA (AND OFTEN COLLENCHYMA OR
SCLERENCHYMA) ONE OR MORE CELL LAYERS
THICK. IN SOME LEAVES WALLS OR RIBS OF
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TISSUE CALLED BUNDLE SHEATH EXTENSIONS
RUN FROM THE VASCULAR BUNDLE TO THE
EPIDERMIS – BOTH UPPER AND LOWER. THE
EXTENSIONS OFTEN CONSIST OF
SCLERENCHYMA AND ARE SUPPORTING
STRUCTURES; THEY MAY ALSO BE PATHWAYS
OF TRANSPORT BETWEEN THE VEINS AND
EPIDERMIS. BUNDLE SHEATHS EXTEND TO THE
ENDS OF THE VASCULAR BUNDLES SO THAT NO
PART OF THE VASCULAR TISSUE IN THE LEAF
IS EXPOSED TO INTERCELLULAR AIR. WHY?
THERE IS USUALLY A DISTINCTION BETWEEN
BUNDLE SHEATH CELLS OF C3 AND C4 PLANTS.
THE BUNDLE SHEATH CELLS OF C3 PLANTS
HAVE FEW ORGANELLES AND SMALL
CHLOROPLASTS AND APPEAR EMPTY AT LOW
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MAGNIFICATION. OFTEN THEY MAY NOT
APPEAR TO BE ALL THAT DISTINCT FROM THE
MESOPHYLL. THE MESOPHYLL CELLS
SURROUNDING THEM ALSO SHOW NO SPECIFIC
ARRANGEMENT.
IN C4 PLANTS THE BUNDLE SHEATH CELLS ARE
PROMINENT, OF RELATIVELY LARGE SIZE AND
HAVE THICK WALLS. THEY CONTAIN MANY
LARGE CHLOROPLASTS (LARGER THAN THOSE
IN CELLS OF THE ADJACENT MESOPHYLL). THE
MESOPHYLL CELLS ADJACENT TO THE
BUNDLE SHEATH CELLS ARE FREQUENTLY
ARRANGED IN AN ORDERLY ARRAY. BECAUSE
OF THE PROMINENCE OF THESE CELLS AND
THEIR USE OF C4 PHOTOSYNTHESIS THIS IS
CALLED THE KRANZ SYNDROME.
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SOME LEAVES HAVE AN EXODERMIS
(=HYPODERMIS) – A LAYER OF CELLS JUST
UNDER THE EPIDERMIS, THAT USUALLY AIDS
IN PREVENTING WATER LOSS. AN ENDODERMIS
MAY ALSO BE FOUND IN SOME LEAVES. BOTH
OF THESE ARE TYPICALLY FOUND IN PLANS
THAT GROW IN DRY HABITATS. IDIOBLASTS
MAY BE PRESENT IN THE MESOPHYLL OR
ELSEWHERE IN LEAVES.
GYMNOSPERM LEAVES
THE LEAVES (NEEDLES) OF CONIFERS
TYPICALLY HAVE A THICK WALLED
EPIDERMIS, HEAVY CUTICLE AND SUNKEN
STOMATA. THE STOMATA OCCUR ON ALL
SIDES AND ARE GENERALLY IN VERTICAL
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ROWS. A SCLERIFIED FIBROUS HYPODERMIS IS
BELOW THE EPIDERMIS. THE MESOPHYLL IS
HOMOGENEOUS USUALLY; PARENCHYMA
CELLS HAVE FOLDS IN THEM THAT PROTRUDE
INTO THE CELL LUMEN. RESIN DUCTS ALSO
OCCUR IN THE MESOPHYLL.
THE VASCULAR TISSUE USUALLY OCCURS AS
ONE BUNDLE OR TWO BUNDLES SIDE BY SIDE,
IN A CENTRAL POSITION IN THE NEEDLE. THE
VASCULAR BUNDLES ARE USUALLY
SURROUNDED BY TRANSFUSION TISSUE. THIS
IS A SPECIALIZED TISSUE OF PARENCHYMA
CELLS INTERMIXED WITH SHORT TRACHEIDLIKE CELLS (WALLS HAVE CIRCULAR
BORDERED PITS). THIS IN TURN IS ENCLOSED
BY AN ENDODERMIS. THE CELLS OF THE
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TRANSFUSION TISSUE LIKELY TRANSPORTS
WATER & SOLUTES FROM THE XYLEM TO THE
MESOPHYLL AND PHOTOSYNTHATE IS
TRANSPORTED BACK TO THE PHLOEM.
THERE IS A GREAT DEAL OF VARIABILITY IN
CONIFER LEAVES WITH REGARD TO THE
NUMBER OF BUNDLES, RESIN CANALS,
DIFFERENTIATION OF THE MESOPHYLL,
AMOUNT AND ARRANGEMENT OF THE
TRANSFUSION TISSUE AND PRESENCE OF A
PLICATE (ROSETTE) MESOPHYLL.
PHYLLOTAXIS (=PHYLLOTAXY)
PHYLLOTAXIS IS THE ARRANGEMENT OF
LEAVES ON A STEM. IN A SPIRAL, OR
ALTERNATE, PHYLLOTAXIS THERE IS ONE
LEAF PER NODE. THE LEAVES FORM A SPIRAL
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OR HELIX GOING UP (DOWN) THE STEM. IN
THIS WAY LEAVES ARE NOT DIRECTLY OVER
EACH OTHER. WHY IS THIS IMPORTANT?
HOWEVER, IF THE LEAVES DO FORM TWO
PARALLEL RANKS ALONG THE STEM THE
PHYLLOTAXIS IS DISTICHOUS.
IN AN OPPOSITE PHYLLOTAXIS THERE ARE
TWO LEAVES PER NODE. WHEN PAIRS OF
LEAVES AT SUCCESSIVE NODES FORM AT
RIGHT ANGLES TO EACH OTHER THE
PHYLLOTAXIS IS DECUSSATE.
PLANTS WITH A WHORLED PHYLLOTAXIS
HAVE THREE OR MORE LEAVES PER NODE.
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LEAF ABSCISSION
THE PROCESS OF SHEDDING OF PLANT PARTS
IS CALLED ABSCISSION. IT IS CONTROLLED BY
THE HORMONES AUXIN AND ETHYLENE, AND
CORRELATED WITH ENVIRONMENTAL
FACTORS SUCH AS PHOTO-PERIOD, OZONE,
WOUNDING/ATTACK BY PATHOGENS, WATER
STRESS AND SENESCENCE. STRUCTURE &
CHEMICAL CHANGES OCCUR NEAR THE BASE
OF THE PETIOLE RESULTING IN THE
FORMATION OF AN ABSCISSION ZONE. IN
WOODY DICOTS TYPICALLY TWO LAYERS CAN
FOUND: 1. SEPARATION LAYER –STRUCTURAL
CHANGES FACILITATE THE FALL OF THE LEAF
– CELLS ARE SHORT WITH POORLY
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DEVELOPED CELL WALLS. THIS IS WHERE THE
SPLIT ACTUALLY OCCURS.
2. PROTECTIVE LAYER – OCCURS BENEATH
THE SEPARATION LAYER AND PROTECTS THE
EXPOSED SURFACE FROM WATER LOSS AND
PATHOGENS. SUBERIN IS DEPOSITED ON THE
CELLS WALLS, ALONG WITH WOUND GUM –
INTERCELLULAR SPACES ARE FILLED WITH
THE SAME SUBSTANCES. THIS AREA IS
RECOGNIZED AS THE LEAF SCAR ONCE THE
LEAF HAS FALLEN.
THE ACTUAL SEPARATION RESULTS FROM THE
LOSS OF ADHESION BETWEEN CELLS CAUSED
BY THE DISSOLUTION OF THE MIDDLE
LAMELLA BY THE ACTION OF ENZYMES. THIS
OCCURS IN THE GROUND TISSUE. THE
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VASCULAR TISSUES ARE USUALLY BROKEN
MECHANICALLY (FREEZING OR WIND) AT THE
END OF THE PROCESS OF SEPARATION.
LEAF FUNCTIONS
1. PRIMARY SITE OF PHOTOSYNTHESIS.
2. TENDRILS AID IN SUPPORT OF CLIMBING
PLANTS, VINES.
3. LEAVES MAY BE SPECIALIZED FOR WATER
STORAGE – THE LEAVES MAY BE SUCCULENT
(FLESHY). BROMELIADS ARE EPIDPHYTES
THATHAVE THEIR LEAVES ARRANGED AND
MODIFIED TO STORE WATER IN TANKS –
FORMED BY A ROSETTE.
4. LEAVES MAY BE SITES OF FOOD STORAGE.
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5. LEAVES MAY BE USED FOR THE DEFENSE OF
THE PLANT – SPINES ON CACTI. LEAVES ARE
THE PART OF THE PLANT MOST SUSCEPTIBLE
FOR HERBIVORY, SO MANY PLANTS STORE
SECONDARY COMPOUNDS IN THE LEAF TO
MAKE THEM LESS PALATABLE OR EVEN TOXIC
– POISON HEMLOCK (CONIUM) AND
MILKWEEDS (ASCLEPIAS). TRICHOMES CAN
DETER SOME HERBIVORES JUST BY THEIR
PRESENCE OR DENSITY. SOME TRICHOMES
MAY ALSO BE HARSH AND STINGING –
STINGING NETTLE (URTICA).
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MODIFIED LEAVES
1. TENDRILS ARE LEAVES MODIFIED FOR
SUPPORT. TENDRILS MAY ALSO BE FORMED BY
MODIFIED STEMS.
2. STIPULES ARE SMALL PAIRED APPENDAGES
AT THE BASE OF THE PETIOLE – SOMETIMES
APPEARING LEAFLIKE, BUT OFTEN NOT. SOME
PLANTS HAVE STIPULES OTHERS DO NOT.
THEY MAY BE LARGE AND PHOTOSYNTHETIC;
MAY BE SPINE-LIKE FOR PROTECTION OR
EVEN FORM TENDRILS.
3. SPINES ARE MODIFIED LEAVES AS IN CACTI.
SPINES CAN BE A FORM OF PROTECTION BUT
ALSO REDUCE WATER LOSS FROM THE PLANT.
4. LEAVES CAN BE MODIFIED TO FORM TRAPS
FOR CARNIVOROUS PLANTS. LEAVES TRAP
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INSECTS TO SUPPLEMENT THE NUTRIENTS
PLANTS REQUIRE (NITROGEN).
5. BUD SCALES ARE TOUGH OVERLAPPING
WATERPROOF LEAVES THAT PROTECT BUDS
FROM FROST, DESSICATION AND PATHOGENS.
6. BRACTS ARE FLORAL LEAVES THAT FORM
AT THE BASE OF FLOWERS OR FLOWER
STALKS; SOMETIMES THEY ARE SMALL AND
PROTECT DEVELOPING FLOWERS; OTHER
TIMES LARGE AND COLORFUL AND AID IN
ATTRACTING POLLINATORS – POINSETTA
(EUPHORBIA), DOGWOOD (CORNUS), INDIAN
PAINTBRUSH (CASTILLEJA).
7. LEAVES MAY BE MODIFIED FOR ASEXUAL
REPRODUCTION – OFTEN SUCCULENT – BY
PRODUCING TINY PLANTLETS AT THEIR
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MARGINS THAT CAN DROP OFF AND BECOME
NEW PLANTS – MATERNITY PLANT
(KALANCHOE), BEGONIA, STONE CROP (SEDUM).
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