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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 1 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 2 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 3 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. 4 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 5 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 6 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. 7 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 8 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 9 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. 10 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 11 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 12 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 13 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. 14 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 15 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 16 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. 17 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). 18 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 19 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 20 MARGINS THAT CAN DROP OFF AND BECOME NEW PLANTS – MATERNITY PLANT (KALANCHOE), BEGONIA, STONE CROP (SEDUM). 21