* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download STEMS PLANT BODIES ARE COMPOSED OF CELLS, TISSUES
Survey
Document related concepts
Plant nutrition wikipedia , lookup
Plant secondary metabolism wikipedia , lookup
Plant physiology wikipedia , lookup
Plant stress measurement wikipedia , lookup
Ornamental bulbous plant wikipedia , lookup
Plant reproduction wikipedia , lookup
Venus flytrap wikipedia , lookup
Flowering plant wikipedia , lookup
Plant morphology wikipedia , lookup
Evolutionary history of plants wikipedia , lookup
Perovskia atriplicifolia wikipedia , lookup
Transcript
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 1 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 2 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 3 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 4 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 5 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 6 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 7 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. 8 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. 9 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. 10 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 11 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. 12 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 13 VASCULAR CYLINDER CALLED THE LEAF TRACE LACUNAE (=LEAF GAP). 14 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. 15 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. 16 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. 17 18 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. 19 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. 20 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. 21 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 22 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 23 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. 24 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. 25 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. 26 27