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PRIMARY GROWTH A PLANT STARTS OFF LIFE AS AN EMBRYO. AS IT GROWS THE ADDITION OF NEW CELLS IS GRADUALLY RESTRICTED TO CERTAIN PARTS OF THE PLANT BODY. AN ADULT PLANT IS A COMPOSITE OF ADULT AND JUVENILE TISSUES. THE PERPETUALLY YOUNG TISSUE – PRIMARILY CONCERNED WITH THE FORMATION OF NEW CELLS – ARE THE MERISTEMS. LIVING TISSUES OTHER THAN MERISTEMS MAY PRODUCE NEW CELLS BUT THE MERISTEMS DO SO INDEFINITELY. MERISTEMS NOT ONLY ADD CELLS TO THE PLANT BODY THEY ALSO PERPETUATE THEMSELVES. GROWTH IN PLANTS GENERALLY CONSISTS OF TWO PARTS: 1. AN 1 INCREASE IN THE NUMBER OF CELLS. 2. AN INCREASE IN CELL SIZE. MERISTEMS OCCUR AT THE APICES OF ALL PLANT SHOOTS AND ROOTS SO THEIR NUMBER ON A SINGLE PLANT MAY BE LARGE. APICAL MERISTEMS PRODUCE PRIMARY TISSUE (GROWTH), EXPANDS THE PLANT BODY (PARTICULARLY IN LENGTH), INCREASES ITS SURFACE (AREA OF CONTACT WITH AIR & SOIL), AND EVENTUALLY PRODUCES THE REPRODUCTIVE ORGANS. TWO OTHER MERISTEMS – THE VASCULAR AND CORK CAMBIA – ARE RESPONSIBLE FOR SECONDARY GROWTH – AN INCREASE IN THE THICKNESS OF THE AXIS. THEY AID IN MAINTENCE OF THE PLANT BODY BY INCREASING THE VOLUME OF 2 THE VASCULAR TISSUE AND FORMING SUPPORTING AND PROTECTING CELLS. THE MERISTEMS AND CAMBIA ARE NOT ALWAYS ACTIVE. THEY CAN SHOW SEASONAL FLUCUATIONS IN THEIR GROWTH, SLOWING DOWN OR STOPPING DURING WINTER. IN SOME PLANTS THE GROWTH OF LATERAL MERISTEMS IS SUPPRESSED AS LONG AS THE MAIN TERMINAL AXIS IS ACTIVELY GROWING. SHOOT APICAL MERISTEMS APICAL MERISTEMS VARY IN MORPHOLOGY AND ACTIVITY IN THE MAJOR TAXA OF VASCULAR PLANTS. ALL CELLS ARE MERISTEMATIC BUT NOT ALL ARE APICAL INITIALS – THE SINGLE CELL(S) THAT ARE THE 3 SOURCE OF ALL OTHER CELLS IN THE SHOOT SYSTEM. MOST SPECIES IN THE FERNS AND THEIR ALLIES (HORSETAILS, ETC.) ARE CHARACTERIZED BY A SINGLE APICAL CELL CONSIDERED TO BE AN APICAL INITIAL – AT THE TOP OF THE MERISTEM. IN GYMNOSPERMS THE APICALMERISTEM IS BOUNDED BY A SINGLE SURFACE LAYER, BELOW WHICH IS A SMALL CLUSTER OF LARGE VACUOLATE CELLS COMPRISING THE CENTRAL MOTHER CELL ZONE. THIS ZONE IS FLANKED BY A PERIPHERAL ZONE COMPRISED OF SMALL CONSPICUOUSLY NUCLEATE AND DENSELYSTAINING CELLS WITH ACTIVE CELL DIVISION. THE PERIPHERAL ZONE ENCLOSES A CENTRAL REGION CALLED THE TRANSITION 4 ZONE (BELOW THE CENTRAL MOTHER CELL ZONE), BELOW WHICH IS A RIB MERISTEM – CONSISTING OF LONGITUDINALLY FILES OF CELLS. THESE GIVE RISE TO THE TISSUES OF THE ADULT PLANT. IN ANGIOSPERMS THE MERISTEM HAS A TUNICA-CORPUS TYPE OF ORGANIZATION. THE APICAL MERISTEM REFERS TO THAT REGION OF THE SHOOT APEX IMMEDIATELY DISTAL TO THE FIRST LEAF PRIMORDIUM. THIS AREA IS ALSO CALLED THE APICAL DOME. THE TUNICA CONSISTS OF THE OUTER ONE OR TWO LAYERS OF CELLS. THESE CELLS DIVIDE IN PLANES PERPENDICULAR TO THE SURFACE OF THE MERISTEM AND CONTRIBUTE TO SURFACE GROWTH. THE CORPUS IS A BODY OF CELLS 5 ENCLOSED BY THE TUNICA; THESE CELLS DIVIDE IN VARIOUS PLANES AND ADD BULK TO THE SHOOT. THE CORPUS CORRESPONDS TO THE CENTRAL MOTHER CELLS OF GYMNOSPERMS AND IS OFTEN CALLED THAT. CELLS OF THE TUNICA FORM THE PROTODERM – WHICH GIVES RISE TO THE EPIDERMIS, TRICHOMES, GUARD CELLS, ETC. LAYERS OF THE TUNICA AND/OR THE CORPUS PARTICIPATE IN THE FORMATION OF THE GROUND MERISTEM AND PROCAMBIUM. THE GROUND MERISTEM PRODUCES PARENCHYMA, COLLENCHYMA, SCLERENCHYMA AND IDIOBLASTS. THIS WILL FORM THE PITH AND CORTEX, IF PRESENT, IN THE ADULT PLANT. 6 THE PROCAMBIUM (=PROVASCULAR STRANDS) IS EMBEDDED IN THE GROUND MERISTEM AND SURROUNDS THE IMMATURE PITH. THIS WILL FORM THE PRIMARY XYLEM AND PRIMARY PHLOEM. IMMEDIATELY BELOW THE CORPUS IS THE PITH MERISTEM – NOT EASY TO SEPARATE FROM THE GROUND MERISTEM AND IS OFTEN INCLUDED WITHIN THAT. CELLULAR DIVISIONS OCCURING IN THE SECOND OR THIRD LAYER OF CELLS EITHER IN THE TUNICA OR CORPUS LEAD TO THE FORMATION OF A LEAF PRIMORDIUM. THESE ADD CELLS TOWARD THE EDGE TO FORM A BUMP – THE PRIMORDIUM. LEAF PRIMORDIA ARISE IN POSITIONS AROUND THE 7 CIRCUMFERENCE OF THE APICAL MERISTEM IN ACCORDANCE WITH THE PHYLLOTAXY OF THE SHOOT. AXILLARY BUDS ORIGINATE AT VARIABLE DISTANCES FROM THE APICAL MERISTEM. FREQUENTLY THEIR INITIATION OCCURS IN RELATION TO THE SECOND LEAF FROM THE APEX. THE DIVISION OF CELLS OCCURS AS THEY DO FOR LEAF PRIMORDIA, BUT IN THE PROCESS THE CELLS BECOME ARRANGED AS THEY ARE IN APICAL MERISTEMS OF THE SHOOT. INTERCALARY MERISTEMS ARE REGIONS OF MERISTEMATIC TISSUE BETWEEN REGIONS OF MATURE, DIFFERENTIATED TISSUE. THEY ARE FOUND IN THE STEMS OF GRASSES AND 8 HORSETAILS (EQUISETUM). IN THESE PLANTS AS DEVELOPMENT PROCEEDS THE FIRST INTERNODES OF THE SHOOT ARE VERY SHORT AND ENTIRELY MERISTEMATIC. FURTHER DEVELOPMENT RESULTS IN MATURE TISSUE IN DISTAL REGIONS WITH MERISTEMATIC TISSUE RESTRICTED TO BASAL REGIONS OF THE INTERNODES. WITH PRODUCTION OF NEW CELLS BY THE INTERCALARY MERISTEMS THE INTERNODES ELONGATE AND THE LEAVES ATTACHED AT THE NODES SEPARATE. THE INTERCALARY MERISTEMS REMAIN THE NODE AND PRODUCE NEW CELLS ON THEIR UPPER MARGIN ONLY. INTERCALARY MERISTEMS ARE ALSO FOUND IN GRASS LEAVES AND PINE NEEDLES. 9 ROOT APICAL MERISTEMS MUCH OF THIS WAS COVERED PREVIOUSLY. GROWING ROOTS HAVE FOUR REGIONS: 1. ROOT CAP – MASS OF PARENCHYMA CELLS COVERING THE TIP OF THE ROOT 2. REGION OF CELL DIVISION = APICAL MERISTEM 3. REGION OF ELONGATION – HERE CELLS ELONGATE GREATLY AND GENERALLY BECOME WIDER 4. REGION OF MATURATION – ROOT HAIR ZONE – AREA WHERE ROOT HAIRS DEVELOP, PRIMARY TISSUES MATURE. THE QUIESCENT CENTER OR ZONE IS A REGION OF LOW CELL DIVISION SURROUNDED BY REGIONS OF ACTIVE CELL DIVISIONS, 10 TYPICALLY FOUND IMMEDIATELY BEHIND THE APEX OF THE APICAL MERISTEM. ROOT APICAL MERISTEMS GIVE RISE TO THE PROTODERM WHICH PRODUCES EPIDERMAL TISSUE; GROUND MERISTEM WHICH PRODUCES THE TISSUES OF THE CORTEX; AND THE PROCAMBIUM WHICH PRODUCES THE PRIMARY XYLEM AND PHLOEM. DIFFERENTIATION WE HAVE TALKED ABOUT MERISTEMATIC CELLS BECOMING PROTODERM AND THEN EPIDERMIS, THE PROCAMBIUM BECOMING PRIMARY XYLEM AND PHLOEM. THIS IS THE PROCESS OF DIFFERENTIATION – THE PHYSIOLOGICAL AND MORPHOLOGICAL 11 CHANGES THAT OCCUR IN A CELL, TISSUE, OR ORGAN DURING DEVELOPMENT FROM A MERISTEMATIC, OR JUVENILE STAGE, TO A MATURE OR ADULT STAGE. THIS IS USUALLY ASSOCIATED WITH AN INCREASE IN SPECIALIZATION. THIS CAN BE LOOKED UPON AS A PROCESS OF 1. BECOMING DIFFERENT FROM THE MERISTEMATIC PRECURSORS AND 2. BECOMING DIFFERENT FROM THE NEIGHBORING CELLS OR TISSUES. THE SECOND STEP IMPLIES THAT SIMILAR MERISTEMATIC CELLS MAY PASS THROUGH DIFFERENT STEPS IN THE THEIR DEVELOPMENT INTO MATURE CELLS, THUS BRINGING ABOUT DIVERSITY OF STRUCTURE 12 IN WHAT WAS INITIALLY RELATIVELY HOMOGENEOUS TISSUE. DIFFERENTIATION VARIES WITH INDIVIDUAL CELLS – SOME BECOME QUITE SPECIALIZED WHILE OTHERS DO NOT. A HIGH DEGREE OF CHANGE OCCURS, FOR EXAMPLE, IN CELLS THAT BECOME XYLEM AND PHLOEM. A LESS PROFOUND CHANGE OCCURS IN CELLS THAT BECOME THE MESOPHYLL OF THE LEAF AND EVEN LESS CHANGE WHEN THE CELL BECOMES NON-PHOTOSYNTHETIC PARENCHYMA. THE CAUSES OF DIFFERENTIATION ARE DIVERSE AND COMPLEX. HOWEVER, IN PLANTS ONE ASPECT OF THIS IS POLARITY – THE ORIENTATION OF ACTIVITIES IN SPACE. 13 WHERE THE CELLS ARE FOUND WITHIN THE PLANT SPATIALLY IS IMPORTANT. THIS ALSO RELATES TO THE PHENOMENON OF GRADIENTS – THE DIFFERENCE BETWEEN REGIONS OF A PLANT BODY APPEAR IN A GRADED SERIES. THERE ARE PHYSIOLOGIC GRADIENTS – RATES OF METABOLIC PROCESSES, CONCENTRATION OF HORMONES, CONCENTRATIONS OF SUGARS, ETC. THERE ARE ALSO GRADIENTS IN ANATOMIC DIFFERENTIATION AND DEVELOPMENT OF EXTERNAL FEATURES OF THE PLANT. AN EXAMPLE IS THE CHANGE IN THE FORM OF SUCCESSIVE LEAVES ALONG THE AXIS FROM JUVENILE TO ADULT. 14 INTERNAL FACTORS MODIFY THE POTENTIALITIES OF A CELL DURING ITS DIFFERENTIATION AND THE MODIFICATIONS MAY BE INDUCED BY CELLS IN BOTH DISTANT AND PROXIMAL POSITIONS. BOTH INDUCTIVE AND REPRESSIVE STIMULI MAY BE RECOGNIZED. THE EFFECTS OF INTERNAL FACTORS CAN BE DIFFICULT TO DISTINGUISH FROM THOSE OF THE EXTERNAL. 15