Download Primary Growth, Meristems

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
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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
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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
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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
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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
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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.
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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
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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
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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.
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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,
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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
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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
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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.
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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.
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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.
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