PLANT SCIENCE

INTRODUCTION

THE PURPOSE OF THIS COURSE IS TO PROVIDE YOU WITH AN OVERVIEW OF THAT GROUP OF ORGANISMS THAT WE CALL, CORRECTLY AND INCORRECTLY, PLANTS.THE OVERVIEW WILL COVER GENERAL ORGANIZATION OF PLANTS, THEIR INTERNAL AND EXTERNAL STRUCTURE AND HOW THIS STRUCTURE ORIGINATES OR DEVELOPS THE DIFFERENT KINDS OF PLANTS, THE
ORGANIZATION OF THE PLANT WORLD THE WAY PLANTS HARVEST AND UTILIZE ENERGY FOR THEIR OWN BENEFIT, AND SECONDARILY TO THE BENEFIT OF HUMANS AND OTHER ANIMALS.
ADDITIONALLY, I WANT YOU TO APPRECIATE THE MAGNIFICENT CONTRIBUTIONS THAT
PLANTS MAKE IN THE CYCLE OF LIFE ON EARTH BY PROVIDING THE OXYGEN THAT ALL AEROBIC ORGANISMS, INCLUDING HUMANS, REQUIRE, REMOVING CARBON DIOXIDE FROM THE ATMOSHPERE, TRANSFORMING LIGHT ENERGY INTO CHEMICAL ENERGY THAT IS THEN USED,
DIRECTLY OR INDIRECTLY, BY MOST OTHER ORGANISMS CONTRIBUTING TO BOTH UTILITARIAN AND AESTHETIC ASPECTS OF HUMAN LIFE. AS A BEGINNING, LETS ESTABLISH THE ORGANISMS GENERALLY TREATED AS COMPOSING THE BOTANICAL WORLD: [HISTORICALLY, LIVING ORGANISMS WERE DIVIDED INTO TWO GROUPS, PLANTS AND ANIMALS, WITH ANYTHING THAT DIDN'T FIT INTO THE ANIMAL WORLD BEING PLACED INTO THAT OF PLANTS;THEREFORE, FUNGI, BACTERIA AND VIRUSES WERE PLACED INTO THE PLANT KINGDOM. W ITH OUR
GENERAL ACCEPTANCE OF THE FIVE KINGDOM WORLD [OR THREE DOMAIN ORGANIZATION] THE
ORGANISMS TRADITIONALLY FORMING THE PLANT KINGDOM NOW FALL INTO FOUR OF THE FIVE
KINGDOMS, MONERA, PROTISTA, FUNGI AND PLANTS. FOR OUR PRESENT DISCUSSION WE'LL NOT BE
CONCERNED WITH THE FORMAL KINGDOM INTO WHICH THE ORGANISMS ARE PLACED, ONLY WITH
THE BASIS THAT THESE ORGANISMS CONSTITUTE THE PLANT WORLD].
PLANT GROUPS
FUNGI – NOW PLACED IN OWN KINGDOM
ALGAE – 9 GROUPS [NOW DISTRIBUTED AMONG MONERA AND PROTISTA]
NON-VASCULAR
PLANTS – [MOSSES AND LIVERWORTS – 3 GPS]
ANTHOCEROPHYTA – HORNWORTS
HEPATOPHYTA – LIVERWORTS
BRYOPHYTA – MOSSES
VASCULAR PLANTS: LYCOPODIOPHYTA — PSILOTUM; EQUISETUM: SCOURING RUSHES;
SPORE PRODUCING LYCOPODIUM — CLUB MOSSES; SELAGINELLA, RESURRECTION PLANT.
ONLY PTERIDOPHYTA – FERNS.
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VASCULAR PLANTS:
SEED PRODUCING
[NON-FLOWERING -- 4GPS]
CYCADOPHYTA
GINKGOPHYTA
CONIFEROPHYTA
GNETOPHYTA
[FLOWERING]
ANTHOPHYTA
THE WHOLE PLANT
JUST AS WE'VE PUT THE PLANT WORLD INTO AN ORGANIZATIONAL PERSPECTIVE,
IT BEHOOVES US TO PUT A TYPICAL PLANT INTO ITS ORGANIZATIONAL PERSPECTIVE.
THE PLANTS THAT I REFER TO AS "TYPICAL" ARE GENERALLY FLOWERING PLANTS. ALL THESE
PLANTS HAVE AN IDENTICAL ORGANIZATIONAL PLAN, ALTHOUGH THE SPECIFIC ORGANS AMONG
FLOWERING PLANTS MAY HAVE A VERY DIFFERENT FORM.
TO PUT THIS ORGANIZATIONAL PLAN INTO PERSPECTIVE WE CAN SAY THAT ALL FLOWERING PLANTS
HAVE SIX ORGANIZATIONAL UNITS:
1. ROOTS--GENERALLY UNDERGROUND AND FUNCTION TO
(A) ANCHOR THE PLANT
(B) ABSORB AND CONDUCT WATER AND DISSOLVED MINERALS TO OTHER
PLANT PARTS.
2. STEMS--GENERALLY ABOVE GROUND AND FUNCTION TO
(A) SUPPORT OTHER ORGANS
(B) CONDUCT FOOD AND WATER TO OTHER PLANT PARTS.
3. LEAVES--GENERALLY ABOVE GROUND AND ALWAYS ATTACHED TO STEM .
FUNCTION IN PRODUCING FOOD THROUGH PHOTOSYNTHESIS. MAJOR
PHOTOSYNTHETIC ORGAN OF THE PLANT.
4. FLOWERS--ABOVE GROUND AND ATTACHED TO SOME PORTION OF STEM.
THEY FUNCTION IN REPRODUCTION.
5. FRUITS--ARISE FROM A PARTICULAR PART OF THE FLOWER, THE OVARY, AND HOUSE THE
SEEDS. OFTEN THEY AID IN SEED DISPERSAL.
6. SEEDS--FOUND IN THE FRUIT. CONTAIN AN EMBRYONIC PLANT WHICH IS MEANT TO
EVENTUALLY BECOME A MATURE PLANT OF THE SAME SPECIES. IT IS BOTH A
DISPERSAL UNIT AND REPRODUCTIVE UNIT. IT ARISES FROM A STRUCTURE CALLED
AN OVULE.
WE CAN RELATE THESE ORGANIZATIONAL FEATURES TEMPORALLY IN A SIMPLE CIRCULAR DIAGRAM
AS FOLLOWS:
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SEED
FRUITS [GERMINATION]
ROOTS
REPRODUCTIVE VEGETATIVE
PHASE STEMS PHASE
[FERTILIZATION]
LEAVES
FLOWERS
SO, THE WHOLE PLANT HAS TWO "PHASES", A VEGETATIVE AND A REPRODUCTIVE PHASE. DURING THE
VEGETATIVE PHASE, THE PLANT BODY "GROWS", INCREASES IN SIZE, MASS OR BOTH. THIS GROWTH IS
FUELED BY PHOTOSYNTHESIS (OCCURS PRIMARILY IN THE LEAVES -REQUIRES CO2 - ABSORBED FROM
THE AIR BY THE LEAVES - GENERALLY, AND H2O -ABSORBED BY ROOTS AND TRANSPORTED THROUGH
ROOTS AND STEMS TO LEAVES - AND THE ENERGY IN SUNLIGHT - TRAPPED BY THE CHLOROPLASTS.
DURING THE VEGETATIVE PHASE THE PLANT IS PREPARING FOR THE REPRODUCTIVE PHASE.
REPRODUCTIVE PHASE BEGINS WHEN FLOWERS ARE PRODUCED -- GROWTH DOESN'T NECESSARILY
END -- AND ENDS WHEN SEEDS ARE PRODUCED.
EXAMINE EACH OF THE ORGANIZATIONAL FEATURES OF PLANTS FOR THEIR OWN EXTERNAL
ORGANIZATION.
ROOTS-- VERY LITTLE EXTERNAL ORGANIZATION TO ROOTS. THEY HAVE NO DEFINITE MERISTIC
[REPEATING] STRUCTURE EXTERNALLY. IN SOME CASES, IN ADDITION TO THEIR FUNCTION OF
ANCHORAGE, ABSORPTION AND CONDUCTION, ROOTS MAY ALSO FUNCTION IN STORAGE OF VARIOUS
PRODUCTS. CARROTS AND RADISHES ARE EXAMPLES OF STORAGE ROOTS -- BUT NOT POTATOS.
STEMS-- STEMS ARE COMPOSED OF NODES AND INTERNODES.
A NODE IS THE PORTION OF THE STEM WHERE A LEAF ATTACHES.
AN INTERNODE IS THE PORTION OF A STEM BETWEEN NODES.
IN ADDITION TO THEIR SUPPORT - CONDUCTION FUNCTION, STEMS MAY BE MODIFIED INTO STORAGE
ORGANS FOR WATER (EG., CACTI) OR FOOD (EG., POTATO, AN UNDERGROUND STEM CALLED A TUBER)
OR BE MODIFIED IN OTHER WAYS (EG., TENDRILS, THORNS). IN SOME CASES A PLANT MAY HAVE
UNDERGROUND STEMS, CALLED RHIZOMES, THAT SPREAD THROUGH THE SUBSTRATE AND FROM
WHICH ABOVE GROUND SHOOTS ARISE, THEREFORE RESEMBLING ROOTS; THEY CAN ALWAYS (NEARLY)
BE IDENTIFIED AS STEMS, HOWEVER, DUE TO THEIR NODE - INTERNODE ORGANIZATION. LET ME GO
BACK QUICKLY AND TELL YOU THAT IN ADDITION TO BEING MODIFIED FOR STORAGE [EITHER FOOD OR
WATER], STEMS MAY BE MODIFIED IN OTHER WAYS, MOST NOTABLY INTO
THORNS -- POINTED BRANCHES
TENDRILS -- TWINING EXTENSIONS MEANT TO PROVIDE SUPPORT
LEAVES - LEAVES ARE GENERALLY ORGANIZED INTO AN EXPANDED, FLAT PORTION TERMED THE BLADE
OR LAMINA, AND A NARROWED ELONGATE PORTION, WHICH ATTACHES THE BLADE TO THE STEM,
TERMED A PETIOLE. WHEN ONE EXAMINES THE POINT OF ATTACHMENT OF A LEAF TO THE STEM, THE
AREA IMMEDIATELY ABOVE THE PETIOLE-STEM JUNCTION IS TERMED THE AXIL OF THE LEAF. ALL
LEAVES WILL HAVE A BUD IN THEIR AXIL. THIS BUD WILL PRODUCE EITHER A FLOWER OR A BRANCH
STEM. THE BUD IS AN IMPORTANT FEATURE BOTH FOR THE PLANT AND FOR HUMANS TRYING TO
DETERMINE WHAT CONSTITUTES A LEAF, SINCE LEAVES CAN BE MODIFIED IN A NUMBER OF WAYS.
USING OUR PREVIOUS EXAMPLE, THE LEAF ILLUSTRATED IS A SIMPLE LEAF, MEANING THAT ITS BLADE IS
NOT DIVIDED INTO INDIVIDUAL PARTS. MANY PLANTS PRODUCE COMPOUND LEAVES, HOWEVER,
MEANING THAT THE BLADE IS DIVIDED INTO INDIVIDUAL PARTS, TERMED LEAFLETS, AND EACH LEAFLET
MAY RESEMBLE IN ORGANIZATION A SIMPLE LEAF, SINCE EACH LEAFLET MAY HAVE AN ATTACHMENT
STRUCTURE TERMED A PETIOLULE. A LEAFLET NEVER HAS A BUD IN ITS AXIL.
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COMPOUND LEAVES CAN BE PINNATELY COMPOUND OR PALMATELY COMPOUND. PINNATELY
COMPOUND LEAVES HAVE AN ELONGATE RACHIS, REPRESENTING THE MID-VEIN, WITH LEAFLETS
ATTACHED ALONG THE RACHIS (FYI-SOME PINNATELY COMPOUND LEAVES HAVE VERY SHORT RACHISES
AND MIMIC PALMATELY COMPOUND LEAVES) WHILE PALMATELY COMPOUND LEAVES HAVE THE
LEAFLETS ATTACHED TO THE APEX OF THE PETIOLE--NO EXTENDED RACHIS.
PINNATELY COMPOUND PALMATELY COMPOUND
PINNATELY COMPOUND LEAVES MAY BE ONCE PINNATELY COMPOUND, TWICE PINNATELY COMPOUND,
THRICE(THREE TIMES) PINNATELY COMPOUND, ETC. THE DEGREE OF COMPOUNDNESS IS DETERMINED
BY THE NUMBER OF BRANCHES ONE ENCOUNTERS BEFORE ARRIVING AT A LEAFLET:
ONCE TWICE THRICE
LEAVES MAY BE MODIFIED IN OTHER WAYS:
SPINE--LEAVES MAY BE MODIFIED EITHER WHOLLY OR IN PART INTO A
SHARP, POINTED STRUCTURE TERMED A SPINE (EG., CACTI, HOLLYS)
TENDRIL--AN ELONGATED, USUALLY TW INING STRUCTURE THAT FUNCTIONS
IN SUPPORT (EG., GRAPES, .
BRACT--USUALLY GREEN, FLATTENED STRUCTURES ATTACHED TO A STEM
AND USUALLY SMALLER THAN THE NORMAL LEAVES. BRACTS WHICH OCCUR IN A
REGULAR POSITION ARE OFTEN NAMED, FOR EXAMPLE:
STIPULE--BRACTS (MODIFIED LEAVES) OCCURRING AT THE BASE OF A LEAF AND
ATTACHED TO THE STEM, AT LEAST PARTIALLY.
LEAVES MAY BE ARRANGED ON THE STEM IN THREE DIFFERENT WAYS, THESE WAYS BEING CALLED THE
LEAF ARRANGEMENT OR PHYLLOTAXY:
(A) ALTERNATE--ONE LEAF PER NODE
(B) OPPOSITE--TWO LEAVES PER NODE
(C) WHORLED--THREE OR MORE LEAVES PER NODE
FLOWERS--FLOWERS ARE COMPOSED OF FOUR SERIES OF PARTS, EVOLUTIONARILY DERIVED FROM
LEAVES. THE SERIES OCCUR IN THE SAME ORDER IN ALL FLOWERS, BUT MODIFICATIONS MAY OBSCURE
THE RELATIONSHIPS IN SOME CASES. THE PARTS ARE, FROM OUTSIDE TO INSIDE:
1. SEPALS--USUALLY GREEN BUT MAY BE MODIFIED INTO BRIGHTLY COLORED
STRUCTURES. FUNCTION TO PROTECT THE OTHER PARTS IN BUD.
2. PETALS--COLORED FLOWER PARTS WHICH FUNCTION TO ATTRACT
POLLINATORS.
3. ANDROECIUM--THE MALE REPRODUCTIVE PARTS COMPOSED OF STAMENS.EACH STAMEN IS
STRUCTURED OF A
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FILAMENT-A USUALLY NARROWED, ELONGATE STRUCTURE
SUPPORTING AN
ANTHER - A SAC-LIKE STRUCTURE AT THE APEX OF THE FILAMENT
[GENERALLY] WITHIN WHICH THE POLLEN GRAINS ARE FORMED; THE
POLLEN GRAINS WILL EVENTUALLY PRODUCE THE SPERM.
4. GYNOECIUM - THE FEMALE REPRODUCTIVE STRUCTURE(S); IT IS FORMED FROM STRUCTURES
CALLED CARPELS THAT HAVE AN EVOLUTIONARY ORIGIN FROM LEAVES; HOWEVER, THE
GYNOECIUM IS COMMONLY REFERRED TO AS A:
PISTIL; CONSISTS OF THREE MORPHOLOGICAL AREAS:
(A) STIGMA--APICAL, USUALLY SOMEWHAT EXPANDED PORTION THAT IS
THE RECEPTIVE SITE FOR POLLEN.
(B) STYLE--ELONGATED INTERMEDIATE PORTION OF PISTIL TOPPED BY
STIGMA AND ATTACHING TO THE OVARY.
(C) OVARY--EXPANDED BASAL PORTION OF PISTIL IN WHICH THE OVULES
DEVELOP; THE OVULES WILL EVENTUALLY PRODUCE THE EGG;
AFTER FERTILIZATION THE OVULES WILL MATURE INTO SEEDS.
THERE ARE MANY MODIFICATIONS IN PRESENCE OR ABSENCE OF PARTS IN A FLOWER,
RELATIONSHIPS [PLACEMENT] OF PARTS ONE TO ANOTHER, SIZE AND COLOR IN FLOWERS, ETC;
MANY OF THE VARIATIONS ARE RELATED TO LIFESTYLE OF A PARTICULAR SPECIES AND WE WILL
DISCUSS THESE ASPECTS LATER IN THE SEMESTER.
FRUITS-- DEVELOP FROM THE OVARY; THEREFORE, A FRUIT IS A RIPENED OVARY. SO TOMATOES ARE
FRUITS. OFTEN BRIGHTLY COLORED, TO ATTRACT DISPERSAL AGENTS (COLOR MAKES FRUIT
STAND OUT AGAINST BACKGROUND), AND OFTEN EDIBLE, (GIVES AGENT A MEAL). MANY
MODIFICATIONS IN STRUCTURE WHICH WE MIGHT DISCUSS AT A LATER DATE BUT NOT
IMPORTANT PRESENTLY.
SEEDS -- DEVELOP FROM THE OVULES AFTER FERTILIZATION. AT MATURITY CONTAINS AN EMBRYONIC
PLANT CAPABLE OF BECOMING A NEW PLANT. MANY VARIATIONS IN STRUCTURE WHICH WE
MIGHT DISCUSS LATER. HOWEVER, ALL SEEDS WILL HAVE
SEED COAT -- PROTECTIVE COVERING WHICH DEVELOPS FROM THE INTEGUMENTS
ENDOSPERM -- FOOD SOURCE FOR THE EMBRYO. [ENDOSPERM MAY OR MAY NOT BE
PRESENT IN THE MATURE SEED AS SUCH; IT CAN BE UTILIZED DURING
DEVELOPMENT OR STORED IN THE COTYLEDONS OF THE EMBRYO]
EMBRYO -- THE IMMATURE PLANT.
THE EMBRYO AND PLANT DEVELOPMENT
WE HAVE NOW DISCUSSED THE GENERAL FORM AND FEATURES OF A FLOWERING PLANT. WE WANT TO
GO ON AND DISCUSS THE METHOD BY WHICH PLANTS ACHIEVE THEIR EXTERNAL FORM AND THEIR
UNDERLYING INTERNAL STRUCTURE. TO UNDERSTAND THIS PROCESS IT SEEMS BEST TO BEGIN WITH
THE MATURE EMBRYO, AS IT'S FOUND IN THE SEED. WE'LL DISCUSS THE PROCESS OF HOW THE
EMBRYO FORMS LATER IN THE SEMESTER. BUT FOR NOW JUST RECALL THAT THIS STRUCTURE IS
FOUND IN THE SEED.
THE MATURE EMBRYO OF FLOWERING PLANTS CONSISTS OF A STEM-LIKE AXIS UPON WHICH ARE
FOUND EITHER ONE OR TWO COTYLEDONS, DEPENDING ON WHETHER THE SPECIES BELONGS TO THE
MONOCOTYLEDONS OR DICOTYLEDONS. THE COTYLEDONS ARE OFTEN CALLED SEED LEAVES AND
FUNCTION IN PROVIDING NUTRITION, EITHER PHOTOSYNTHETICALLY OR DIGESTIVELY, FOR THE
DEVELOPING SEEDLING. AT EITHER END OF THE STEM-LIKE AXIS IS AN APICAL MERISTEM, ONE THE
SHOOT APICAL MERISTEM AND ONE THE ROOT APICAL MERISTEM.
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MERISTEMS ARE FORMED OF PHYSIOLOGICALLY YOUNG CELLS-CAPABLE OF REPEATED MITOTIC
DIVISIONS; THESE CELLS ARE REFERRED TO AS MERISTEMATIC CELLS.
THE APICAL MERISTEM OF THE SHOOT TERMINATES THE
EPICOTYL - THE PORTION OF THE ROOT - SHOOT AXIS ABOVE THE ATTACHMENT POINT OF THE
COTYLEDONS. IN SOME EMBRYOS THE EPICOTYL CONSISTS OF LITTLE MORE THAN AN
APICAL MERISTEM--IN OTHERS THERE ARE ONE OR MORE YOUNG LEAVES. IN THE
LATTER CASE, THE EPICOTYL PLUS ITS YOUNG LEAVES IS REFERRED TO AS
THE PLUMULE.
THE EMBRYONIC AXIS BELOW THE COTYLEDONS IS TERMED THE HYPOCOTYL.
THE LOWER END OF THE HYPOCOTYL MAY CONTAIN AN EMBRYONIC ROOT--TERMED THE
RADICLE -- OR THERE MAY BE ONLY AN APICAL MERISTEM. IF A RADICLE CANNOT BE
DISTINGUISHED, THE EMBRYO AXIS BELOW THE COTYLEDON(S) MAY BE REFERRED TO AS THE
HYPOCOTYL-ROOT AXIS.
THE MATURE EMBRYO CONSISTS THEN OF THE TWO APICAL MERISTEMS, THE COTYLEDONS, [THE
PLUMULE IF YOUNG LEAVES ARE PRESENT], AND THREE PRIMARY MERISTEMS -- MERISTEMS
THAT ARISE DIRECTLY FROM THE ZYGOTE'S DEVELOPMENT INTO AN EMBRYO OR FROM THE TWO
APICAL MERISTEMS OF THE EMBRYO.
THE THREE PRIMARY MERISTEMS ARE:
1. THE PROTODERM -- GIVES RISE TO THE EPIDERMIS;
2. THE GROUND MERISTEM -- GIVES RISE TO THE GROUND TISSUESPITH
AND CORTEX;
3. PROCAMBIUM -- GIVES RISE TO VASCULAR TISSUE.
RESTATE AND SUMMARIZE:
(A) THE EARLY EMBRYO CONSISTS OF A MASS OF ESSENTIALLY UNDIFFERENTIATED CELLS, ALL
CAPABLE OF REPEATED MITOTIC DIVISIONS; EVENTUALLY, AFTER DEVELOPING INTO THE
MATURE EMBRYO, EXCEPT FOR THE COTYLEDONS, THE EMBRYO CONSISTS OF TWO APICAL
MERISTEMS, AND THREE PRIMARY MERISTEMS.
(B) CERTAIN CELLS IN THE APICAL MERISTEMS WILL REMAIN MERISTEMATIC THROUGHOUT THE LIFE
OF THE PLANT, UNLESS INJURED; THEY WILL ALWAYS REMAIN UNDIFFERENTIATED.
(C) CELLS OF THE PRIMARY MERISTEMS WILL UNDERGO MITOTIC DIVISIONS AND EVENTUALLY FORM
DIFFERENTIATED TISSUES, AS LISTED PREVIOUSLY, BUT THE MERISTEMS WILL CONSTANTLY BE
REPLENISHED BY THE APICAL MERISTEM AS THE EMBRYO BECOMES A MATURE PLANT.
(D) TISSUES DERIVED FROM THE PRIMARY MERISTEMS ARE ALWAYS PRIMARY TISSUES. TO PUT
PRIMARY MERISTEMS--PRIMARY TISSUES INTO AN OVERALL PERSPECTIVE AS REGARDS A
MATURE PLANT, ASK YOURSELF THE QUESTION "HOW DOES A PLANT GROW?". DO ALL PLANTS
GROW IN THE SAME WAY?
THE ANSWER IS BOTH YES AND NO. ALL PLANTS EXHIBIT A GROWTH THAT IS CALLED:
PRIMARY GROWTH--DEFINED AS INCREASE IN LENGTH OF THE STEM(S) AND ROOT(S).
THERE IS A SECOND TYPE OF GROWTH EXHIBITED BY SOME– PERHAPS THE MAJORITY OF– PLANTS CALLED:
SECONDARY GROWTH--DEFINED AS INCREASE IN DIAMETER OF THE STEM OR ROOT.
INCREASE IN LENGTH– PRIMARY GROWTH--IS ALWAYS ACCOMPLISHED THROUGH ADDITION OF PRIMARY
TISSUES WHICH ARISE FROM PRIMARY MERISTEMS WHICH ARISE FROM APICAL MERISTEMS.
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INCREASE IN DIAMETER– SECONDARY GROWTH– IS ALWAYS ACCOMPLISHED THROUGH ADDITION OF
SECONDARY TISSUES WHICH ARISE FROM A SECONDARY MERISTEM [= LATERAL MERISTEM]– W HICH
ULTIMATELY HAS ITS ORIGIN IN CELLS PRODUCED BY THE APICAL MERISTEM. (THERE ARE TWO
SECONDARY OR LATERAL MERISTEMS). THEREFORE, PRIMARY MERISTEMS, OF WHICH THERE ARE
THREE, ARE RESPONSIBLE FOR PRIMARY GROWTH--INCREASE IN LENGTH. PLANTS WHICH UNDERGO
NO SECONDARY GROWTH ARE COMPOSED SOLELY OF PRIMARY TISSUES AND PRIMARY MERISTEMS
WHICH ARISE FROM APICAL MERISTEMS.
PLANTS WHICH UNDERGO SECONDARY GROWTH ALSO UNDERGO PRIMARY GROWTH BEFORE
SECONDARY GROWTH OCCURS, AND HAVE BOTH PRIMARY AND SECONDARY TISSUES AND
PRIMARY AND SECONDARY MERISTEMS.
AFTER THE EMBRYO REACHES MATURITY AND BEGINS TO GROW (SEED GERMINATES), ADDITION OF
TISSUES WHICH WILL INCREASE THE SIZE OF THE YOUNG PLANT IS RESTRICTED TO MERISTEMS. IN ALL
MERISTEMS (PRIMARY AND SECONDARY), RECALL, SOME CELLS REMAIN PERPETUALLY
PHYSIOLOGICALLY "YOUNG" AND CAPABLE OF REPEATED MITOTIC DIVISIONS. IN EACH DIVISION, ONE
OF THE TWO CELLS RESULTING WILL ENTER THE PLANT BODY WHILE ONE WILL REMAIN IN THE
MERISTEM. THE CELL REMAINING IN THE MERISTEM IS TERMED AN INITIAL AND THE CELL ENTERING THE
PLANT BODY (IN PRIMARY GROWTH THE CELL "ENTERS" THE PLANT BODY BY WAY OF A PRIMARY
MERISTEM) IS TERMED A DERIVATIVE. THEREFORE, THE APICAL MERISTEM AND ALL SECONDARY
MERISTEMS ARE PERPETUATED BY INITIALS. DERIVATIVES COMMONLY DIVIDE 1 TO SEVERAL TIMES
BEFORE DIFFERENTIATING INTO A SPECIFIC CELL TYPE (PRIMARY MERISTEMS IN GENERAL CONSIST OF
DERIVATIVES FROM THE APICAL MERISTEM).
(DUE TO DIFFICULTY OF ESTABLISHING, IN MANY CASES, THE ACTUAL INITIALS AND THEIR IMMEDIATE
DERIVATIVES, APICAL AND LATERAL MERISTEMS GENERALLY CONSIDERED AS CONSISTING OF INITIALS
AND THEIR IMMEDIATE DERIVATIVES).
IMPORTANT TO UNDERSTAND RELATIONSHIPS OF APICAL MERISTEM AND PRIMARY MERISTEMS AS THEY
RELATE TO GROWTH.
NOW, MENTION BRIEFLY LATERAL MERISTEMS, TO KEEP MERISTEMS TOGETHER:
TWO SECONDARY (OR LATERAL) MERISTEMS:
SECONDARY PHLOEM
ASSOCIATED CELLS
VASCULAR CAMBIUM–
SECONDARY XYLEM
ASSOCIATED CELLS
CORK (PHELLEM)
CORK CAMBIUM – [CALLED PHELLOGEN]
PHELLODERM
THESE MERISTEMS ARE FOUND SEVERAL CELL LAYERS UNDER THE SURFACE OF THE STEM OR ROOT
WITH THE VASCULAR CAMBIUM FARTHEST INSIDE. DISCUSS THESE MERISTEMS IN DETAIL LATER.
PLANT CELLS
GENERAL ORGANIZATION
CELL WALL
PRIMARY--MOSTLY CELLULOSE
SECONDARY--COMPOSED OF CELLULOSE, PECTIN, LIGNIN, HEMICELLULOSE AND OCCASIONALLY
SUBERIN, WAXES, ETC.
CELL MEMBRANE--PLASMALEMMA
VACUOLE--GENERALLY– [SOME NOT VACUOLATED--EG., INITIALS]; CONTAINS H2O, SALTS, SUGARS,
PROTEINS, ANTHOCYANINS, ETC.; BOUNDED BY TONOPLAST.
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NUCLEUS – DOUBLY MEMBRANE BOUNDED; GENETIC INFO-DNA;
NUCLEOLUS- RNA {RRNA} SYNTHESIS.
ENDOPLASMIC RETICULUM – ROUGH– SMOOTH. 3-D NETWORK--TRANSPORT.
MITOCHONDRIA – DOUBLE MEMBRANE--RESPIRATORY FUNCTION-ENERGY PRODUCTION.
LYSOSOMES – ENZYME PACKAGES.
GOLGI APPARATI (DICTYOSOMES)– SYNTHESIS– TRANSPORT (RECENT SUGGESTION OF
ORGANIZATION INTO THREE AREAS)
PLASTIDS– CHLOROPLASTS – PHOTOSYNTHESIS
AMYLOPLASTS – STARCH STORAGE
CHROMOPLASTS – CAROTENOIDS, XANTHOPHYLLS
DIAGRAMMATIC REPRESENTATION OF PLANT CELL -- TYPICAL OF ONLY A DIAGRAM.
CELLS MAY BE LIVING OR DEAD IN THEIR FUNCTIONAL STATE AND MAY LACK ONE, OR MOST OF THE
ABOVE LISTED FEATURES.
GENERAL PLANT CELL TYPES
PARENCHYMA--UNSPECIALIZED; USUALLY THIN WALLED-NO SECONDARY WALL-WITH LIVING
PROTOPLAST AT FUNCTIONAL MATURITY; USUALLY HAS WELL DEVELOPED VACUOLE. BASIC
CELL TYPE FROM WHICH OTHERS MAY DEVELOP.
COLLENCHYMA--OFTEN ELONGATE; LIVING AT FUNCTIONAL MATURITY; PRIMARY WALL THICK;
USUALLY VACUOLATE; THICKENINGS OFTEN UNEQUALLY DISTRIBUTED AND LIMITED TO
ANGLES OF THE CELL; MAIN FUNCTION SUPPORT-MECHANICAL; USUALLY THE FIRST
SUPPORTING TISSUE FOUND IN YOUNG STEMS; CAPABLE OF INCREASING IN LENGTH WITH ITS
THICK WALLS OR THICKENINGS.
SCLERENCHYMA--VARIOUSLY SHAPED BUT OFTEN ELONGATE CELLS WITH DEFINITE THICK
SECONDARY WALLS; USUALLY NO PROTOPLAST AT MATURITY--DEAD. SUPPORT; MAY BE
SHORT AND BRANCHED--SCLEREIDS; MAY OCCUR IN OTHERWISE HOMOGENEOUS TISSUE AS
SCATTERED IDIOBLASTS.
TISSUE SYSTEMS
THE CELLS OCCURRING IN THE PLANT BODY ARE ORGANIZED INTO TISSUE SYSTEMS;
TISSUE SYSTEM -- A TISSUE OR GROUP OF TISSUES ORGANIZED INTO A STRUCTURAL AND FUNCTIONAL
UNIT IN A PLANT OR PLANT ORGAN.
THERE ARE THREE TISSUE SYSTEMS:
1. GROUND TISSUE SYSTEM
2. VASCULAR TISSUE SYSTEM
3. DERMAL TISSUE SYSTEM
EACH SYSTEM IS FORMED OF TISSUES:
TISSUE-- GROUP OF CELLS THAT ARE STRUCTURALLY AND/OR FUNCTIONALLY DISTINCT.
A TISSUE MAY BE
SIMPLE AND COMPOSED OF A SINGLE CELL TYPE
OR
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COMPLEX AND CONTAIN TWO OR MORE CELL TYPES.
THE TISSUES FORMING THE THREE SYSTEMS ARE:
GROUND SYSTEM
(A) PARENCHYMA TISSUE < SIMPLE TISSUES WITH
(B) COLLENCHYMA TISSUE A SINGLE CELL TYPE
(D) SCLERENCHYMA TISSUE – FIBERS, SCLEREIDS [MAY BE COMPLEX]
VASCULAR TISSUE SYSTEM
(E) XYLEM TISSUE COMPLEX TISSUE - 2 OR MORE CELL TYPES
(F) PHLOEM TISSUE COMPLEX TISSUE
DERMAL TISSUE SYSTEM
(G) EPIDERMIS (PRIMARY) - COMPLEX TISSUE
(H) PERIDERM (SECONDARY) - COMPLEX TISSUE
WE'VE PREVIOUSLY DISCUSSED THE BASIC PLANT CELL TYPES AND THE GROUND TISSUE SYSTEM IS
FORMED OF TISSUES COMPOSED OF ONE OR ANOTHER OF THOSE CELL TYPES.
THE TWO REMAINING TISSUE SYSTEMS ARE COMPOSED OF A NUMBER OF CELL TYPES. EXAMINE
COMPONENT TISSUES AND CELLS OF THESE SYSTEMS.
ORGANIZATION OF THE VASCULAR TISSUE SYSTEM
XYLEM TISSUE--PRINCIPAL WATER CONDUCTING TISSUE.
ALSO
(A) TRANSPORTS DISSOLVED MINERALS
(B) STORAGE OF FOOD IN SOME INSTANCES
(C) SUPPORT
MAY BE
(D) PRIMARY--DERIVED FROM PROCAMBIUM
OR
(E) SECONDARY– DERIVED FROM VASCULAR CAMBIUM
COMPOSED OF SEVERAL CELL TYPES:
(F) TRACHEIDS
>PRINCIPAL CONDUCTING CELLS
(G) VESSEL MEMBERS
(H) SCLERENCHYMA MECHANICAL SUPPORT
(I) PARENCHYMA STORAGE--LATERAL TRANSPORT
TRACHEIDS AND VESSEL MEMBERS
ELONGATED CELLS
HAVE SECONDARY WALL
LACK PROTOPLAST AT MATURITY
HAVE PITS IN WALLS
VESSEL MEMBERS ALSO HAVE PERFORATIONS
PIT--A RECESS OR CAVITY IN A CELL WALL WHERE THE SECONDARY WALL DOESN'T FORM.
A PIT FORMS AROUND A PRIMARY PIT FIELD--A THIN AREA IN A PRIMARY CELL WALL OVER
WHICH A SECONDARY WALL ISN'TDEPOSITED.
PIT ORGANIZATION:
MIDDLE LAMELLA
PIT
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PIT MEMBRANE
PRIMARY WALL
SECONDARY WALL
PIT MEMBRANE--CELLULOSE MATRIX; WATER THROUGH BY DIFFUSION – ABSORPTION.
PIT PAIR– PITS OCCURRING AT THE SAME SPOT IN ADJACENT CELL WALLS.
A PERFORATION IS AN ACTUAL "HOLE" IN THE WALL OF A CELL.
PERFORATIONS GENERALLY OCCUR IN THE ENDS OF VESSEL MEMBERS BUT MAY OCCUR ON SIDE
WALLS AS WELL. THE PART OF THE CELL WALL WHERE A PERFORATION OCCURS IS CALLED A
PERFORATION PLATE.
TRACHEIDS -- PITS ONLY.
VESSEL MEMBER– PITS AND PERFORATIONS.
VESSEL MEMBERS ARE JOINED INTO LONG "TUBES" CALLED VESSELS. THEY ARE MORE EFFICIENT
AT TRANSPORTING WATER SINCE NO PHYSICAL BARRIER TO IMPEDE WATER "FLOW". YOU
MIGHT SAY THAT VESSESL ARE MORE EFFICIENT SINCE WATER DOESN'T HAVE TO MAKE
ANY "PIT STOPS" WHILE TRAVELLING THROUGH THEM. HOWEVER, IT IS DOUBTFUL THAT
THE PIT MEMBRANE IS A SIGNIFICANT IMPEDIMENT TO WATER SINCE IT IS HYDROLYZED IN
LATTER STAGES OF DEVELOPMENT AND CONSISTS OF A MESH OF CELLULOSE MICROFIBRILS.
SECONDARY AND LATE FORMED PRIMARY TRACHEIDS AND VESSEL MEMBERS HAVE
SECONDARY WALL ALL OVER THE PRIMARY EXCEPT AT PITS AND PERFORATIONS ;
THEREFORE, THEIR WALLS ARE RIGID AND CANNOT STRETCH. BUT EARLY PRIMARY
XYLEM MUST ELONGATE TO SOME DEGREE, SO SECONDARY WALL APPLIED AS ANNULAR
OR HELICAL THICKENINGS.
XYLEM SCLERENCHYMA– FIBERS
PARENCHYMA– USUALLY NOT ELONGATE--THINK OF AS CUBES; TYPICAL ORGANIZATION.
PHLOEM TISSUE--PRINCIPAL FOOD CONDUCTING TISSUE. MAY BE PRIMARY OR SECONDARY.
CELL TYPES:
SIEVE CELLS
>PRINCIPAL CONDUCTING ELEMENTS [SIEVE ELEMENTS]
SIEVE TUBE MEMBERS
ALBUMINOUS CELLS
> FUNCTION WITH RESPECTIVE SIEVE ELEMENT
COMPANION CELLS
PARENCHYMA CELLS — STORAGE
SCLERENCHYMA CELLS — MECHANICAL--SUPPORT--PROTECTION
SIEVE ELEMENTS:
SIEVE--REFERS TO CLUSTER OF PORES, TERMED SIEVE AREAS, THROUGH WHICH THE
PROTOPLASTS OF ADJACENT SIEVE ELEMENTS ARE INTERCONNECTED.
IN SIEVE CELLS THE PORES ARE NARROW AND SIEVE AREAS ARE RELATIVELY
UNIFORM IN STRUCTURE ON ALL WALLS; USUALLY CONCENTRATED ON OVERLAPPING
END WALLS OF VERTICALLY ADJACENT CELLS.
IN SIEVE TUBE MEMBERS, SIEVE AREAS ON SOME WALLS HAVE LARGER PORES THAN
SIEVE AREAS ON OTHER WALLS OF THE SAME CELL; THE PART OF THE WALL BEARING
THE SIEVE AREAS WITH LARGER PORES IS CALLED A SIEVE PLATE. SIEVE PLATE(S) MAY
OCCUR ON ANY WALL BUT GENERALLY OCCUR ON END WALLS. THEREFORE, SIEVE CELLS
LACK SIEVE PLATES WHILE SIEVE TUBE MEMBERS HAVE THEM.
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SIEVE TUBE MEMBERS OCCUR END-ON-END IN LONGITUDINAL SERIES CALLED SIEVE
TUBES.
SIEVE CELLS ARE THE ONLY FOOD CONDUCTING TISSUE FOUND IN MOST LOWER
PLANTS AND GYMNOSPERMS.
SIEVE TUBE MEMBERS ARE THE ONLY FOOD CONDUCTING CELLS IN ANGIOSPERMS.
CELL WALLS OF SIEVE ELEMENTS ARE PRIMARY;
BOTH CELL TYPES ELONGATED; BOTH HAVE A LIVING PROTOPLAST AT MATURITY BUT
THERE IS NO NUCLEUS WITHIN THE CELL OR ONLY FRAGMENTS OF NUCLEUS PRESENT.
MATURE SIEVE ELEMENTS ALSO LACK A CENTRAL VACUOLE; WHEN YOUNG, SIEVE
ELEMENTS DEVELOP SEVERAL VACUOLES EACH WITH A TONOPLAST. HOWEVER,
TONOPLAST OF VACUOLES EVENTUALLY DEGENERATE AND CYTOPLASM-VACUOLE
SEPARATION NO LONGER EXISTS; BUT, AT MATURITY ALL THE REMAINING CELLULAR
COMPONENTS ARE FOUND ALONG THE WALL, PLASMA MEMBRANE, PLASTIDS, SMOOTH
ER, MITOCHONDRIA. NO GOLGI, RIBOSOMES, MICROTUBULES FOUND IN MATURE CELLS.
STM AND SIEVE CELL HAVE A POLYSACCHARIDE TERMED CALLOSE WHICH OFTEN PLUGS PORES
IN SIEVE AREAS-PLATES AS A RESULT OF WOUNDING-FIXATION. STM ALSO CONTAINS A PROTEIN
CALLED S- OR SLIME-PROTEIN OR P-FOR PHLOEM PROTEIN (FOUND IN DICOTS AND SOME MONOCOTS).
THOUGHT TO "PLUG" SIEVE PLATES ON INJURY--PREVENTING LOSS OF STM CONTENTS OR PREVENTING
ENTRY OF BACTERIA ETC. INTO INJURED AREA.
SIEVE ELEMENTS LACK A NUCLEUS, AT LEAST A FUNCTIONAL ONE, AT MATURITY. HOWEVER, EACH
SIEVE ELEMENT HAS AN ASSOCIATED NUCLEATED CELL:
SIEVE CELL — ALBUMINOUS CELL
SIEVE TUBE MEMBER — COMPANION CELL
THESE ASSOCIATED, NUCLEATED CELLS ARE THOUGHT TO BE INVOLVED, INDEED, REQUIRED, FOR
THE ACTIVE SECRETION OF SUBSTANCES INTO AND THEIR REMOVAL FROM THE RESPECTIVE SIEVE
ELEMENT. WHILE BOTH ALBUMINOUS CELLS AND COMPANION CELLS ARE FUNCTIONALLY
EQUIVALENT, EACH HAS A DIFFERENT ONTOGENETIC RELATIONSHIP WITH ITS ENUCLEATED
ASSOCIATE.
STM SIEVE CELL
SINGLE PRECURSOR CELL TWO PRECURSOR CELLS
| | |
MITOSIS MITOSIS MITOSIS
| | |
COMPANION SIEVE TUBE SIEVE ALBUMINOUS
CELL MEMBER CELL CELL
SIEVE ELEMENT AND ASSOCIATED CELL HAVE NUMEROUS CONNECTIONS VIA PLASMODESMATA. MOST
SIEVE ELEMENTS HAVE A FUNCTIONAL LIFE OF LESS THAN ONE YEAR. SOME CASES KNOWN, EG., TILIA,
WHERE SIEVE TUBE MEMBERS FUNCTION FOR 5-10 YEARS. WHEN SIEVE ELEMENT DIES, ITS NUCLEATED
ASSOCIATE DIES, ALSO.
ORGANIZATION OF DERMAL TISSUE SYSTEM
RECALL, THERE ARE TWO TYPES: PRIMARY AND SECONDARY; EITHER CASE, OUTERMOST LAYER(S) OF
CELLS ON PLANT BODY.
PRIMARY
EPIDERMIS--GENERALLY SINGLE CELL LAYER; SOME CASES MULTIPLE LAYERS OF EPIDERMAL
CELLS – CALLED MULTIPLE EPIDERMIS. IN ANY CASE, CONSTITUTES DERMAL TISSUE SYSTEM OF
LEAVES, FLORAL PARTS, FRUITS, SEEDS, ROOTS AND STEMS UNTIL THEY UNDERGO
CONSIDERABLE SECONDARY GROWTH.
CELL TYPES: MANY TYPES--COMMON ONES:
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EPIDERMAL CELL--VARIABLE IN PRECISE SHAPE BUT USUALLY NOT ELONGATE; FUNCTIONS
TO PROTECT PLANT PART--MECHANICAL-- AND RETARD WATER LOSS; UPPER WALL OFTEN
CUTINIZED IN AERIAL PARTS AND MAY BE FURTHER COVERED WITH A CUTICLE OF CUTIN.
GENERALLY TIGHTLY PACKED CELLS.
GUARD CELLS--REGULATE SIZE OF SMALL OPENINGS CALLED STOMATA (SING., STOMA) AND
REGULATE MOVEMENT OF GASES, INCLUDING WATER VAPOR INTO AND OUT OF AERIAL PARTS;
FOUND ON ALL AERIAL PARTS BUT MOST HEAVILY DISTRIBUTED ON LEAVES.
SUBSIDIARY CELLS--EPIDERMAL CELLS DIFFERING IN SHAPE FROM ORDINARY EPIDERMAL
CELLS AND ASSOCIATED WITH GUARD CELLS.
TRICHOMES--SPECIALIZED APPENDAGES OF THE EPIDERMIS WITH VARIOUS FUNCTIONS; THESE
ARE PLANT “HAIRS”.
SECONDARY
PERIDERM--GENERALLY REPLACES EPIDERMIS IN ROOTS AND STEMS WHICH UNDERGO
EXTENSIVE SECONDARY GROWTH.
CELL TYPES --
CORK [PHELLEM] – DEAD CELLS – HEAVILY SUBERIZED – PROTECTION – PREVENT WATER
LOSS.
CORK CAMBIUM [PHEIIOGEN]– PRODUCES CORK AND PHELLODERM; ARISES FROM
VARIOUS PRECURSORS.
PHELLODERM– PARENCHYMATOUS TISSUE INTERNAL TO THE CORK CAMBIUM.
THE ROOT--PRIMARY STRUCTURE
REITERATE SOME OF OUR PREVIOUS STATEMENTS CONCERNING ROOTS.
1. IN MOST VASCULAR PLANTS THE ROOT CONSTITUTES THE UNDERGROUND PORTION OF THE PLANT.
2. FUNCTION IS ANCHORAGE, ABSORPTION AND TO SOME EXTENT– IN ALL CASES – STORAGE
3. THE FIRST ROOT OF THE PLANT ORIGINATES IN THE EMBRYO--FROM THE RADICLE OR THE
ROOT-HYPOCOTYL AXIS. THIS ROOT IS GENERALLY TERMED A PRIMARY ROOT. IN
GYMNOSPERMS AND DICOTYLEDONS THE PRIMARY ROOT IS A TAPROOT– IT GROWS
STRAIGHT DOWN AND GIVES RISE TO BRANCH ROOTS OR LATERAL ROOTS AS IT GROWS
DOWNWARD. THE OLDER LATERAL ROOTS AS YOU KNOW WILL BE FOUND TOWARDS THE UPPER
PART OF THE PRIMARY ROOT AND THE YOUNGER ONES TOWARD THE ROOT TIP. THE TYPE OF
ROOT SYSTEM WHICH HAS A MAJOR CENTRAL ROOT– IE., A TAPROOT– AND BRANCHES FROM
THE CENTRAL ROOT IS CALLED A TAP ROOT SYSTEM.
4. IN MONOCOTYLEDONS THE PRIMARY ROOT IS USUALLY SHORT LIVED AND REPLACED BY
ADVENTITIOUS ROOTS, NONE OF WHICH IS MORE PROMINENT THAN ANOTHER. THIS TYPE
OF ROOT SYSTEM IS TERMED A FIBROUS ROOT SYSTEM--OR DIFFUSE ROOT SYSTEM.
TAP ROOT SYSTEMS USUALLY PENETRATE THE SOIL FARTHER THAN A FIBROUS ROOT
SYSTEM. PLANTS WITH FIBROUS ROOT SYSTEMS ARE PARTICULARLY SUITED AS GROUND
COVERS BECAUSE THEIR ROOTS ARE SHALLOW AND TEND TO CLING TO SOIL PARTICLES
TIGHTLY – PREVENTING EROSION.
THERE IS A NEED FOR A PLANT TO MAINTAIN A BALANCE BETWEEN THE ABSORBING SURFACE
AREA BELOW GROUND AND THE ABOVE GROUND PHOTOSYNTHETIC SURFACE AREA . IN YOUNG
PLANTS THE BALANCE FAVORS THE ROOT SYSTEM, BUT CHANGES AS THE PLANT AGES TO FAVOR
THE ABOVE GROUND PLANT. TRANSPLANTING – SHOULD CONSIDER THIS FACTOR– NEED ROOT
SYSTEM TO SUPPORT ABOVE GROUND PARTS.
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ORIGIN AND GROWTH OF PRIMARY TISSUES
EXAMINE A L.S. OF A PRIMARY ROOT:
ROOT CAP--MASS OF CELLS THAT PROTECTS THE APICAL MERISTEM--BEHIND IT – AND AIDS
THE ROOT AS IT PENETRATES THROUGH THE SOIL. AS THE ROOT PENETRATES THROUGH THE
SOIL THE CELLS ALONG THE PERIPHERY OF THE CAP SLOUGH OFF AND FORM A SLIMY
COVERING AROUND THE ROOT AND LUBRICATE ITS PASSAGE THROUGH THE SOIL. AS THE
ROOT CAP CELLS ARE SLOUGHED OFF THEY ARE REPLACED BY THE APICAL MERISTEM.
LIFE OF A ROOT CAP CELL IS BETWEEN 4-9 DAYS. SLIME IS PRODUCED IN THE PERIPHERAL
ROOT CAP CELLS; HIGHLY HYDRATED POLYSACCHARIDE– PROBABLY A PECTIC SUBSTANCE–
ACCUMULATED IN GOLGI VESICLES – MOVE TO CELL MEMBRANE – FUSE – DUMP CONTENTS
BETWEEN MEMBRANE AND WALL– EVENTUALLY SLIME GETS TO OUTSIDE.
CAN ALSO IDENTIFY GROWTH REGIONS IN ROOT:
REGION OF CELL DIVISION
APICAL MERISTEM– SMALL CELLS AT APEX OF ROOT– INITIALS AND DERIVATIVES– MANY
SIDED, DENSE CYTOPLASM AND LARGE NUCLEI. ORGANIZATION OF MERISTEM AND NUMBER
OF INITIALS IS VARIABLE BETWEEN SPECIES. THERE ARE, HOWEVER, TWO MAJOR
ORGANIZATIONAL TYPES IN SEED PLANTS:
(A) ALL TISSUES ARISE FROM SAME INITIALS; PROBABLY MOST COMMON
(B) DIFFERENT SET OF INITIALS FOR ROOT CAP, VASCULAR CYLINDER AND CORTEX;
EPIDERMIS ARISES FROM INITIALS PRODUCING THE ROOT CAP OR CORTEX
AT ONE TIME, THE REGION OF INITIALS IN THE MERISTEM WAS THOUGHT TO BE A REGION OF ACTIVE
CELL DIVISION; RECENT STUDIES SHOW THAT THAT ISN'T SO– IN FACT– THERE IS AN AREA TERMED A
QUIESCENT CENTER IN THE MERISTEM– INITIAL AREA – WHERE RELATIVELY LITTLE CELL DIVISION
OCCURS; MOST CELL DIVISIONS OCCUR IN THE "DERIVATIVES" AREA. INITIALS CAN, NEVERTHELESS,
QUICKLY REPOPULATE PRIMARY MERISTEM AREA IF IT IS DAMAGED. MOST CELL DIVISIONS OCCUR
BEHIND THE APICAL MERISTEM THEN, IN THE PRIMARY MERISTEMS. THE REGION OF CELL DIVISION
CONTAINS, THEN, THE APICAL MERISTEM AND THE FREQUENTLY DIVIDING AREA OF THE PRIMARY
MERISTEMS.
REGION OF ELONGATION
BEHIND THE REGION OF CELL DIVISION; NOT SHARPLY DIFFERENTIATED FROM THE FORMER;
USUALLY ONLY A FEW MILLIMETERS IN LENGTH; CELLS ELONGATE AND IN SO DOING RESULT
IN MOST OF THE ELONGATION--INCREASE IN LENGTH-- OF THE ROOT; LITTLE
ELONGATION OF CELLS IN AREA BEHIND THIS REGION. THEREFORE ELONGATION OF ROOT
RESULTS IN ONLY A SMALL PORTION OF THE ROOT CONTINUALLY BEING
FORCED THROUGH THE SOIL.
REGION OF MATURATION
REGION WHERE MOST OF THE CELLS OF THE PRIMARY TISSUES MATURE. ROOT HAIRS
ALSO ARISE IN THIS REGION, LEADING TO THE AREA SOMETIMES BEING TERMED
THE ROOT HAIR REGION.
ILLUSTRATE WITH CS OF PRIMARY ROOT, AT VARIOUS POINTS, THE ORGANIZATION OF ROOT:
CS NEAR CS IN UPPER REGION CS THROUGH REGION
ROOT APEX OF CELL DIVISION OF MATURATION
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PRIMARY STRUCTURE OF MATURE ROOT--CS
THE INTERNAL STRUCTURE-ORGANIZATION OF THE ROOT IS RELATIVELY SIMPLE– PARTICULARLY
WHEN COMPARED TO THE STEM. ORGANIZATION OF TISSUES SHOWS VERY LITTLE DIFFERENCE
FROM ONE LEVEL OF THE ROOT TO ANOTHER.
ILLUSTRATE WITH CS OF MATURE PRIMARY ROOT:
EPIDERMIS: ABSORBS WATER IN YOUNG ROOTS; FUNCTION FACILITATED BY ROOT HAIRS;
ROOT HAIRS ARE TUBULAR EXTENSIONS OF EPIDERMAL CELLS; HAIRS INCREASE GREATLY
THE SURFACE AREA OF THE EPIDERMIS. IN ONE STUDY OF FOUR MONTH OLD RYE PLANTS,
IT WAS ESTIMATED THAT THE ROOTS HAD 14 BILLION ROOT HAIRS--THEY HAD A SURFACE
AREA OF 401 SQUARE METERS– PLACED END TO END THE HAIRS WOULD EXTEND 10,000
KILOMETERS. ROOT HAIRS ARE SHORT LIVED AND CONFINED LARGELY TO REGION OF
MATURATION. NEW ROOT HAIRS ARE PRODUCED AT ABOUT THE SAME RATE THAT OLDER
ROOT HAIRS ABOVE THEM DIE. AS THE GROWING ROOT PENETRATES INTO NEW SOIL,
CONTACTING NEW WATER– NUTRIENT SUPPLIES, NEW HAIRS ARE PRODUCED ALLOWING
THE ROOT TO ABSORB THE NEW WATER. IT IS CLEAR THAT THE NEW AND GROWING ROOTS
ARE THE MAJOR SUPPLIERS OF WATER FOR THE PLANT.
GARDENERS, KEEP THIS IN MIND; REMOVE AS MUCH SOIL AND ROOT AS POSSIBLE
WHEN TRANSPLANTING.
EPIDERMAL CELLS OF ROOT ARE PARENCHYMA CELLS AND CLOSELY PACKED. IN YOUNG
PORTIONS OF THE ROOT, EPIDERMAL CELLS HAVE AT MOST A THIN LAYER OF CUTICLE
WHICH OFFERS LITTLE RESISTANCE TO WATER ABSORPTION.
CORTEX: OCCUPIES BY FAR THE GREATEST AREA OF THE PRIMARY ROOT IN CS. (ACTUALLYTHEGREATEST
VOLUME OF ROOT TISSUE AS WELL). CELLS OF THE CORTEX STORE STARCH
BUT USUALLY LACK CHLOROPLASTS; ROOTS THAT UNDERGO CONSIDERABLE AMOUNTS
OF SECONDARY GROWTH, GYMNOSPERMS AND MOST DICOTS, SHED THEIR CORTEX EARLY;
IN SUCH ROOTS, THE CORTICAL CELLS REMAIN PARENCHYMATOUS. IN MONOCOTS, THE
CORTEX IS RETAINED FOR THE LIFE OF THE PLANT, AND MANY OF THE CORTICAL CELLS
DEVELOP SECONDARY WALLS THAT BECOME LIGNIFIED. REGARDLESS, THE CORTICAL
TISSUE CONTAINS MANY INTERCELLULAR SPACES– AIR SPACES FOR AERATION OF THE
ROOT CELLS. ALSO, CORTICAL CELLS HAVE NUMEROUS CONTACTS WITH ONE ANOTHER,
AND THEIR PROTOPLASTS ARE CONNECTED BY PLASMODESMATA. THEREFORE, MOVEMENT
OF SUBSTANCES ACROSS THE CORTEX MAY BE BY WAY OF THE PROTOPLASTS AND
PLASMODESMATA OR THROUGH THE CELL WALLS.
HOWEVER, SUBSTANCES ENTERING INTO OR EXITING FROM THE VASCULAR CYLINDER OF
A ROOT MUST GO THROUGH THE PROTOPLAST OF THE INNERMOST CORTICAL CELL LAYER.
THIS LAYER IS TERMED THE ENDODERMIS. UNLIKE THE OTHER CELLS OF THE CORTEX, THIS
LAYER – ENDODERMIS– HAS NO SPACES BETWEEN ITS CELLS AND IS COMPACTLY ARRANGED.
MOREOVER, ENDODERMAL CELLS ARE CHARACTERIZED BY A BANDLIKE PORTION OF THE
PRIMARY WALL THAT IS IMPREGNATED WITH A FATTY SUBSTANCE CALLED SUBERIN AND
SOMETIMES LIGNIFIED; THIS BAND-LIKE AREA IS TERMED THE CASPARIAN STRIP. THE
PROTOPLAST OF EACH ENDODERMAL CELL IS FIRMLY ATTACHED TO ITS CASPARIAN STRIP.
STRIP IS ATTACHED TO THE ANTICLINAL WALLS OF EACH ENDODERMAL CELL--WALLS AT
RIGHT ANGLES-- PERPENDICULAR--TO THE SURFACE OF THE ROOT. SINCE THE ENDODERMIS
IS COMPACT IN CELLULAR ARRANGEMENT AND SINCE THE CASPARIAN STRIP IS IMPERMEABLE
TO WATER, ANY SUBSTANCE EXITING THE VASCULAR CYLINDER OR ENTERING THE VASCULAR
CYLINDER THROUGH THE ENDODERMIS MUST PASS THROUGH THE PROTOPLAST OF
ENDODERMAL CELLS, EITHER VIA THE CELL MEMBRANE OR PLASMODESMATA. THEREFORE, THE
ENDODERMAL CELLS EXERCISE SELECTIVE CONTROL OVER WHAT ENTERS OR EXITS THE
VASCULAR CYLINDER. [SEVERAL EXPERIMENTS HAVE BEEN COMPLETED RELATING TO THE
CAPACITY OF THE ENDODERMIS TO REGULATE PASSAGE OF SUBSTANCES IN THE ROOT. ONE,
USING ROOTS OF CORN, INVOLVED THE PASSAGE OF THE ELEMENT LANTHANUM, A POSITIVELY
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CHARGED ION THAT CANNOT PENETRATE CELL MEMBRANES; DID ENTER THE ROOT VIA CELL
WALLS BUT WAS STOPPED BY THE ENDODERMIS--VISUALIZED BY TEM.]
IN ROOTS THAT UNDERGO EXTENSIVE SECONDARY GROWTH, THE CORTEX AND ITS
ENDODERMIS ARE LOST. IN THOSE THAT RETAIN THE CORTEX, ENDODERMAL CELL WALLS
EVENTUALLY ARE COMPLETELY SUBERIZED ON THEIR INTERNAL WALL SURFACES, AND
SECONDARY WALL IS THEN LAID DOWN. THESE CHANGES BEGIN IN THE ENDODERMAL
AREAS ADJACENT TO THE PRIMARY PHLOEM STRANDS AND PROCEED TO THOSE CELLS
ADJACENT TO THE PROTOXYLEM. IN SOME ENDODERMAL CELLS OVER THE PROTOXYLEM,
THESE CHANGES MAY NOT OCCUR– CELLS REMAIN THIN WALLED AND WITH A FUNCTIONAL
CASPARIAN STRIP; THESE CELLS ARE TERMED PASSAGE CELLS. IN MOST SPECIES, PASSAGE
CELLS EVENTUALLY BECOME SUBERIZED.
VASCULAR CYLINDER: CONTAINS THE VASCULAR TISSUE– XYLEM AND PHLOEM–
AND IS COMPLETELY SURROUNDED BY ONE OR MORE LAYERS OF CELLS TERMED THE
PERICYCLE – ARISES FROM THE PROCAMBIUM AND, THEREFORE, IS THE OUTERMOST
LAYER OF THE VASCULAR CYLINDER.
IMPORTANT TISSUE WITH SEVERAL IMPORTANT FUNCTIONS:
1. IN MOST SEED PLANTS, LATERAL ROOTS ARISE IN THE PERICYCLE
2. IN PLANTS WITH SECONDARY GROWTH, PERICYCLE CONTRIBUTES TO THE
VASCULAR CAMBIUM
3. PERICYCLE USUALLY GIVES RISE TO THE FIRST CORK CAMBIUM
4. MAY PRODUCE MORE PERICYCLE
[IN YOUNG ROOT TISSUE, PERICYCLE CONSISTS OF PARENCHYMA CELLS WITH PRIMARY WALLS;
AS ROOT AGES, CELLS OF PERICYCLE MAY DEVELOP SECONDARY WALLS.]
CENTER OF VASCULAR CYLINDER OF MOST ROOTS IS OCCUPIED BY A SOLID CORE OF PRIMARY
XYLEM FROM WHICH RIDGELIKE PROJECTIONS EXTEND TOWARD THE PERICYCLE. BETWEEN
THE XYLEM RIDGES ARE FOUND THE PRIMARY PHLOEM STRANDS. THE NUMBER OF XYLEM
RIDGES MAY VARY FROM SPECIES TO SPECIES AND EVEN WITHIN A XYLEM COLUMN AT
DIFFERENT LEVELS WITHIN A ROOT. IF TWO RIDGES OF XYLEM ARE PRESENT, THE ROOT IS
SAID TO BE DIARCH, THREE, TRIARCH, FOUR, TETRARCH, IF MANY RIDGES, POLYARCH.
THE FIRST XYLEM ELEMENTS TO MATURE IN A ROOT ARE LOCATED NEXT TO THE PERICYCLE AND
ARE CALLED PROTOXYLEM; THE APEX OF THE XYLEM RIDGES ARE OFTEN CALLED PROTOXYLEM
POLES. THE METAXYLEM OCCUPIES THE INNER PORTIONS OF THE RIDGES AND CENTRAL
PORTION OF THE CYLINDER AND MATURES AFTER THE PROTOXYLEM. THE ROOTS OF DICOTS
GENERALLY LACK A PITH; ROOTS OF SOME MONOCOTS HAVE A PITH--SOME BOTANISTS
INTERPRET THE PITH AS POTENTIAL XYLEM TISSUE.
BETWEEN THE PHLOEM STRANDS AND THE XYLEM ARE FOUND "PROCAMBIAL CELLS" WHICH
DO NOT DIFFERENTIATE. THEY ARE ESSENTIALLY PARENCHYMA CELLS. IMPORTANT LATER.
LATERAL ROOTS: IN MOST SEED PLANTS-ORIGINATE IN PERICYCLE; GENERALLY OCCURS
IN PERICYCLE SOME DISTANCE BEHIND THE REGION OF ELONGATION WHERE CELLS
ARE PARTIALLY OF FULLY DIFFERENTIATED. IN ANGIOSPERMS, CELLS OF BOTH THE
PERICYCLE AND ENDODERMIS CONTRIBUTE TO LATERAL ROOT PRIMORDIUM
ALTHOUGH THE ENDODERMAL DERIVATIVES ARE SHORT LIVED. AS THE LATERAL
ROOT PRIMORDIUM ENLARGES, IT PUSHES THROUGH THE CORTICAL TISSUES, POSSIBLY
SECRETING ENZYMES THAT DIGEST THE CORTICAL TISSUE LYING IN ITS PATH. THE
YOUNG LATERAL ROOT DEVELOPS A ROOT CAP AND APICAL MERISTEM AND THE
PRIMARY MERISTEMS APPEAR. INITIALLY THE VASCULAR TISSUE OF
THE PRIMARY ROOT AND LATERAL ROOT ARE NOT CONNECTED; THE
TWO CYLINDERS ARE JOINED LATER BY DIFFERENTIATION OF INTERVENING
PARENCHYMA CELLS INTO XYLEM AND PHLOEM.
AERIAL ROOTS: ADVENTITIOUS ROOTS PRODUCED FROM ABOVE GROUND PARTS.
AERIAL ROOTS OF SOME PLANTS ARE PROP ROOTS– FOR SUPPORT– CORN–
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BUT WHEN COME INTO CONTACT WITH SOIL BRANCH AND ABSORB AS
NORMAL ROOTS.
ROOTS REQUIRE OXYGEN FOR RESPIRATION--MOST ROOTS CAN'T GROW IN SOIL LACKING AIR
SPACES-EG., POORLY DRAINED SOIL. SOME PLANTS THAT GROW IN SWAMPS OR AREAS
TYPICALLY UNDER WATER DEVELOP ROOTS THAT GROW OUT OF THE WATER AND SERVE NOT
ONLY TO ANCHOR BUT ALSO TO AEREATE THE SUBMERGED TISSUES.
AVICENNIA, MANGROVE, DEVELOPS EXTENSIONS THAT GROW ABOVE THE SOIL AND
PROVIDE FOR AERATION. TISSUES LOOSELY PACKED– MANY INTERCELLULAR SPACES.
CALLED PNEUMATOPHORES.
SPECIAL ADAPTATIONS: EPIPHYTES--ROOTS MUST ABSORB WATER IN LIQUID FORM--CAN'T
GET IT FROM AIR UNLESS WATER CONDENSES ON ROOT OR SURFACE ROOT
CONTACTS. ONE PLANT, FLOWER POT PLANT– EPIPHYTE– SOME LEAVES FORM
INTO A HOLLOW STRUCTURE THAT CATCHES RAINWATER AND DEBRIS. ANT COLONIES
LIVE IN THE POTS AND ENRICH THE NITROGEN CONTENT OF THE POT. ADVENTITIOUS
ROOTS PRODUCED AT THE NODE ABOVE THE POT LEAF GROW DOWN INTO THE POT
AND ABSORB THE WATER AND MINERALS CONTAINED THEREIN.
PRIMARY STRUCTURE-DEVELOPMENT OF THE STEM
ALREADY FAMILIAR WITH THE ORGANIZATION OF THE EMBRYONIC SHOOT:
PLUMULE-CAN BE THOUGHT OF AS THE FIRST BUD OF PLANT– CONTAINS AN
EPICOTYL (STEM), ONE OR MORE LEAF PRIMORDIA (RUDIMENTARY LEAVES)
AND APICAL MERISTEM. ON GERMINATION OF SEED, EMBRYO ELONGATES
AND SHOOT DIFFERENTIATES INTO NODES AND INTERNODES; NEW LEAVES
DEVELOP FROM APICAL MERISTEM AND BUD PRIMORDIA FORM IN AXILS OF
LEAVES. EVENTUALLY, THESE BUD PRIMORDIA DEVELOP IN ESSENTIALLY
THE SAME PATTERN AS THE FIRST BUD. THIS PATTERN IS REPEATED OVER
AND OVER AS THE SHOOT SYSTEM DEVELOPS.
ORIGIN AND GROWTH OF THE PRIMARY TISSUES OF THE STEM
APICAL MERISTEM OF SHOOT MORE COMPLEX THAN THAT OF
ROOT. SHOOT APICAL MERISTEM NOT ONLY ADDS TISSUE TO
THE DEVELOPING STEM BUT ALSO PRODUCES:
LEAF PRIMORDIA
BUD PRIMORDIA
SHOOT AM ALSO LACKS A PROTECTIVE COVERING AS FOUND
IN ROOT AM.
MOST SHOOT AMS IN FLOWERING PLANTS EXHIBIT
TUNICA-CORPUS
TYPE ORGANIZATION.
TUNICA– OUTERMOST LAYER OR LAYERS; CELLS DIVIDE AT RIGHT ANGLES TO SURFACE OF
MERISTEM; CONTRIBUTES PRIMARILY TO SURFACE GROWTH.
CORPUS-BODY OF CELLS ENCLOSED BY THE TUNICA LAYER(S); CELLS DIVIDE IN VARIOUS
PLANES--CONTRIBUTE TO BULK OF DEVELOPING SHOOT. CORPUS AND EACH TUNICA
LAYER HAVE OWN INITIALS.
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IN MANY ANGIOSPERMS, BULK OF CORPUS CONTAINS HIGHLY VACUOLATED CELLS– CALLED CENTRAL
MOTHER CELLS, CMCS.
CENTRAL MOTHER CELLS SURROUNDED BY — PERIPHERAL MERISTEM— WHICH ARISES PARTLY
FROM TUNICA ANDPARTLY FROM CORPUS (=CMCS). BENEATH THE CMCS IS THE – PITH
MERISTEM.
CELL DIVISION INFREQUENT IN CMCS BUT PERIPHERAL ZONE VERY ACTIVE. REMINISCENT OF
QUIESCENT REGION IN ROOT APICAL MERISTEM.
PROTODERM ALWAYS ARISES FROM OUTERMOST TUNICA LAYER.
PROCAMBIUM AND PART OF THE GROUND MERISTEM (TO BECOME CORTEX AND PART OF THE PITH)
DERIVE FROM PERIPHERAL MERISTEM.
REST OF GROUND MERISTEM (ALL OR MOST OF THE PITH) ORIGINATES FROM PITH MERISTEM.
YOUNG STEM TIP CANNOT BE DIVIDED INTO GROWTH REGIONS AS CAN THE ROOT, ALTHOUGH STEM TIP
PASSES THROUGH GROWTH STAGES SIMILAR TO THE ROOT. IN ACTIVE GROWTH, STEM APICAL
MERISTEM RAPIDLY PRODUCES LEAF PRIMORDIA AND NODE- INTERNODES CANNOT BE DISTINGUISHED.
GRADUALLY, INTERNODES BEGIN TO ELONGATE AND POINT ON STEM TO WHICH LEAVES ATTACH
BECOME DISCERNABLE--NODE-INTERNODE ORGANIZATION BECOMES APPARENT. THEREFORE,
INCREASE IN LENGTH OF STEM IS DUE MAINLY TO INTERNODE ELONGATION. COMMONLY,
MERISTEMATIC ACTIVITY LEADING TO INTERNODE ELONGATION IS MORE INTENSE AT BASE OF
INTERNODE THAN ELSEWHERE. IF INTERNODE ELONGATION TAKES PLACE OVER PROLONGED PERIODS,
THE BASAL MERISTEMATIC REGION IS TERMED AN INTERCALARY MERISTEM. CERTAIN ELEMENTS OF
THE PRIMARY XYLEM AND PHLOEM DIFFERENTIATE WITHIN THE INTERCALARY MERISTEM AND CONNECT
THE MORE HIGHLY DIFFERENTIATED REGIONS OF THE STEM ABOVE AND BELOW THE MERISTEM.
AS IN THE ROOT, THE PRIMARY TISSUES ARISE FROM THEIR RESPECTIVE PRIMARY MERISTEMS.
PRIMARY STRUCTURE OF THE STEM
WE COULD GO THROUGH AND DISCUSS THE DEVELOPMENT OF EACH CELL-TISSUE TYPE IN STEM
MATURATION--SERVE NO PURPOSE. CELLS-TISSUES DEVELOP IN MUCH THE SAME MANNER AS
THOSE OF THE ROO T— ALTHOUGH NOT SO CLEARLY ZONED IN STEM.
CELLS THAT COMPOSE THE VARIOUS TISSUES ARE THE SAME AS THOSE FORMING THE SAME
TISSUE IN THE ROOT.
EG; VASCULAR TISSUE –
PHLOEM -- COMPOSED OF SIEVE TUBE MEMBERS AND COMPANION CELLS
OR
SIEVE CELLS AND ALBUMINOUS CELLS
AND
PARENCHYMA CELLS, SCLERENCHYMA CELLS
XYLEM -- COMPOSED OF VESSEL MEMBERS, TRACHEIDS, PARENCHYMA
AND SCLERENCHYMA
SUFFICE IT TO SAY– TISSUES FOUND ARE:
1. EPIDERMIS– COMPACT CELLS; CUTICLE LAYER; SOME STOMATES BUT NOT AS MANY AS
LEAVES.
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2. CORTEX– PARENCHYMA; COLLENCHYMA; PERHAPS SOME SCLERENCHYMA; PARENCHYMA
CELLS LOOSELY PACKED WITH NUMEROUS INTERCELLULAR SPACES FOR GAS
EXCHANGE-AERATION. FUNCTION AS STORAGE AREAS, ETC.
3. VASCULAR TISSUE– TYPICAL CELLS-TISSUES
4. PITH – PARENCHYMA – STORAGE
MUCH VARIATION IN PRIMARY ORGANIZATION OF STEMS OF SEED PLANTS, BUT THREE BASIC TYPES OF
ORGANIZATION CAN BE RECOGNIZED:
1. PROCAMBIUM AND THEREFORE THE PRIMARY VASCULAR TISSUE APPEARS AS A MORE OR
LESS CONTINUOUS HOLLOW CYLINDER WITHIN THE GROUND TISSUE; SOME CONIFERS AND
DICOTS; THE OUTER REGION OF GROUND TISSUE IS CALLED THE CORTEX – THE INNER REGION–
PITH.
2. PROCAMBIUM AND PRIMARY VASCULAR TISSUES DEVELOP AS A CYLINDER OF DISCRETE
STRANDS SEPARATED FROM ONE ANOTHER BY GROUND TISSUE; GROUND TISSUE SEPARATING
THE VASCULAR BUNDLES IS CONTINUOUS WITH THE CORTEX AND PITH AND IS CALLED –
INTERFASCICULAR PARENCHYMA; INTERFASCICULAR REGIONS ARE OFTEN TERMED-- PITH RAYS;
SOME CONIFERS AND DICOTS.
3. PROCAMBIUM AND VASCULAR TISSUES DEVELOP AS MORE THAN ONE RING OF BUNDLES OR,
MORE GENERALLY, AS A SYSTEM OF STRANDS SCATTERED THROUGHOUT THE GROUND TISSUE;
IN THIS LATTER INSTANCE THE GROUND TISSUE CANNOT BE DISTINGUISHED AS CORTEX AND
PITH; FEW HERBACEOUS DICOTS AND MOST MONOCOTS.
IN STEMS WITH THE FIRST TWO TYPES OF ORGANIZATION, THERE ARE PROCAMBIAL CELLS
WHICH DO NOT MATURE; AFTER PRIMARY XYLEM AND PHLOEM DIFFERENTIATE, THESE
PROCAMBIAL CELLS DIVIDE AND GIVE RISE TO SOME DEGREE OF SECONDARY GROWTH EVEN IN
ENTIRELY HERBACEOUS PLANTS; IE., THE PROCAMBIAL CELLS FORM A CAMBIUM. IN PLANTS IN
WHICH THE VASCULAR TISSUE CONSISTS OF DISCRETE BUNDLES AND IN WHICH CAMBIAL
ACTIVITY OCCURS THE BUNDLES ARE SAID TO BE OPEN BUNDLES.
IN THE BUNDLES OF MONOCOTS AND A FEW HERBACEOUS DICOTS, (RANUNCULUS, E.G.), WHERE
ALL PROCAMBIAL CELLS DIFFERENTIATE INTO XYLEM OR PHLOEM, THE BUNDLES ARE REFERRED
TO AS CLOSED BUNDLES.
RELATION BETWEEN VASCULAR TISSUE OF STEM AND LEAF
VASCULAR TISSUE OF STEMS (BRANCHES) AND LEAVES MUST BE CONTINUOUS SO FOOD
MANUFACTURED IN LEAF CAN BE TRANSPORTED TO OTHER PARTS OF PLANT AND SO WATER AND
DISSOLVED MINERALS CAN GET FROM ROOTS TO THE LEAVES AND BRANCHES.
SO, WHEN WE EXAMINE THE ORGANIZATION OF THE VASCULAR TISSUE IN THE STEM WE CAN SEE
STRANDS OF VASCULAR TISSUE "BRANCHING OFF" FROM THE MAIN STEM BUNDLES TO FEED THE
LEAVES AND BUDS (WHICH WILL BECOME BRANCHES OR FLOWERS).
THE STRANDS OF VASCULAR TISSUE THAT BRANCH OFF TO FEED A LEAF OR BUD ARE CALLED TRACES;
LEAF TRACES IF THEY ENTER A LEAF
OR
BRANCH TRACES IF THEY ENTER A BUD.
WHEN THESE TRACES DEPART FROM THE VASCULAR BUNDLE OF A STEM THEY LEAVE A "SPACE" IN THE
VASCULAR TISSUE OF THE STEM AT THE POINT WHERE THEY BRANCH OFF THAT IS FILLED BY GROUND
TISSUE; THIS "SPACE" IS REFERRED TO AS A GAP;
A LEAF GAP IF THE SPACE IS CAUSED BY THE DIVERGENCE OF TRACES TO A LEAF,
OR
A BRANCH GAP IF THE SPACE IS DUE TO DIVERGENCE TO A BUD.
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LEAF TRACES MAY DIVERGE DIRECTLY AT A NODE, OR THEY MAY ARISE SEVERAL NODES BELOW THE
NODE AT WHICH THEY ENTER A LEAF; SO, LEAF TRACES MAY VARY IN LENGTH.
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BIOLOGY 3327
SAMPLE QUESTIONS FOR EXAM 1
1. WHICH OF THE FOLLOWING WOULD BE LACKING IN A WOODY ROOT? (A) ENDODERMIS (B) CORK
CAMBIUM (C) VASCULAR CAMBIUM (D) CORK (E) PERIDERM.
2. WHICH OF THE FOLLOWING IS A PRIMARY MERISTEM? (A) PITH (B) CORTEX (C) PROTODERM
(D) A&B (E) A,B&C.
3. WHICH OF THE FOLLOWING WOULD BE ASSOCIATED WITH A COMPANION CELL?
(A) PERFORATIONPLATE (B) XYLEM VESSEL (C) SIEVE CELL (D) ANNULAR THICKENINGS (E) SIEVE
TUBE MEMBER.
4. WHICH OF THE FOLLOWING CELLS WOULD HAVE ONLY A THIN PRIMARY WALL? (A) PARENCHYMA
(B) A DERIVATIVE (C) AN INITIAL (D) COMPANION CELL (E) ALL THE ABOVE.
5. WHICH OF THE FOLLOWING WOULD BE LACKING IN A WOODY STEM? (A) CORTEX (B) CORK
CAMBIUM (C) VASCULAR CAMBIUM (D) CORK (E) PERIDERM.
6. THE TISSUE CONTAINING THE PHELLOGEN AND PHELLODERM IS THE (A) PERICYCLE (B) XYLEM
(C) PHLOEM (D) PERIDERM (E) PITH.
7. A PLANT WITH A WHORLED LEAF ARRANGEMENT WOULD HAVE HOW MANY LEAVES AT EACH
NODE? (A) ONE (B) TWO (C) THREE OR MORE.
8. ASSUME YOU HAVE A LARGE, MATURE ROOT CONSISTING ONLY OF PRIMARY TISSUES. A LINE
EXTENDS FROM THE CENTER OF THE ROOT, BETWEEN THE ARMS OF THE XYLEM, TO THE
OUTSIDE SOIL AROUND THE ROOT. OF THE FOLLOWING TISSUES, AND MOVING FROM THE
CENTER OF THE ROOT OUTWARD, WHICH WOULD THE LINE ENCOUNTER FIRST?
(A) ENDODERMIS (B) CORTEX (C) PERICYCLE (D) PHLOEM (E) EPIDERMIS.
9. WHICH OF THE FOLLOWING PRODUCES THE PRIMARY MERISTEMS: (A) APICAL MERISTEM
(B) CORTEX (C) PITH (D) PROCAMBIUM (E) PROTODERM.
10. WOOD IS COMPOSED OF (A) XYLEM AND PHLOEM (B) CORTEX AND PHLOEM (C) BARK
(D) SECONDARY XYLEM (E) SECONDARY PHLOEM.
11. WHICH OF THE FOLLOWING WOULD BE CHARACTERISTIC OF A SIEVE CELL: (A) PERFORATION
PLATE (B) SIEVE PLATE (C) SIEVE AREA (D) ANNULAR THICKENINGS (E) THICK SECONDARY WALL.
12. PLANTS IN WHICH ALBUMINOUS CELLS ARE FOUND WOULD (A) BE SOFTWOODS (B) BE
HARDWOODS, IF WOODY (C) HAVE VESSEL ELEMENTS (D) A&C (E) B&C.
13. IF YOU WERE EXAMINING A PLANT STRUCTURE COMPOSED OF NODES AND INTERNODES YOU
WOULD BE EXAMINING A (A) ROOT (B) LEAF (C) FLOWER (D) SEED (E) STEM.
14. A PRIMARY PIT FIELD IS (A) A THIN AREA IN A PRIMARY WALL (B) AN AREA OVER WHICH
SECONDARY WALL WILL NOT BE DEPOSITED (C) THE AREA OF A PRIMARY WALL OVER WHICH
SECONDARY WALL WILL BE DEPOSITED (D) A&B (E) A&C.
15. IN WHICH OF THE FOLLOWING IS A QUIESCENT REGION FOUND: (A) STEM APICAL MERISTEM
(B) CORTEX (C) PROCAMBIUM (D) ROOT APICAL MERISTEM (E) PROTODERM.
16. WHICH OF THE FOLLOWING WOULD BE ABSENT IN PLANTS CATEGORIZED AS SOFTWOODS:
(A) TRACHEIDS (B) VESSELS (C) RAY PARENCHYMA (D) FUSIFORM INITIALS (E) PHLOEM.
17. IN A STEM [TREE TRUNK] THAT IS 150 YEARS OLD, WHICH OF THE PRIMARY TISSUES MIGHT STILL
BE FOUND? (A) PHLOEM (B) CORTEX (C) EPIDERMIS (D) XYLEM (E) INTERFASCICULAR
PARENCHYMA.
18. THE GYNOECIUM REFERS TO THE (A) ANTHERS (B) PETALS (C) SEPALS (D) CARPELS (E) XYLEM.
19. IF THE MALE PARTS OF A FLOWER WERE ABSENT THE FLOWER WOULD BE LACKING (A) THE
GYNOECIUM (B) THE PETALS (C) THE SEPALS (D) THE ANDROECIUM.
20. IN AN EMBRYO THE RADICLE GIVES RISE TO (A) THE STEM (B) LEAVES (C) COTYLEDONS (D) THE
ROOT (E) THE STEM APICAL MERISTEM.