Download FLOWERS, ETC

FLOWERS, ETC.
SEXUAL REPRODUCTION IN THE KINGDOM
PLANTAE IS RELATIVELY COMPLEX,
INVOLVING A LIFE CYCLE CONSISTING OF
TWO PHASES, A DIPLOID SPOROPHYTE AND A
HAPLOID GAMETOPHYTE PHASE, COMPRISING
WHAT IS CALLED AN ALTERNATION OF
GENERATIONS. THE DIPLOID BODY OR
SPOROPHYTE DEVELOPS FROM A ZYGOTE AND
EVENTUALLY PRODUCES, BY MITOSIS, SPORE
MOTHER CELLS, EACH OF WHICH UNDERGOES
MEIOSIS PRODUCING FOUR SPORES. THE
SPORES ARE HAPLOID AND DEVELOP INTO
HAPLOID BODIES CALLED GAMETOPHYTES.
THESE EVENTUALLY FORM SEX ORGANS IN
WHICH GAMETES ARE PRODUCED BY MITOSIS.
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THE GAMETES UNITE (FERTILIZATION)
FORMING A DIPLOID ZYGOTE. THIS BASIC
PLAN CAN BE SEEN IN THE PROTISTS AND
FUNGI BUT NOT IN ANIMALS.
IN THE PLANT KINGDOM THE GAMETOPHYTE
OF PRIMITIVE SPECIES CONSTITUTE A LARGE
PART OF THE VISIBLE PLANT BODY (THE
SPOROPHYTE MAY CONSIST ONLY OF THE
ZYGOTE), BUT AS THAT PROGRESSES TOWARD
MORE COMPLEX PLANTS THE GAMETOPHYTE
BECOMES REDUCED IN SIZE UNTIL IT MAY BE
ONLY MICROSCOPIC. THE SPOROPHYTE THEN
IS THE VISIBLE PLANT BODY.
ALTERNATION OF GENERATIONS CAN BE
SUMMARIZED AS:
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1. THE FIRST CELL OF ANY GAMETOPHYTE
GENERATION IS NORMALLY A SPORE AND THE
LAST CELL IS NORMALLY A GAMETE.
2. ANY CELL OF A GAMETOPHYTE
GENERATION IS USUALLY HAPLOID (n).
3. THE FIRST CELL OF ANY SPOROPHYTE
GENERATION IS NORMALLY A ZYGOTE AND
THE LAST CELL IS NORMALLY A SPORE
MOTHER CELL.
4. ANY CELL OF A SPOROPHYTE GENERATION
IS USUALLY DIPLOID (2n).
5. THE CHANGE FROM A SPOROPHYTE TO A
GAMETOPHYTE GENERATION USUALLY
OCCURS AS A RESULT OF MEIOSIS.
6. THE CHANGE FROM A GAMETOPHYTE TO A
SPOROPHYTE GENERATION USUALLY OCCURS
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AS A RESULT OF FERTILIZATION (FUSION OF
GAMETES).
THE WORD GENERATION AS USED IN
ALTERNATION OF GENERATIONS SIMPLY
MEANS PHASES OF A LIFE CYCLE.
THE BEST WAY TO UNDERSTAND THIS
CONCEPT IS TO LOOK AT EXAMPLES OF
VARIOUS PLANT GROUPS AND THE STAGES IN
THEIR LIFE CYCLES.
MOSSES – BRYOPHYTA
IN MOSSES SPORES GERMINATE INTO A
FILAMENTOUS STAGE CALLED PROTONEMA,
THIS PRODUCES ADULT FEMALE AND MALE
GAMETOPHYTES. THIS IS THE DOMINANT
GERNERATION. THE GAMETOPHYTES ARE
MULTI-CELLULAR AND PHOTOSYNTHETIC –
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THIS IS WHAT WE SEE WHEN LOOKING AT A
MOSS. THE GAMETOPHYTE PRODUCES
GAMETANGIA (A MULTICELLULAR
STRUCTURE WHICH PRODUCES GAMETES.)
FEMALE GAMETANGIA ARE CALLED
ARCHEGONIA AND FORM EGGS; MALE
GAMETANGIA ARE ANTHERIDIA AND FORM
SPERM.
SPERM RELEASED FROM ANTHERIDIA MUST
SWIM IN WATER TO THE ARCHEGONIA TO
FERTILIZE THE EGG. THE RESULTING ZYGOTE
GROWS INTO A SPOROPHYTE WHICH
PRODUCES A SPORANGIUM (A STRUCTURE
THAT FORMS SPORES). IN MOSSES THE
SPOROPHYTE REMAINS ATTACHED TO THE
GAMETOPHYTE AND IS WHOLLY OR
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PARTIALLY DEPENDENT ON THE
GAMETOPHYTE FOR NUTRITION. THE
SPORANGIUM PRODUCES INSIDE OF IT SPORE
MOTHER CELLS THAT UNDERGO MEIOSIS TO
PRODUCE SPORES.
FERNS
IN FERNS SPORES GERMINATE AND FORM A
FREE LIVING GREEN PHOTOSYNTHETIC PLANT
BODY CALLED A PROTHALLUS. THE
PROTHALLUS IS TYPICALLY VERY SMALL. THE
PROTHALLUS FORMS ON ITS BODY
GAMETANGIA – ANTHERIDIA AND
ARCHEGONIA. THE SPERM PRODUCED BY THE
ANTHERIDIA MUST SWIM IN WATER TO THE
ARCHEGONIUM TO FERTILIZE THE EGG. THE
RESULTING ZYGOTE THEN GROWS INTO THE
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LARGE PLANT WE ALL RECOGNIZE AS A FERN.
IN FERNS THE DOMINANT GENERATION IS THE
SPOROPHYTE. THE SPOROPHYTE THEN
PRODUCES A SPORANGIUM WITH SPORE
MOTHER CELLS THAT UNDERGO MEIOSIS TO
PRODUCE SPORES. IN FERNS SPORANGIA ARE
USUALLY FORMED IN CLUSTERS CALLED SORI
(SORUS) FORMED ON THE UNDERSIDE OF THE
FERN LEAF (FROND).
IN MOSSES AND FERNS THE SPORES PRODUCED
TEND ALL TO BE THE SAME SIZE AND SHAPE
AND SO THESE PLANTS ARE HOMOSPOROUS.
IN THE REST OF THE PLANT KINGDOM –
CONIFERS AND FLOWERING PLANTS – SPORES
OF TWO DIFFERENT SIZES – WITH DIFFERENT
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POTENTIALS – ARE PRODUCED; LARGE
MEGASPORES FROM A MEGASPORANGIUM;
AND SMALL MICROSPORES FROM A
MICROSPORANGIUM. THESE PLANTS ARE
HETEROSPOROUS.
GYMNOSPERMS
IN THE PINES THE LIFE CYCLE GETS MORE
COMPLEX AND SO IT IS EASIEST TO START
WITH THE ZYGOTE. THE ZYGOTE DEVELOPS
INTO AN EMBRYO FOUND WITHIN THE SEED.
THE SEED GERMINATES AND THE EMBRYO
GROWS INTO A SEEDLING AND THEN THE
ADULT PINE TREE – THE SPOROPHYTE. IN
GYMNOSPERMS THE DOMINANT GENERATION
IS THE SPOROPHYTE.
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THE PINE TREE PRODUCES MALE CONES
(POLLEN CONES) WHICH ARE SMALL AND
DELICATE. AFTER POLLEN HAS BEEN
RELEASED THE MALE CONES SHRIVEL AND
FALL OFF THE TREE. ON SCALES IN THE MALE
CONES ARE PRODUCED MICROSPORANGIA
THAT PRODUCE INSIDE OF THEM MICROSPORE
MOTHER CELLS; THESE UNDERGO MEIOSIS TO
PRODUCE FOUR MICROSPORES THAT DEVELOP
INTO POLLEN GRAINS. A POLLEN GRAIN
CONSISTS OF A CENTRAL BODY WITH TWO AIR
SACS. THE AIR SACS GIVE THE POLLEN GRAIN
BUOYANCY AND ALLOW IT TO BE CARRIED BY
THE WIND SOME DISTANCE. POLLEN GRAINS
ARE CARRIED BY THE WIND TO FEMALE
CONES.
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FEMALE CONES ON PINES CAN BE LARGE AND
ARE USUALLY VERY WOODY AND PERSIST FOR
A LONG TIME. WOODY SCALES IN THE CONE
PRODUCE ON THEIR UPPER SURFACE OVULES
WHICH ARE USUALLY LARGE.
THE OVULE CONSISTS OF AN OUTER LAYER OF
TISSUE – THE INTEGUMENT – THAT IS DERIVED
FROM THE SPOROPHYTE. THE INTEGUMENT
MATURES INTO THE SEED COAT. THE
INTEGUMENT HAS A PORE IN IT AT ONE END
CALLED THE MICROPYLE. INSIDE THE
INTEGUMENTS IS THE MEGASPORANGIUM
(=NUCELLUS).
THE MEGASPORANGIUM HAS ONE MEGASPORE
MOTHER CELL THAT UNDERGOES MEIOSIS TO
PRODUCE FOUR MEGASPORES. THREE OF
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THESE DEGENERATE. THE REMAINING ONE
DEVELOPS INTO THE FEMALE GAMETOPHYTE
WHICH WILL CONSIST OF THOUSANDS OF
CELLS. THIS PRODUCES ARCHEGONIA, EACH
WITH ONE EGG – FACING THE MICROPYLE.
OUT OF THE MICROPYLE OOZES A STICKY
FLUID (POLLEN DROPS).
THE POLLEN GRAIN FLOATING IN THE AIR CAN
STICK TO THE POLLEN DROP AT THE END OF
THE MICROPYLE. AS THE FLUID EVAPORATES
THE POLLEN GRAIN IS DRAWN DOWN
THROUGH THE MICROPYLE TO THE TOP OF
THE MEGASPORANGIUM. ONCE THERE THE
POLLEN GRAIN FORMS AN OUTGROWTH – THE
POLLEN TUBE – WHICH SLOWLY DIGESTS ITS
WAY THROUGH THE MEGASPORANGIUM
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TOWARDS THE ARCHEGONIUM AND ITS EGG. A
CELL IN THE POLLEN GRAIN – THE
GENERATIVE CELL – DIVIDES INTO TWO CELLS
– THE STERILE CELL AND THE
SPERMATOGENOUS CELL. THE
SPERMATOGENOUS CELL THEN DIVIDES TO
PRODUCE TWO SPERM. THE GERMINATED
POLLEN GRAIN WITH THE POLLEN TUBE AND
TWO SPERM IS THE MALE GAMETOPHYTE.
(NOTE NO ANTHERIDIUM HAS BEEN FORMED.)
ONCE THE POLLEN TUBE REACHES THE
ARCHEGONIUM THE SPERM ARE RELEASED
AND ONE UNITES WITH THE EGG (OTHER
SPERM AND THE REST OF THE POLLEN GRAIN
DEGENERATE). THE ZYGOTE DEVELOPS INTO
THE EMBRYO.
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DURING GROWTH AND DEVELOPMENT OF THE
EMBRYO THE YOUNG SPOROPHYTE OBTAINS
NUTRIENTS FROM THE PARENT PLANT THAT
ARE STORED IN THE COTYLEDONS. FOOD
STORAGE TISSUE IN THE SEED IS PROVIDED BY
THE FEMALE GAMETOPHYTE. IT IS OFTEN
FORMED BEFORE FERTILIZATION OCCURS.
NOTE ALSO THAT WATER IS NO LONGER
REQUIRED FOR FERTILIZATION. WIND
CARRIES THE POLLEN GRAINS RANDOMLY
THROUGHOUT THE ATMOSPHERE – WITH
LUCK ONE WILL LAND ON THE POLLEN FLUID.
ANGIOSPERMS
IN FLOWERING PLANTS THE LIFE CYCLE IS
SIMILAR TO PINES IN THAT THE DOMINANT
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GENERATION IS THE SPOROPHYTE. THE
GAMETOPHYTIC GENERATION IS REDUCED TO
SMALL BODIES PARASITIC ON THE
SPOROPHYTE.
AGAIN START WITH THE ZYGOTE IN THE SEED
THAT DEVELOPS INTO AN EMBRYO. THE SEED
GERMINATES AND THE EMBRYO GROWS INTO
A SEEDLING AND ADULT PLANT – THE
SPOROPHYTE. THIS PLANT PRODUCES
FLOWERS WHICH ARE INVOLVED IN SEXUAL
REPRODUCTION. THE ANTHER IN THE FLOWER
PRODUCES MICROSPORE MOTHER CELLS
INSIDE MICROSPORANGIA. EACH MICROSPORE
MOTHER CELL UNDERGOES MEIOSIS TO YIELD
FOUR MICROSPORES. THESE DEVELOP INTO
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POLLEN GRAINS. EACH POLLEN GRAIN HAS A
GENERATIVE CELL AND A TUBE NUCLEUS.
AS IN GYMNOSPERMS THE OVULE CONTAINS
THE MEGASPORANGIUM. IT CONSISTS OF AN
OUTER LAYER OF TISSUE, THE INTEGUMENT,
WHICH WILL DEVELOP INTO THE SEED COAT.
INSIDE THE INTEGUMENT IS THE
MEGASPORANGIUM (=NUCELLUS); IT
PRODUCES ONE MEGASPORE MOTHER CELL
THAT DIVIDES THROUGH MEIOSIS TO
PRODUCE 4 MEGASPORES. THREE OF THEM
DEGENERATE; THE REMAINING MEGASPORE
UNDERGOES THREE DIVISIONS RESULTING IN 8
NUCLEI IN ONE LARGE CELL. THE NUCLEI
ARRANGE THEMSELVES IN TWO GROUPS OF
FOUR AT OPPOSITE ENDS OF THE
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MEGASPORANGIUM. ONE NUCLEUS FROM
EACH END MIGRATES TO THE CENTER. THE
TWO NUCLEI IN THE CENTER ARE CALLED THE
POLAR NUCLEI. THE THREE NUCLEI AT THE
END OPPOSITE THE MICROPYLE ARE THE
ANTIPODALS. AS IN THE GYMNOSPERM OVULE
THERE IS A GAP IN THE INTEGUMENT CALLED
THE MICROPYLE. THE THREE REMAINING
NUCLEI AT THE MICROPYLAR END ARE THE
EGG PLUS THE TWO SHORT LIVED SYNERGIDS.
THIS EIGHT NUCLEATE STRUCTURE IS THE
MATURE MEGAGAMETOPHYTE OR EMBRYO
SAC.
IN ANGIOSPERMS THE OVULE IS ENCLOSED
INSIDE A STRUCTURE CALLED THE OVARY.
THE OVULE IS ATTACHED BY A STALK (THE
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FUNICULUS) TO THE WALL OF THE OVARY.
THE REGION ON THE OVARY THAT BEARS THE
OVULES IS THE PLACENTA. TYPICALLY
CONNECTED TO THE OVARY IS A STALK CALL
THE STYLE; AT ONE END IS THE OVARY AT THE
OTHER IS THE STIGMA. THE FUNCTION OF THE
STYLE IS TO CONNECT THE STIGMA AND
OVARY. THE STIGMA IS THE SITE WHERE THE
POLLEN GRAIN LANDS AND GERMINATES. THE
TRANSFER OF POLLEN GRAINS FROM AN
ANTHER TO A STIGMA IS POLLINATION AND
CAN OCCUR IN MANY DIFFERENT WAYS.
WHEN A POLLEN GRAIN LANDS ON A STIGMA
IT GERMINATES WITH THE TUBE NUCLEUS
FORMING A POLLEN TUBE. THE GENERATIVE
CELL DIVIDES TO PRODUCE TWO SPERM. THIS
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IS THE MATURE MALE GAMETOPHYTE. THE
POLLEN TUBE GROWS DOWN THROUGH THE
STYLE INTO THE LOCULES AND ENTERS THE
MICROPYLE.
THE SYNERGID DISINTEGRATES, ONE SPERM
UNITES WITH THE EGG AND THE OTHER
SPERM UNITES WITH THE TWO POLAR NUCLEI.
THIS EVENT IS CALLED DOUBLE
FERTILIZATION AND IS UNIQUE TO
ANGIOSPERMS (PLUS TO GYMNOSPERMS –
EPHEDRA & GNETUM [GNETOPHYTA]). THE
UNION OF THE EGG AND SPERM RESULTS IN
THE FORMATION OF THE ZYGOTE – WHICH
WILL DEVELOP INTO THE EMBRYO. THE UNION
OF THE OTHER SPERM AND THE POLAR
NUCLEI RESULTS IN A TRIPLOID (3n)
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ENDOSPERM NUCLEUS – THIS IS THE START OF
ENDOSPERM FORMATION. THE TUBE NUCLEUS
AND THE ANTIPODALS DEGENERATE.
POLLINATION IS THE TRANSFER OF POLLEN
FROM ANTHER TO STIGMA. IN ANGIOSPERMS
IT MAY BE ACCOMPLISHED BY GRAVITY,
WIND, WATER, BIRDS, BATS, OTHER MAMMALS
BUT PRIMARILY BY INSECTS. IT IS AN
ENTIRELY SEPARATE EVENT FROM
FERTILIZATION (UNION OF GAMETES) WHICH
MAY NOT OCCUR UNTIL DAYS, WEEKS,
MONTHS (OR NEVER) AFTER POLLINATION.
HOWEVER, ONCE DOUBLE FERTILIZATION
OCCURS IT SETS INTO MOTION A SERIES OF
EVENTS WHICH WILL TAKE PLACE OVER TIME.
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1. THE FUSION OF GAMETES (EGG & SPERM)
RESULTS IN A ZYGOTE THAT WILL DEVELOP
INTO THE EMBRYO, FOUND INSIDE THE OVULE
(IMMATURE SEED).
2. THE OVULE WILL DEVELOP INTO THE SEED.
3. THE FUSION OF THE SPERM AND POLAR
NUCLEI RESULT IN THE FORMATION OF
ENDOSPERM. IN MANY PLANTS THE
ENDOSPERM REMAINS IN THE SEED TO SERVE
AS A SOURCE OF FOOD MATERIAL FOR THE
DEVELOPING EMBRYO. IN OTHER PLANTS THIS
FOOD SOURCE IS QUICKLY ABSORBED BY THE
DEVELOPING EMBRYO AND FOOD IS STORED IN
THE COTYLEDONS. IN OTHER SPECIES THE
NUCELLUS PROLIFERATES INTO A FOOD
STORAGE TISSUE CALLED PERISPERM.
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INITIALLY, AT LEAST IN ANGIOSPERMS, THE
FOOD STORAGE TISSUE IS NEITHER
GAMETOPHYTIC NOR SPOROPHYTIC AND IS
FORMED AFTER FERTILIZATION OCCURS.
4. THE INTEGUMENT WILL DEVELOP INTO THE
SEED COAT.
5. THE OVARY WILL DEVELOP INTO THE FRUIT.
IN ANGIOSPERMS, SEEDS ARE ALWAYS FOUND
INSIDE FRUITS. THE FRUIT IS FORMED BY
MATURATION OF THE OVARY WALL OR
PERICARP.
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