FLOWERS, ETC. SEXUAL REPRODUCTION IN THE KINGDOM PLANTAE IS RELATIVELY COMPLEX, INVOLVING A LIFE CYCLE CONSISTING OF

Transcription

1 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. 1

2 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: 2

3 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 3

4 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 4

5 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 5

6 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 6

7 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 7

8 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. 8

9 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. 9

10 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 10

11 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 11

12 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. 12

13 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 13

14 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 14

15 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 15

16 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 16

17 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 17

18 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) 18

19 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. 19

20 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. 20

21 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. 21