Biological Foundations 112, Lecture 14


Flower Structure and Ovule Development

Chapter 27, pages 564-565
Chapter 34, pages 727-731
Basic Botany-5 4:55-15:23


I.   Flower Structure

     A.   Gross flower structure

            1.   Bract

            2.   Peduncle

            3.   Receptacle

     B.    A Flower is Four Whorls of Modified Leaves

            1.   Calyx

                  a.   Name of outermost whorl

                  b.   Sepals, name of individual floral leaves of this whorl

            2.   Corolla

                  a.   Name of second whorl going inward

                  b.   Petals, name of individual floral leaves of this whorl 

            3.   Androecium

                  a.   Name of third whorl going inward

                  b.   Male whorl

                  c.   Stamens, name of individual floral leaves of this whorl

                        1.   Anther - knobby structure that holds the pollen

                        2.   Filament - stalk on which the anther stands

            4.   Gynoecium

                  a.   Innermost whorl (4th going inward)

                  b.   Female whorl

                  c.   Carpel, name of the individual floral leaves of this whorl

                        1.   Stigma - knob at the tip of the style to which pollen sticks

                        2.   Style - stalk attached to the tip of the ovary

                        3.   Ovary - the lower portion containing the ovules

                              a.   Ovules - structures that will be fertilized and become seeds

                              b.   Placenta - the structure to which the ovules are attached

                              c.   Locule - the chambers inside the ovary

     C.   Other Term Referring to Flowers

           1.   Hypanthium

                 a.   Fusion of the calyx, corolla, and androecium into a single structure

                 b.   Characteristics of dicots only

           2.   Perianth

                 a.   A single whorl of floral leaves that corresponds to the calyx and corolla

                 b.   Loosely used to refer to the calyx and corolla whorls

                 c.   Specifically characteristic of monocots only

           3.   Tepals - the individual floral leaves of the perianth

     D.   Variations in Flower Structure

           1.   Complete flower - flower with all four whorls of floral leaves

           2.   Incomplete flower - flower with one or more whorls of floral leaves missing

           3.   Perfect flowers - bisexual flowers (both androecium and gynoecium present)

           4.   Imperfect flowers - unisexual flowers (either the androecium or gynoecium missing)

                 a.   Staminate flowers - flowers with an androecium whorl and no gynoecium whorl

                 b.   Pistillate flowers - flowers with a gynoecium whorl and no androecium whorl

                 c.   Monoecious flowers - staminate and pistillate flowers on the same plant

                 d.   Dioecious flowers - staminate and pistillate flowers on different plants

           5.   Regular flowers - flowers with all their flower parts the same size and shape (rose)

           6.   Irregular flowers - flowers with different shaped flower parts (sweet pea or orchid)

           7.   Coalescence of flower parts - floral leaves of the same whorl united

           8.   Adnation of flower parts - fusion of floral leaves of two different whorls

     E.   Elevation of flower parts

           1.   Hypogynous - floral parts attached below the ovary (ovary superior)

           2.   Perigynous - floral parts attached to a hypanthium

           3.   Epigynous - floral parts attached above the ovary (ovary inferior)

II.  Development of Ovules

     A.   Structural development

            1.   Dome-shaped cluster of cells appears on the placenta called the nucellus

            2.   From dome of cells two ring-shaped layers of cells grow from sides out to the apex  
                  and don't completely fuse.

                  a.   Inner integument

                  b.   Outer integument

                  c.   Megaspore mother cell - cell in the center (megasporocyte)

            3.   Micropyle - small circular opening where integuments don't fuse

            4.   Nucellus - general name for the tissue surrounding the megasporocyte

     B.   Embryo Sac Development

           1.    Megaspore mother cell undergoes meiosis and produces 4 megaspores

                  a.   The megaspore mother cell is diploid (2N)

                  b.   The megaspores are haploid (1N)

                  c.   Meiosis has reduced the chromosomes from the diploid number (megaspore
                        mother cell) to the haploid number (megaspores).

           2.    The three megaspores nearest the micropyle disintegrate

           3.    The surviving megaspore develops into a mature embryo sac

                  a.   Mitotic divisions

                        (1)  Megaspore divides forming a two-nucleated embryo sac

                        (2)  Each of these two nuclei divide to produce a total of 4 nuclei

                        (3)  Each of these four nuclei divide again to produce a total of 8 nuclei

                  b.   Migration of nuclei

                        (1)  After the first mitotic division the two daughter nuclei migrate to the opposite
                               poles of the embryo sac

                        (2)  After the last mitotic division, one nucleus of each set migrates back to the
                               center of the embryo sac

                  c.   Cell formation around the nuclei

                        (1)  The three nuclei at each end take some cytoplasm and form cell walls                                                around themselves to become a cell

                        (2)  The two nuclei in the center take the "lion's share" of the cytoplasm and  
                               become a binucleated cell

                         (3)  Thus a 7-celled embryo sac is the result

                         (4)  Names of the cells:

                                (a)  Three cells at the micropylar end

                                       i)   Egg cell - in the center of the three

                                       ii)  One synergid cell on either side of the egg cell (2 synergids total)

                                       iii)  These three cells are called the egg apparatus

                                (b)  One cell in the center

                                       i)   Called polar cell

                                       ii)  Binucleate

                                (c)   Three cells at the chalazal end (end opposite micropyle) are called
                                        the antipodal cells 


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