Chapter 13.  The Flower and Sexual Reproduction

• Flower = structure responsible for sexual reproduction in angiosperms ("flowering plants"), which are the dominant plant group today.
• Flower structure
• Flower can be interpreted as a modified short shoot.  The leaves have been modified to form petals, etc.
• "Typical" (i.e., primitive, relatively unspecialized) flower has four  whorls of modified leaves:
• Calyx composed of sepals.
• Corolla composed of petals.
• Androecium composed of stamens.
• Anther - contains pollen (which will contain sperms).
• Filament - supports anther.
• Gynoecium (pistil) composed of carpels (usually fused together to for a single structure).
• Stigma - receptive surface for pollen grains
• Style - supports stigma
• Ovary - contains ovules, each of which ultimately produces a single egg.
• Flowering - vegetative (indeterminate) shoot apex converts to floral (determinate) shoot apex (Fig. 13.8).  What would be leaf primordia become primordia of sepals, petals, stamens, carpels (in that order).  Environmental stimulus: day length and temperature.
• The angiosperm life cycle - alternation of generations between:
• Dominant (large, complex) diploid sporophyte generation.  Sporophyte plants produce spores.
• Reduced (microscopic, simple) haploid gametophyte generation.  Gametophyte plants produce gametes (eggs and sperms).
• Female contribution to reproduction (eggs), see Fig. 13.7)
• One cell of ovary (megasporocyte, diploid) undergoes meiosis, producing 4 haploid megaspores.
• 3 of the megaspores degenerate, leaving 1
• Surviving megaspore undergoes three mitotic divisions to produce 8-nucleate female gametophyte (=embryo sac).
• female gametophyte has 8 nuclei:
• 1 is the egg (will be fertilized by sperm cell)
• 2 are synergids
• 2 are polar nuclei (will be fertilized by sperm cell)
• 3 are antipodal nuclei
• Male contribution to reproduction (sperms), see Fig. 13.2
• Each chamber of anther contains sporogenous tissue.  This develops into many microsporocytes.
• Microsporocyte (diploid) undergoes meiosis to producing, 4 haploid microspores (all survive).
• Microspore divides mitotically to produce 2-celled male gametophyte.  Contained in single structure, the pollen grain.
• Pollen grains germinate on stigma.  Mitosis of one cell produces 2 sperm nuclei.
• Double fertilization
• Pollen tube grows down stigma, sperm nuclei enter female gametophyte.
• Egg fertilized by one sperm to produce diploid zygote (which becomes embryo).
• Polar nuclei (together) fertilized by other sperm to produce triploid cell which develops into endosperm (nourishes embryo).
• Flower architecture - fantastically diverse.  Some examples of specializations relative to the "typical" (primitive) flower:
• Incomplete flowers - missing parts
• Many wind-pollinated flowers lack conspicuous corollas
• Unisexual flowers
• Monoecious - unisexual flowers of both sexes on same plant (e.g. oaks).
• Dioecious - male flowers on some individuals, female flowers on others (e.g. willows).
• Symmetry - radial (regular) or bilateral (irregular).
• Parts can be fused, e.g. fusion of petals to form a corolla tube.
• Inflorescences - clusters of flowers (see Fig. 13.13).  Most common types:
• Raceme:  the main axis has short branches (pedicels), each with a single flower
• Panicle:  a branched raceme.
• Spike or catkin:  like a raceme, but the flowers lack pedicels.
• Umbel:  the flowers are on branches which arise at the same place on the main axis.
• Head:  the flowers are crowded together in a very tight cluster (hallmark of the sunflower family)
• Cyme:  each branch of the inflorescence terminates in a flower, with younger flowers developing from lower branches.
• Flowers with asexual reproduction.
• Apomixis = asexual reproduction in angiosperms.  Meiosis of megasporocyte is bypassed, diploid egg develops directly into embryo without being fertilized.
• Has evolved many times (e.g. blackberries, dandelions, arctic tundra plants).
• In some cases pollination is required (even though no fertilization occurs!).
• Pollination = transfer of pollination from stamen to stigma.  Vector = agent responsible for pollen transfer (biotic or abiotic).
• Biotic pollen vectors (animals)
• Plant provides "reward" - nectar or pollen.  Flower morphology and vector biology have co-evolved into mutually productive relationship.
• Insects - beetles, bees, butterflies
• Birds - hummingbirds
• Mammals - bats
• Abiotic pollen vectors (wind, water).  Flowers are relative inconspicuous, no investment in brightly-colored petals, etc.  Compared to animal-pollinated flowers, wind-pollinated flowers produce more pollen grains and more pollen (quantity vs. quality).