This animation (Audio - Important) describes the angiosperm life cycle.
Components of Flowers
Accessory structures form the nonreproductive parts of the flower. Petals (collectively called
the corolla) are the colored parts located between the reproductive structures and the receptacle.
Sepals are the outermost green leaflike parts. Epidermal oils impart fragrance and pigments give
colors.
Reproductive parts include the stamens (male) and
carpels (female).
Male parts are called stamens. Often the stamen consists of a slender stalk (filament) capped with
an anther. Inside the anthers are pollen sacs in which pollen grains develop.
Pollen is basically sperm packed inside a nutritious package. When it first evolved it was
transferred by wind currents. Later it was transferred by insects. Most species of angiosperms
have coevolved with pollinators attracted to their pollen and nectar. Coevolution refers to two
(or more) species jointly evolving as an outcome of close ecological interactions. Plants with
flowers that attracted insect pollinators had a reproductive advantage. Plant structures that were
more attractive to pollen-delivering insects were favored. The more attractive plants proved to be
good sources of food for the insects.
Female parts are located in the center of the flower. The carpel is the vessel-shaped structure with an expanded lower chamber (ovary), slender column (style), and upper surface (stigma) for pollen landing. In the ovary, eggs develop, fertilization occurs, and seeds mature.
So called "perfect" flowers have both male and female parts (may be on the same plant). "Imperfect" flowers lack the parts of one sex. Imperfect flowers are either male or female. The same plant may have both male and female flowers or the sexes may be on separate plants.
The moon flower is an example of an insect pollinated flower.
Pollen Formation
Each anther has
four pollen sacs. In anthers, each diploid mother cell divides by meiosis followed by cytoplasmic
division to form four haploid microspores. Each microspore will divide by mitosis to form two
haploid cells. One cell will produce the sperm and the other will form a pollen tube.
Egg Formation
Ovules form inside the ovary of a flower. Protective integuments form around the ovule. Inside,
cells divide by meiosis to form haploid megaspores. All megaspores but one disintegrate. The
remaining megaspore undergoes mitosis three times without cytoplasmic division. The result is a cell
with eight nuclei. Then cell division produces a seven-cell gametophyte. One cell has two nuclei
and will become the endosperm, another is the egg.
In the carpel, a mass of tissue forms an ovule (potential seed) enclosed by integuments. A diploid mother cell divides by meiosis to produce haploid megaspores, one of which will undergo mitosis three times to produce a cell with eight nuclei.
The nuclei migrate resulting in an embryo sac (female gametophyte) with seven cells. One cell has two nuclei and will become the endosperm (nutrition for embryo). Another cell will be the egg.
Pollination is the transfer of pollen grains to the surface of a receptive stigma. Wind, insects, birds, or other agents are often required for the transfer. After a pollen grain lands on a stigma, it germinates and a pollen tube forms, creating a path that the two sperm nuclei will follow to the ovule.
Guided by chemical cues, the pollen tube grows through the tissues of the ovary to an ovule. It carries two sperm nuclei. When the pollen tube reaches an ovule, it penetrates the embryo sac and deposits two sperm. The two sperm are released to accomplish double fertilization.
One sperm fuses with (fertilizes) the egg nucleus to form a diploid zygote. The other sperm nucleus fuses with the two endosperm nuclei to yield a triploid "primary endosperm cell," (endosperm mother cell) that will nourish the young sporophyte seedling.
Endosperm formation occurs only in angiosperms. The fusion of a sperm nucleus with the two nuclei of the endosperm mother cell produces a triploid (3n) cell. This cell will give rise to the endosperm, the nutritive tissue of the seed.
This animation (Audio - Important) describes egg and pollen formation and fertilization.
This animation (Audio - Important) describes double fertilization.
REVIEW: The evolution of flowers and insects is an example of
REVIEW: The male part of a flower is the
REVIEW: Which of the following are produced within the anthers?
REVIEW: The endosperm is
REVIEW: The zygote is
REVIEW: A _____ is a vessel, the lower portion of which is an ovary in which eggs develop,
fertilization occurs, and seeds mature.
REVIEW: After meiosis within pollen sacs, haploid _____ form.
REVIEW: After meiosis in ovules, _____ megaspores form.
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