The life cycle for a common flowering plant, corn, is partially shown here...
The stamens of the male flowers contain diploid microsporocytes.
These divide by meiosis to produce four microspores each.
The cell inside the microspore (pollen) wall divides once
to produce a two-celled microgametophyte: the pollen grain.|
In the process called pollination, pollen is carried by a vector (wind in this case) to the stigma of the female flower. The tube cell of the pollen germinates and digests a pathway through the stigma, style and into the ovule of the female flower. The generative cell follows this pathway, divides into two sperm cells, and one will unite with the egg cell to form the zygote.
The carpel of the female flowers contains an ovule. One cell inside
this ovule is the megasporocyte. This diploid cell divides by meiosis
to produce four haploid megaspores. Three of these degenerate.
The remaining functional megaspore nucleus divides three times to make an 8-nucleate cell. After cytokinesis, the embryo sac consists of:
three antipodal cells, a central cell with two polar nuclei, the egg, and two synergids.
The pollen tube finds the micropyle opening into the ovule and
encounters the synergids.
The synergids enzymatically digest the tip of the pollen tube, releasing the two sperm cells.
One sperm cell unites in syngamy with the nearby egg, forming a zygote.
The other sperm cell unites with the central cell in a second syngamy event. The resulting triploid cell is called the endosperm.
The endosperm cell accumulates lipids, polysaccharides, and protein and divides to form a storage tissue.
The zygote later uses this storage material to sprout and grow into a mature corn plant, completing the life cycle.
The purpose of the images and text above is to communicate that plants can reproduce by sexual processes. In the example here, the plant has separate male and female flowers that are present on the same plant. This is just one of many breeding systems that operate among plants. Plants exhibit tremendous diversity in reproductive biology; these assist us in deciphering evolutionary relationships among the various organisms known as plants.
The images and text above show a life cycle, so one may start at any point. The diploid mature sporophyte plant is the one we see in the field, so starting there is natural. Meiosis occurs in cells called sporocytes and this produces four meiotic products called spores. In plants, these spores can and do divide mitotically to make a multicellular haploid body, the gametophyte plant. The endosporic microgametophyte is a two-celled pollen grain. Inside the ovule, the megagametophyte is a seven-celled embryo sac. After pollination and growth of the pollen tube, the egg of the embryo sac and the sperm from division of the pollen's generative cell will join in syngamy to produce a zygote. A second syngamy occurs between the second sperm from the generative cell's division and the central cell of the embryo sac to produce the primary endosperm cell. This double syngamy is an autapomorphy of the flowering plant clade.
A diagram of a flower is depicted below:
The flower is an organ system...a short branch on a plant. As with any shoot it consists of a stem with leaves. The stem here is the pedicel under the flower which terminates in a sometimes-swollen receptacle. The leaves are represented by four separate whorls. The lowest whorl is the calyx made up of sepals; while sometimes green and leaf-like, they can sometimes be almost indistinguishable from petals. Above this is the corolla whorl made up of petals; sometimes showy and fragrant. Above that is the androecium whorl made up of stamens; which produce the pollen grains in the anther at the end of a filament. In the center of the flower is the gynoecium whorl made up of one or more carpels. These may be separate or fused into a single pistil. Inside the ovary of the carpel is a chamber (the locule) where one or more ovules reside. The ovule contains the embryo sac.
The actual form and substance of the flower parts vary tremendously from species to species and assist us in determining the pathways of evolution among the flowering plants. As an example of real flowers, here is the buttercup in side view and face view:
A unique part of sexual reproduction is the movement of the pollen from the anther of the stamen to the stigma of the carpel. This process is called pollination. This process may be achieved by the flower itself (self-pollination; a severe form of inbreeding), by means of an animal vector (a pollinator), or by means of environmental vectors (water, wind). So the form and function of the flower has evolved to facilitate the pollination event. Because of the resulting diversity among flowers, flowers hare critical morphological features used to distinguish and group various species in identification, taxonomy, and phylogeny.
After syngamy, much of the flower goes through senescence processes which might culminate in abscission. However, the ovary at least will remain attached to the plant to develop into a fruit. The ovules inside this ovary mature and develop into seeds. The zygote inside the ovule matures into an embryonic plant. Typically the embryo is sent hormonal signals to halt growth and to become dormant. These developmental processes culminate in a modified senescence process called fruit ripening, and fruit abscission. Plants have also evolved ways to distribute these seeds either by means of actions of the fruit itself, some animal vector (fruit or seed disperser) or some environmental factor (water, wind).
In the next lecture we shall start with an examination of a "typical" cell.