Features of the Angiosperm Life Cycle
Like other plants, the angiosperms alternate a sporophytic generation with a gametophytic one, a sporic meiosis (see Figure ). Angiosperm sporophytes are the common plants around us—trees, grasses, and garden vegetables. They produce through meiosis (reduction division) two kinds of spores in specialized structures of their flowers, microspores in the anthers and megaspores in the ovules contained within the ovaries. The gametophytes, which develop from the spores, are much reduced in size. The microgametophyte consists of only three cells and is the germinated pollen grain. The megagametophyte is the mature embryo sac, a seven‐celled structure in the ovule surrounded by, and dependent upon, sporophyte tissue.
Development of the gametophytes
Haploid microspores develop from microsporocytes in the anthers and give rise to pollen grains containing two cells: the tube cell and the generative cell. At about the time of pollination, the latter cell divides and produces two sperm. This three-celled pollen grain is the immature male gametophyte (microgametophyte).
The female gametophyte, the megagametophyte, develops in the ovary at the same time the male gametophyte is developing in the anthers. While the process is exceedingly variable among taxa, about three-quarters of the flowering plants go through the following steps.
One megasporocyte is contained in each of the young ovules within the ovaries in the flower buds. The ovule is attached by a stalk, the funiculus, to the placenta on the ovary wall and, at this stage, is essentially a lump of tissue, the nucellus, covered by two tissue layers, the integuments (which wrap almost completely over the ovule but leave a small opening, the micropyle, at one end). Three of the haploid megaspores produced by the megasporocyte disintegrate almost immediately and the remaining one divides by three successive mitotic divisions to produce eight nuclei in an embryo sac within the elongated, swollen megaspore. The nuclei cluster, first in groups of four at either end of the sac and then one nucleus from each end, migrates to the center. The two migrating nuclei are called the polar nuclei and they form a polar cell when walls develop around them. Cell walls also form around the three nuclei left at the end of the cell opposite the micropyle, the chalazal end. The chalazal end cells are theantipodals. At the micropylar end, the three nuclei are organized into the egg apparatus and walls form around each of them also. One cell is the egg cell, the two others are synergids (helpers). All three look alike, but only the egg continues to develop; the synergids deteriorate as do the antipodals on the opposite end of the sac. The embryo sac at this stage is the female gametophyte (megagametophyte). Before further development can occur and seeds are produced in the ovary, two events must occur: pollination followed by fertilization.
Pollination is the mechanical transfer of pollen grains from an anther to a stigma, the receptive end of a carpel. Pollination is accomplished by a variety of physical dispersal agents such as wind, water, and gravity or many kinds of animals including insects, bats, birds, and small rodents. The variations in floral structure are, in large part, adaptations to achieve pollination success. Most pollination is between flowers located on separate plants ( cross-pollination), but in some taxa self-pollination occurs when pollen from the anthers falls on stigmas of the same plant.
If the pollen grain lands on the stigma of a genetically compatible flower, it absorbs moisture and a pollen tube emerges through a pore in the wall. The germinated pollen grain with its pollen tube and three nuclei is the mature male gametophyte. The tube grows downward toward the ovary through special tissues in the style, penetrates the embryo sac, usually through the micropyle (destroying a synergid in the process), and discharges its contents. The tube nucleus disintegrates while one of the sperm nuclei fuses with the egg nucleus, forming a zygote. The other sperm fuses with the polar cell, forming the endosperm nucleus. In other words, a double fertilization occurs: Both sperms fuse with embryo sac nuclei. Double fertilization is a characteristic of the angiosperms and results in a polyploid endosperm tissue. (Polyploidy refers to the number of sets of chromosomes the cell contains; plants with more than the diploid two sets are polyploids. The endosperm tissue may be triploid [3 n] or more depending upon the species.)
If no pollen tube and its contents reach an ovule in the ovary, the ovule aborts with no further development. Lacking chemical signals (hormones) from a developing seed, the ovary, too, may wither and die. If double fertilization does occur, the ovule develops into a seed and the entire ovary into a fruit.