Like all plants, flowering seed plants exhibit alternation of generations. We will look at some of the important features in this life cycle in the figure below. The anthers are made up of diploid (2n) cells that undergo meiosis to form haploid (1n) cells (microspores). A microspore will undergo a series of changes (including the acquisition of a tough cell wall), leading to the formation of a pollen grain, the male gametophyte that contains two nuclei.
Meanwhile, in the female ovary each diploid ovule undergoes meiosis to produce a megaspore that is haploid. This megaspore divides by mitosis, three times, to produce eight haploid nuclei in seven cells. One of these cells is the egg, and one cell is larger than the others and contains two nuclei. Together, the seven cells constitute the embryo sac, which is the female gametophyte. This gametophyte is totally enclosed within the ovary of the parent plant.
At this point, the egg is ready to be fertilized and the pollen grain is ready for dispersal. Pollen grains need their tough outer wall to survive the process of pollination (the transfer of pollen to the female flower parts). Once the pollen grain lands on the stigma, it germinates. One nucleus begins to grow a pollen tube that extends down into the style. The other nucleus then divides by mitosis to form two haploid sperm cells. It is at this point (with two sperm cells and one pollen tube) that the male gametophyte is mature and contains three haploid cells. If successful, the pollen tube will grow into an ovule and discharge its sperm cells. There are mechanisms to prevent more than one pollen tube from entering a given ovule.
Once inside the ovule, one sperm cell fuses with the egg and the other fuses with the large cell that has two nuclei. Recall, this is known as double fertilization. The diploid zygote forms from the fusion of the sperm and the egg. A triploid cell forms from the fusion of the other sperm and the central cell. This triploid cell will then divide by mitosis to produce endosperm , which provides nourishment for the developing embryo. The zygote grows within the seed and eventually the seed germinates and grows into a mature, diploid, sporophytic plant.
Plants III Part 2 VoiceThread TranscriptUnlike most animals, many species of plants are self-fertile, which means that pollen from a plant can fertilize ovules of the same plant. While this can be very beneficial if a plant germinates in a place away from other members of the same species, so it has no other plant to exchange pollen with, the result is extreme inbreeding which can have harmful results. Dioecious plants must cross-pollinate. However, monoecious and synecious plants have the potential to self-pollinate.
Some plants have physical mechanisms that prevent or reduce the chance of self-pollination. For instance, the pollen may mature before the stigma is receptive (sunflowers), or the stigma may be receptive before the pollen matures (magnolias). In others, the placement of the stigmas relative to the anthers makes self-pollination difficult. For example, in this hibiscus the stigmas are positioned above the anthers so that pollen cannot drop onto them. What type of animal do you think pollinates this plant? How could the pollinator help to ensure cross-pollination?
Some plants are unable to self-pollinate because they are genetically self-incompatible. This system was first discovered by Dr. The-hui Kao and his research team at Penn State.
In this system, if the pollen that lands on a stigma shares an allele at the S locus with the stigma, the pollen tube cannot grow to reach the ovule. However, if the pollen has a different allele than the stigma, then the pollen tube will be able to grow and fertilization can take place. This requires the interaction of a molecule produced in the carpal and a similar molecule produced in the pollen. Dr. Kao’s group discovered that the two genes producing these interacting molecules are located very close together on the same chromosome.