32.4: Pollination and Fertilization - Introduction
- Determine the differences between self-pollination and cross-pollination, and describe how plants have developed ways to avoid self-pollination
Pollination: An Introduction
In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the same or a different flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm that fertilize the egg.
Self-Pollination and Cross-Pollination
Pollination takes two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time and are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators. These types of pollination have been studied since the time of Gregor Mendel. Mendel successfully carried out self-pollination and cross-pollination in garden peas while studying how characteristics were passed on from one generation to the next. Today’s crops are a result of plant breeding, which employs artificial selection to produce the present-day cultivars. An example is modern corn, which is a result of thousands of years of breeding that began with its ancestor, teosinte. The teosinte that the ancient Mesoamericans originally began cultivating had tiny seeds, vastly different from today’s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the genetic difference between them is minuscule.
Genetic Diversity
Living species are designed to ensure survival of their progeny; those that fail become extinct. Genetic diversity is, therefore, required so that in changing environmental or stress conditions, some of the progeny can survive. Self-pollination leads to the production of plants with less genetic diversity since genetic material from the same plant is used to form gametes and, eventually, the zygote. In contrast, cross-pollination leads to greater genetic diversity because the male and female gametophytes are derived from different plants. Because cross-pollination allows for more genetic diversity, plants have developed many ways to avoid self-pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. The primrose employs this technique. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower and the thrum-eyed flower. In the pin-eyed flower, anthers are positioned at the pollen tube’s halfway point, and in the thrum-eyed flower, the stigma is found at this same location. This allows insects to easily cross-pollinate while seeking nectar at the pollen tube. This phenomenon is also known as heterostyly. Many plants, such as cucumbers, have male and female flowers located on different parts of the plant, thus making self-pollination difficult. In other species, the male and female flowers are borne on different plants, making them dioecious. All of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. The majority of pollinators are biotic agents such as insects (bees, flies, and butterflies), bats, birds, and other animals. Other plant species are pollinated by abiotic agents, such as wind and water.
Key Points
- Pollination, the transfer of pollen from flower-to-flower in angiosperms or cone -to-cone in gymnosperms, takes place through self-pollination or cross-pollination.
- Cross-pollination is the most advantageous of the two types of pollination since it provides species with greater genetic diversity.
- Maturation of pollen and ovaries at different times and heterostyly are methods plants have developed to avoid self-pollination.
- The placement of male and female flowers on separate plants or different parts of the plant are also barriers to self-pollination.
Key Terms
- pollination : the transfer of pollen from an anther to a stigma that is carried out by insects, birds, bats, and the wind
- heterostyly : the condition of having unequal male (anther) and female (stigma) reproductive organs
- cross-pollination : fertilization by the transfer of pollen from an anther of one plant to a stigma of another
- self-pollination : pollination of a flower by its own pollen in a flower that has both stamens and a pistil