THE LIVING WORLD
Unit Seven. Plant Life
34.3. Gametes Combine Within the Flower
Pollination is the process by which pollen is transferred from the anther to the stigma (figure 34.3 1). The pollen may be carried to the flower by wind or by animals, or it may originate within the individual flower itself. When pollen from a flower’s anther pollinates the same flower’s stigma, the process is called self-pollination, which can lead to self-fertilization. For some plants, self-pollination and self-fertilization occur because self-pollination eliminates the need for animal pollinators and maintains beneficial phenotypes in stable environments. However, other plants are adapted to outcrossing, the crossing of two different plants of the same species. The presence of only male or female flowers on a plant (a dioecious plant,) requires outcrossing, as does the different timing of appearance of the male and female parts on a flower. Even when a flower’s stamen and stigma mature at the same time, some plants exhibit self-incompatibility. Self-incompatibility results when the pollen and stigma recognize each other as being genetically related, and pollination of the flower is blocked, as shown on the left in figure 34.4a.
Figure 34.3. Pollination and fertilization.
When pollen lands on the stigma of a flower, the pollen tube cell grows toward the embryo sac, forming a pollen tube. While the pollen tube is growing, the generative cell divides to form two sperm cells. When the pollen tube reaches the embryo sac, it bursts through one of the synergids and releases the sperm cells. In a process called double fertilization, one sperm cell nucleus fuses with the egg cell to form the diploid (2n) zygote, and the other sperm cell nucleus fuses with the two polar nuclei to form the triploid (3n) endosperm nucleus.
In many angiosperms, the pollen grains are carried from flower to flower by insects and other animals that visit the flowers for food or other rewards or are deceived into doing so because the flower’s characteristics suggest such rewards. A liquid called nectar, which is rich in sugar as well as amino acids and other substances, is often the reward sought by animals. Successful pollination depends on the plants attracting insects and other animals regularly enough that the pollen is carried from one flower of that particular species to another (figure 34.4b).
Figure 34.4. Incompatibility in pollination.
(a) Self-pollination is not permitted in some plant species. If a pollen grain from the same plant lands on the stigma, pollination is blocked, but in outcrossing, pollination occurs. (b) Controlled- pollination by insects or animals increases the likelihood that a plant will be pollinated by its same species and not by a different species.
The relationship between such animals, known as pollinators, and flowering plants has been important to the evolution of both groups, a process called coevolution. By using insects to transfer pollen, the flowering plants can disperse their gametes on a regular and more or less controlled basis, despite their being anchored to the ground. The more attractive the plant is to the pollinator, the more frequently the plant will be visited. Therefore, any changes in the phenotype of the plant that result in more visits by pollinators offer a selective advantage. This has resulted in the evolution of a wide variety of angiosperm species that have very different flowers, as you saw on page 703.
For pollination by animals to be effective, a particular insect or other animal must visit plant individuals of the same species. A flower’s color and form have been shaped by evolution to promote such specialization. Yellow and blue flowers are particularly attractive to bees (figure 34.5a), whereas red flowers attract birds but are not particularly noticed by most insects. Some flowers have very long floral tubes with the nectar produced deep within them; only the long, slender beaks of hummingbirds or the long, coiled proboscis of moths or butterflies can reach such nectar supplies. You can see the long proboscis in the butterfly reaching down inside a flower in figure 34.5b.
Figure 34.5 Insect pollination.
(a) Bees are usually attracted to yellow flowers. (b) This alfalfa butterfly (Colias eurytheme) has a long proboscis that allows it to feed on nectar deep in the flower.
In certain angiosperms and all gymnosperms, pollen is blown about by the wind and reaches the stigmas passively. For such a system to operate efficiently, the individuals of a given plant species must grow relatively close together because wind does not carry pollen very far or very precisely, compared to transport by insects or other animals. Because gymnosperms, such as spruces or pines, grow in dense stands, wind pollination is very effective. Wind-pollinated angio- sperms, such as birches, grasses, and ragweed, also tend to grow in dense stands. The flowers of wind-pollinated angio- sperms are usually small, greenish, and odorless, and their petals are either reduced in size or absent altogether. They typically produce large quantities of pollen.
Once a pollen grain has been spread by wind, an animal, or self-pollination, it adheres to the sticky, sugary substance that covers the stigma and begins to grow a pollen tube, which pierces the style. The pollen tube, nourished by the sugary substance, grows, and the generative cell within the pollen grain tube cell divides to form two sperm cells, shown in figure 34.3 2. Growth of the pollen tube continues until it reaches the ovule in the ovary 3.
When the pollen tube reaches the entry to the embryo sac in the ovule, one of the nuclei flanking the egg cell degenerates, and the pollen tube enters that cell. The tip of the pollen tube bursts and releases the two sperm cells 4. One of the sperm cells fertilizes the egg cell, forming a zygote. The other sperm cell fuses with the two polar nuclei located at the center of the embryo sac, forming the triploid (3n) primary endosperm nucleus 5. The primary endosperm nucleus eventually develops into the endosperm, which will nourish the developing embryo (see chapter 32). This process of fertilization in angiosperms in which two sperm cells are used is called double fertilization.
Key Learning Outcome 34.3. In pollination, pollen is transferred to the female stigma. After growth of the pollen tube, double fertilization leads to the development of an embryo and endosperm.