18.10: Most plants can avoid self-fertilization.

The efficiency of flower structure is remarkable. Flowers commonly contain both the male reproductive parts and gametes and the female reproductive parts and gametes, so a pollinator can easily assist in sexual reproduction. A visiting bee, for example, can pollinate a flower such as an apple blossom with pollen from another blossom it previously visited, while simultaneously picking up pollen for later fertilization of other individuals. But this hermaphroditism—having both male and female reproductive parts—in flowering plants can lead to some unintended and detrimental consequences. What happens if a plant’s male gamete fertilizes one of its own eggs? Such self-fertilization leads not only to less genetically varied offspring but also to offspring that are more likely to express one or more lethal or harmful genes because of the extreme inbreeding. How can plants avoid self-fertilization?

Plants have numerous mechanisms for avoiding self-fertilization, including (as we saw in Section 18-9) chemical reactions within the cells of the style that destroy a growing pollen tube if it is too closely related. Three of the most effective general mechanisms for reducing self-fertilization are described here (FIGURE 18-21).

1. Separate male and female flowers. In some plants, an individual produces some flowers with only male reproductive parts and others with only female reproductive parts. Overall, the plant is still a hermaphrodite, but these unisexual flowers minimize the likelihood of self-pollination and self-fertilization. Examples of plants with separate male and female flowers include corn and most fig species.

2. Staggered maturation of male and female reproductive parts. To prevent self-fertilization in hermaphrodite flowers, some plants have flowers with male parts that develop before the female parts, or vice versa. An example of this pattern occurs in geraniums.

3. Separate male and female plants. The most extreme method of avoiding self-fertilization is for some plants within a population to produce only flowers with male reproductive parts, while others produce only flowers with female reproductive parts. As in humans, then, there are two separate sexes. This makes self-fertilization impossible, but it also reduces the likelihood that any given visit by a pollinator will result in pollination and fertilization. Examples of plants with separate male and female plants include poplars, American holly, and willows.