12.0.5 12.9: A flower is nothing without a pollinator.

Picture a flower. What does it look like? Flowers differ hugely from one to another, and we all have our favorites. There are giant sunflowers, colorful roses, and some rotten-smelling orchids and lilies, to name just a few. The label “flower” would seem to indicate little beyond the fact that flowers are usually brightly colored, frequently have a prominent odor, and generally sit at the end of a plant’s branches. Closer inspection, however, reveals that, with very few exceptions, all flowers have the same four fundamental structures: sepals, petals, stamens, and carpels. Here we examine the importance of flowers to sexual reproduction in plants.

Fertilization occurs when the male gamete merges with the female gamete. But that’s a lot easier said than done: the male gamete has to get to where the female gamete is. The first step toward this is pollination—when a pollen grain makes the journey to the stigma.

A small number of angiosperms achieve pollination by releasing tremendous amounts of pollen into the wind, as do the gymnosperms, or into water—on the slim chance that some will land on the female reproductive organs of another plant of the same species. Most angiosperms, though, have a different way of moving pollen from the anthers of one flower to the stigma of another: they use animals to carry it (FIGURE 12-21).

To ensure that animals will visit a flower, picking up and delivering their pollen cargo, two strategies for achieving pollination have evolved among the flowering plants.

1. Trickery. The plant deceives some animals into carrying its pollen from one plant to another. Some orchid species, for example, produce flowers that resemble female wasps. The mimicry is so good that male wasps mount the flower and attempt to mate with it. The male wasp twirls wildly on the flower, like a cowboy on a bucking bronco, repeatedly whacking his head against the strategically located anthers and getting pollen stuck all over his head and body. That is not enough for the plant to achieve pollination—but it’s a start. If that male wasp gets fooled again by another orchid flower and mounts it in an attempt to mate, he will inadvertently deposit some of the pollen from his body onto the also strategically placed stigma of that flower. In the end, the wasp does not gain from his actions, but the orchids have an effective system of pollination.

2. Bribery. The plant bribes some animals to carry its pollen from one plant to another. Rather than just using trickery, the plant offers something of value to the animal. For this mutually beneficial method (a “mutualism”; see Section 15-14) to work, the plant must produce (a) a sticky pollen, (b) a flower that catches the attention of the pollinator, and, most important, (c) something of value to the pollinator. The payoff for the pollinator can be food, such as nutritious nectar rich in sugars and amino acids, or perhaps a safe, hospitable location for an insect to lay its eggs.

The variety of flower structures is tremendous. They differ in shape, color, smell, time of day they are open, whether or not they produce nectar, and whether or not their pollen is edible. Pollinators also vary widely and include birds (mostly hummingbirds), bees, flies, beetles, butterflies, moths, and even some mammals (mostly bats) (FIGURE 12-22). In each case, there has been strong coevolution between the plants and their pollinators: the plants become increasingly effective at attracting the pollinators and deterring other species from visiting the flower, while the pollinators become increasingly effective at exploiting the resources offered by the plants. Because of this strong coevolution between the plant and animal species, for most flowers we can determine, just by examining the features of the flower, the type of animal that will pollinate it.

Pollination is just one step toward fertilization. Fertilization itself doesn’t happen until the male and female sex cells meet and fuse. We explore this process in the next section.