You may not think of a plant as sexually active. Yet plants include some of the most promiscuous organisms on the planet. “Plants can produce hundreds of seeds sired by many other plants,” said Dr. Spencer Barrett, a professor at U of T’s faculty of botany. Barrett studies the evolution of plant reproductive systems in angiosperms. Angiosperms, or flowering plants, include weeds, fruiting trees and crops.

Plant sex, like human sex, involves the fertilization of a female sex cell, or egg, by a male sex cell, or sperm. However differences abound. In humans, males and females inhabit separate bodies, males producing sperm and females producing eggs. To produce offspring, males impregnate females by fertilizing eggs with sperm. However, noted Barrett, “most angiosperm plants are hermaphrodites producing male and female sex cells.” So a single plant can act as both father and mother. As a father, it can disperse its sperm (packaged in powdery granules called pollen) and fertilize the eggs, or ovules, of other plants. As a mother, the same plant can get fertilized by sperm from another plant and become pregnant with seed.

For humans and plants, sexual success involves looking attractive and smelling good. But plants do not attempt to entice their sexual partners, because of their immobility. Instead, plants lure mobile middlemen, called pollinators, with their attractive floral displays. Common pollinators include birds, bats, beetles and butterflies. The floral display, tailored to a plant’s particular pollinators or pollinators, advertises the energy-laden nectar found inside. “The plant bribes the pollinator with nectar and the pollinator unknowingly carries the plants’ pollen to other plants,” said Barrett.

In a study recently published in the journal Nature, Barrett described the South African plant Babiana ringens. The plant sprouts a stalk that functions solely as a perch for its pollinator, the sunbird Nestarinia famosa. The perch positions the bird to ensure pollen stuck to the bird contacts the receptive female part, or stigma, of another plant.

By depending on pollinators to shuttle pollen between plants, plants promote outcrossing-sex with other plants. You may wonder why they bother; if a plant produces pollen and ovules, why doesn’t it fertilize itself? This process of self-sex, called selfing, would save the plant the time and energy required to produce floral displays and nectar. In fact, many plants deliberately discourage selfing, and Barrett has made major contributions to our understanding of why plants refuse to self.

“One reason is inbreeding depression,” said Barrett. This occurs in sexual species when individuals procreate with close genetic relatives-such as cousins, siblings or in the case of selfing plants, themselves. The resultant offspring suffer from high mortality. In plants, it reduces the quality and number of seeds a plant produces. By avoiding selfing, plants avoid inbreeding depression.

Another reason plants avoid selfing concerns the number of offspring a plant can father with other plants. When a plant fertilizes its own ovules with its own pollen, it cannot use that pollen to fertilize ovules of other plants. So plants that self produce fewer offspring with other plants. This may seem an odd reason given that a plant produces millions of sperm packaged in millions of pollen granules.

Even if a plant uses some of its pollen to fertilize itself, shouldn’t it have more than enough left to fertilize other plants? “Only 1% of pollen successfully fertilizes other plants,” said Barrett. When a plant fertilizes its own ovules with its own pollen, it reduces the amount of pollen that would potentially fertilize ovules of other plants. Since only a small percentage of pollen successfully fertilizes other plants, any reduction of that number compromises the chance a plant will fertilize other plants.

Why should we care whom plants have sex with? Consider genetically modified (GM) crops. Many people fear modified plants can escape company enclosures and compete with natural non-modified plants. “These genes would mainly escape in pollen,” said Barrett. Knowing how, when and why crops produce and receive pollen would allow for better containment of GM crops.