Weird and wonderful members of the plant kingdom - plant sex

Flowering plants have evolved one of the most complicated sexual systems in the natural world. In fact, they have 'double fertilization' involving two sperm rather than the usual one. In a mature seed, the embryo originates from a zygote formed by the fusion of sperm #1 with an egg inside the embryo sac. Sperm #2 fuses with the two polar nuclei forming the nutrient-rich endosperm tissue. So, complex, in fact that botanists have been forced to create special terms to explain plant sexuality including: unisexual, bisexual, asexual, self-fertile, self-sterile and polygamous. At least 90% of all flowering plants have bisexual flowers containing male and female organs, and most of these species avoid inbreeding and incest by having their sex organs mature at different times. The term homosexual is not correct for plants, although many plants are unisexual with flowers of only one sex. Figs are fascinating, not just for their own sex lives, but because they inspire sex in other creatures! They possess tiny unisexual male and female flowers inside a fleshy structure called a syconium. A female wasp squeezes into the syconium to pollinate the flowers and lay her eggs. After a few months, the new generation of wasps have an orgy inside the syconium and fertile females exit and start the entire cycle over again.

Up until the late nineteenth century, sexuality in plants was denounced by theologians who preferred to believe in the literal translation of the Bible. According to Genesis, plants were created on the third day. Because it wasn't until the sixth day that animals and people were created - at the time of plant creation there couldn't have been any words for male and female. Obviously, without male and female there could be no sex, therefore plants did not have sex!

Believe it or not, plants also have sexually transmitted diseases. One of the most serious is anther smut, a venereal disease of the plant world. This debilitating fungal disease causes the male organs (anthers) to blacken and shrivel, resulting in sterility. Anther smut is spread by unsanitary, promiscuous insect pollinators who carry the infectious spores from one plant to another. The spores infect healthy individuals during the spring flowering season, emerging a year later when the plants re-enter their mating cycle. Wise plants avoid the risk of sexually transmitted diseases by developing an asexual life style. They simply clone themselves vegetatively. Some can even produce seeds without sex. The embryo develops parthenogenetically from an unfertilized egg or from other cells in or surrounding the embryo sac.

Plant sex photograph by markresch, used under a creative commons attribution licence