How Plants Get Dirty

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Somewhere in Australia, a Cryptostylis orchid is about to trick an unsuspecting wasp into getting down and dirty with it. Pseudocopulation, where a plant has evolved to resemble a certain insect in order to hoodwink others of the same species into having (pseudo)sex with it, is only one of the hundreds of techniques employed by plants to spread their pollen.

While the wasp may think it’s having sex, it is also inadvertently doing the orchid’s dirty work by contracting its pollen and distributing it to the waiting gynoecium — the female reproductive organ of most plants — of other flowers.

The sexual exploits of plants have a surprising amount in common with human coitus. By referring to plant reproduction by a familiar term like “plant sex,” scientists hope to make plant reproduction more palatable to the public.

“The females produce the offspring — seeds that have been fertilized by the [pollen of the] males,” said UC Santa Cruz Arboretum succulents curator Stephen McCabe.

More explicitly, UCSC Arboretum director Brett Hall said plants release pollen, which sticks to the plant’s pollen collector, the stigma. The pollen moves down a thin sheath called a style, to the base of the flower, where it then fertilizes the ovaries.

“Just like in many organisms where the eggs are fertilized and continue to grow into embryos,” Hall said, “this is where the seed forms, and then you have your germ of a new plant.”

However, some scientists shy away from the phrase “plant sex” out of fear people will improperly attribute other animalistic qualities to plants, such as advanced cognitive abilities.

“When plant people start talking about studying plants’ neurobiology [the biology of neurons and nervous systems in animals] and plants’ supposed brains, thinking, choices and feelings, then things start getting muddy and other scientists are not as supportive,” McCabe said.

Other scientists embrace the term “plant sex” for making plant reproduction more relatable and approachable.

“If you define sex as the various activities leading up to and going through the reproductive process, it’s sex,” Hall said. “Pollination and everything, it’s just plant sex.”

Animals and insects are beguiled into fertilizing plants through two primary tactics.

“You can have various rewards, like nectar or pollen in the case of some bees,” Hall said. “The organisms, whether birds or insects, will go for the reward. They are cued into it over a millennia of evolutionary development.”

Similar to humans, plants also use trickery, or mimicry, in order to pollinate. Certain plants, like the Cryptostylis orchid, will adopt appearances or other characteristics that trick insects into carrying pollen from flower to flower.

However, not all plant reproduction cycles are so easily compared to human processes.

Some plants are asexual and do not need to reproduce, while some have both male and female organs and can reproduce independently.

“Some plants reproduce without sex by having bulbs divide,” McCabe said. “Others will send out runners above ground like strawberries or send out underground stems to make new plants like in many iris species.”

However, most plant species persist through the same sexual reproduction methods as animals. Strawberries, for example, also have flowers for normal reproduction techniques, McCabe said.

“Most plant species have some aspect of sexual reproduction in their life cycle,” said the Jean H. Langenheim Chair in Plant Ecology and Evolution Ingrid Parker. “Self-fertilizing is like a ‘back-up plan’ for some species.”

Despite its variety of methods, plant reproduction is still generally similar to the sex of animals.

Parker suggests the myriad ways plants interact sexually help broaden our conceptions of reproduction.

“You could say an understanding of plant sex makes you more appreciative of the range of possibilities out there for how reproduction happens,” Parker said.