Nature is a ruthless place. Wild animals live in a kill-or-be-killed world. Survival depends on finely tuned weapons, whether they be sharp teeth or camouflage. We expect these in the Animal Kingdom, where they’re key to defending turf and securing a mate, but recent research shows that animals aren’t the only ones to develop “weapons.”
A study published in New Phytologist has uncovered the first evidence that the struggle for propagation leads to the evolution of weaponry in plants as well. That study focused on the milkweed plant (which we’ll discuss below), but the milkweed’s reproductive weapons are only the tip of the iceberg. Read on for a handful of other plants that have developed surprisingly brutal defense systems.
We’ve cultivated this plant for decades. We consider it benign, common, and lord knows it’s been genetically-engineered to the hilt. Yet few realize that corn possesses the ability to call down a reign of terror on those who would munch upon its leaves. See, when an herbivore or insect starts to feast on the corn plant, it triggers a chemical response. The plant immediately produces what are referred to as “green leaf volatile compounds.” This cocktail of chemicals is like catnip to parasitic wasps. “These wasps fly to the plant that is being eaten, find the culprits and, depending on the species, employ a few different protection strategies: some wasps, like digger wasps (genus Sphex), will actually pick up the host and put it somewhere else,” explains Mental Floss. “Other wasps will lay eggs in the creature munching on the plant; those eggs hatch a la the Xenomorphs from Alien, often chewing their way out of the host’s body cavity within a day or two.” Corn 1, enemy 0.
As I hinted at in the introduction, the milkweed plant recently surprised scientists by demonstrating a sophisticated system whereby males of the species fight each other for a mate. Milkweed are known to reproduce by hooking sacs of pollen grains, known as pollinia, to the bodies of birds and other pollinators, which can be unwittingly dropped into another flower to complete pollination. During in-depth analysis of the South America milkweed genus Oxypetalum, the research team discovered horn-like structures on these pollinia sacs which have no obvious biological use. The paper suggests that these horns are used to prevent the sacs from being hooked together with competing pollinia from other parent plants. This means milkweeds may physically compete for access to the most ideal attachment points on the pollinator’s body.
3. The Stinging Tree of Australia
Imagine you’re walking through the Australian rainforest, minding your own business. Suddenly a searing pain shoots up your leg. It feels like you’re being burned alive while simultaneously being stabbed. You spin around, half expecting to see a wild animal with half your leg in its mouth, or at least a swarm of fire ants intent on eating you for lunch. All you see is green foliage, swaying gently in the breeze. Unbeknownst to you, the source of your searing pain isn’t a lethal animal but a plant known as the Stinging Tree of Australia (Dendrocnide moroides) or the Gympie Gympie. To protect itself, this plant developed a covering of microscopic hairs, all of which are loaded with an incredibly potent neurotoxin. Brush up against it, and those hairs become miniature hypodermic needles, burying themselves into your flesh.
They look so beautiful and innocent hanging there on the tree, but apples hide a deadly secret among their blossoms. Apples are one of over 3,000 different plants—including spinach and lima beans—that can cause the animals that eat them to choke to death. These plants compartmentalize hydrogen cyanide (HCN) in certain sugar and fat cells. After eating an apple, the HCN is released when cells are ruptured during digestion. Respiration is disrupted, and in some cases, the animal ceases to breathe. Lucky for us, the concentration of HCN is low enough that our bodies can handle it. Apples also hide cyanogenic glycosides in their seeds, and if you eat too many of them, even humans can suffer fatal effects.
This common garden veggie creates an interesting conundrum for those who say we shouldn’t eat “conscious” things. I’d say sounding the alarm for your community during an attack shows a definite desire to stay alive. “When a cabbage leaf is cut or eaten by a predator, the cabbage will emit a gas—methyl jasmonate,” explains the Harvard Community Garden page. “This gas sends a signal to the cabbages around it. When the signal is received, the nearby cabbages exude toxic chemicals in their leafs to ward off the potential predator that is noshing on the cabbages.” (For more on this fascinating topic, see “Sorry, Vegans: Brussels Sprouts Like to Live, Too” in the New York Times.)
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