Although plants do not have a central nervous system and it is unlikely that they can feel pain the same way animals and humans do, they do not lie idly by and watch their greenery get munched on by hungry critters — they protect themselves. Some species have thorns, while others are loaded with poisons. And some have very interesting abilities such as emitting a horrible stench and folding their leaves.
Here are a few examples of plants with strange, but useful, defense mechanisms:
Mimosa pudica, also known as the sensitive plant, is a creeping herb of the pea family, Fabaceae, that is often grown out of curiosity — the compound leaves fold inward and droop when they are touched or shaken, a way of defending themselves from harm. This makes them appear dead and thus unappetizing. They re-open a few minutes later.
The types of movement the plant undergoes are termed seismonastic, and the movements occur when specific regions of cells lose turgor pressure, which is the force that is applied onto the cell wall by water within the cell vacuoles. So when the plant is disturbed, the stems release chemicals that force water out of the cell vacuoles, leading to a loss of pressure and cell collapse.
It is not clear why Mimosa pudica evolved this trait, but many scientists think that the plant uses the ability to shrink as a defense mechanism from herbivores and dangerous insects.
Mimosa pudica is also known for emitting a foul smell (similar to a common bodily function) when its roots are disturbed by humans.
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Stinging nettles (Urtica dioica) grow a bristling fur called trichomes, which are pointed structures that shield the plant from hungry predators. The trichomes act like hypodermic needles whose tips come off when touched, injecting histamine, acetylcholine, serotonin and other chemicals that produce a stinging sensation when touched by humans, animals or insects.
Some of the plants even inject poison into the trichome-inflicted wounds causing permanent nerve damage or death.
Photo credit: Fayes4Art/Flickr (CC BY 2.0)
Not all plants wear their defenses on the surface. “Idioblasts” are cells within the plant that store specialized chemicals and are needed when the first line of defense has been breached.
Dieffenbachia, a common houseplant, contains idioblasts that fire barbed calcium oxalate crystals into the mouths of predators and then release an enzyme similar to reptilian venom known as raphides. This can cause paralysis and a loss of speech, hence the plant’s common name, “dumb cane.”
Bullhorn Acacia Tree
Photo credit: Feroze Omardeen/Flickr (CC BY 2.0)
Some plants hire mercenaries to do their dirty work in a process known as commensalism. Bullhorn acacia trees (Vachellia cornigera) both house and feed aggressive ants. The ants dwell inside the trees stipular spines, or thorns, and feed off food bodies produced especially for them by the plant.
The ants will viciously defend the trees against everything that comes near them, including animals, plants and fungi. They have even been known to snip off the foliage of any other plants that get too close to the tree. In experiments where researchers removed the ant colonies, the trees have died.
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