Humans are fascinated by gore and violence, but even more so the mysterious and unsolved. Interest in these disturbing and unpleasant subjects is called morbid curiosity, and it has gripped millions of people throughout the ages. I am one of those people. My name is Hallie, and this is the Morbid Curiosity Podcast.
To the uninitiated, plants are beautiful, serene, stationary background actors in a world of mammals and other moving, active beings. They are alive, but only just. But to those in the know, plants are living things that actively participate in the world around them. Most surprising is the fact that while beautiful, most plants are not particularly friendly.
Like most creatures that have survived for millions of years, plants have evolved sophisticated defense mechanisms to protect themselves from the creatures that might eat them, damage them, or otherwise harm them. Plant defense mechanisms are mostly invisible to the naked eye. Toxic chemical agents hidden inside, released when the plant is eaten, and only apparent when it's too late. However, some plants have obvious defenses, such as the thorns of a rose or the spikes of a cactus.
Some of these look small, but have a mighty effect, such as the stinging hairs on the nettle and the oil on the leaves of poison ivy, which causes an intensely itchy rash. Plant defense mechanisms are fascinating, and in this episode, we're going to explore the different ways plants protect themselves. We'll also explore some of the more extreme examples of these defenses. A staggering number of animal species eat plants.
Some are large grazing mammals, including deer or cattle, but most are insect species and arthropods, as in hundreds of them. Plants can't move to get away from their predators, so they've evolved to survive as best they can, just like animals do. Many times, plants and animals evolve together, each developing adaptations that help them defeat each other. For example, a plant develops thorns to protect itself from being eaten, and the animal that preys upon it develops a thinner face in order to eat between the thorns.
This is often called an evolutionary arms race, but the scientific term is coevolution. Another version of coevolution are plants that produce special chemicals in order to attract certain insects or herbivores that will protect them from predators. I'll get into this more in a moment. Coevolutionary theory suggests that this arms race between plants and herbivores is the reason behind the expansive diversity among plants.
Plant defenses are classified as either constitutive or induced. Constitutive defenses are always present, while induced defenses are those that spring into action at the site of an injury. There's lots of variation in both types, and some plants have a combination of the two. Constitutive defenses range from mechanical defenses, like thorns or spikes, to chemicals that hinder digestion or poison the predator.
Constitutive defenses are usually part of the plant and take much of the plant's energy to produce, but the benefits outweigh that cost. The plant has constant protection from predators, which is beneficial if predators are numerous and also constant. Thorns are sharp, pointy branches or stems. Their defensive function is somewhat obvious.
They can stab or poke predators that try to eat them. These are positioned similarly to branches in a pattern along the stem and share the plant's vascular system, or nutrition channels. Plants with thorns often have smaller leaves as well, making them difficult to get to through the thorns. If a predator is persistent, thorns slow down the rate at which they consume the plant, as well as wear down their teeth.
Some examples of plants with thorns include many varieties of citrus fruit trees, hawthorns, and acacia trees. Spines are another stabby defense mechanism. Spines are modified leaves or parts of leaves. They often replace leaves, especially in drier climates.
These spines have vascular tissue, just like thorns do. Many plants have spines, but the most obvious example are cacti. Spines not only defend the juicy stems of cacti against predators, they also shade them from the sun. Cacti that grow in non-desert environments tend to have fewer spines.
Holly plants also have spines on the tips of their leaves. Another defense that can poke and stab are prickles. With the naked eye, they look the same as thorns, but unlike thorns, prickles are pointed protuberances from the plant's epidermis, its outer tissue layer, not its stem. They're also positioned randomly along the stem.
They deter predators and aid the plant in growing over competing plants. Fun fact, roses actually have prickles, not thorns. If you've ever had the misfortune of brushing against a stinging nettle, you've felt the power of the tiny trichome. Trichomes resemble tiny hairs or fur on the leaves of a plant.
They can impale or eviscerate smaller predators like insects. Some trichomes, like those of the nettle, also have glands that inject toxic chemicals into the wounds they create, just like a hypodermic needle. Stinging nettles inject histamine that causes contact dermatitis, a type of inflammation characterized by itchy red bumps and blisters. The trichomes of some topical plants can cause permanent nerve damage or even death.
More on that after the break. If thorns, spines, and prickles are the fortresses that defend these plants, then idioblasts are the landmines. Idioblasts are specialized cells that contain defensive compounds, which vary from razor-sharp crystals to pain-inducing chemicals that detonate when the first line of defense has been breached. The Dieffenbachia, a common houseplant also known as the leopard lily or dumb cane, contains idioblasts that fire tiny needle-like calcium oxalate crystals, called raphids, into the mouths of predators.
These then release an enzyme that functions similar to reptilian venom, which can cause a numbing sensation, swelling, difficulty swallowing, and burning on the mouth that can last from several minutes to weeks. Some less potent raphids can be found in the idioblasts of pineapples, kiwis, agave, and monstera plants. Not all plants maintain their defenses on the surface. Many plants hold certain chemicals inside their stems, leaves, and fruit to deter herbivores.
The evolution of chemical defenses inside plants is thought to stem from the production of secondary metabolites, chemical substances that aren't necessary for the plant to live. These substances are often byproducts of essential processes like photosynthesis. For example, tannins, also called tannic acid, which cause the plant to taste bitter, deterring some predators. They often also prevent absorption of certain nutrients.
These low-impact chemical substances might not have been enough of a deterrent for some plants, which resulted in the development of higher-impact substances, highly toxic compounds that can poison predators. Many of these compounds are alkaloids, which we've discussed in our previous plant episodes, Plants That Kill, Plants That Kill Too, and Plants That Thrill. In summary, some plants contain chemical substances, such as alkaloids, which can cause extreme physiological and neurological reactions in predators, including humans. For example, the leaves and berries of the belladonna plant, which contain the alkaloid atropine, scopolamine, and hyoscyamine, can cause delirium and hallucinations in humans if eaten.
The seeds of the strychnine tree contain the alkaloids strychnine and brucine, which can cause convulsions and death via asphyxia in humans, but is tolerated by some animal species. Plant defensive chemicals are dosage-dependent. They may have little negative effects on herbivores at low or moderate doses, but are more dramatic at higher concentrations. Plants with constitutive defensive secondary metabolites are always toxic.
Plants with inducible defenses create their defensive chemicals in response to certain situations, such as predation. Some induced defensive mechanisms can also cause morphological and physiological changes in the plant themselves, such as leaves crinkling up. Inducible defenses are useful in situations when the plant isn't always in danger from herbivores and or cannot spare the nutrients on constant protection. Induced chemical defenses occur only when the plant reacts to an infection, a parasite, or a predator.
A plant's ability to sense and respond to its environment is called plant perception. They can sense a broad range of stimuli, including chemicals, gravity, light, moisture, temperature, air quality, parasite infestation, disease, physical disruption, sound, and touch. They can respond to these stimuli by adjusting their chemical signals, morphology, and physiology. For example, some plants can induce rapid cell death to prevent the spread of a pathogen, sort of like an amputation.
An example of chemical signaling to prevent further injury is systemic acquired resistance. This is a reaction that occurs when herbivores eat a plant and leave saliva behind on the leaves. The plants react to the saliva by releasing chemical signals to their cells, which then produce a higher concentration of salicylic acid, a hormone that's essential for regulating plants' immune systems. This hormone then signals all the leaves to increase the production of tannins, making them bitter and difficult to digest.
Some plants react in a more mechanical way. The mimosa pudica, also called the sensitive plant, sleepy plant, and touch-me-not, is a creeping plant that folds its leaves inward and droops when touched or shaken. The leaves reopen a few minutes later. The mechanism that controls this is similar to that of a Venus flytrap, which we talked about in our Plants That Bite episode.
Specialized cells on the top and bottoms of the leaf expand and contract when stimulated by touch, causing it to curl up. Other plants produce volatile organic compounds that attract the predators of their predators. Volatile organic compounds, or VOCs, are another form of secondary metabolite, and over 1700 VOCs have been identified so far. They are often released from flowers as a scent, but can sometimes come from Parts of Tanzania where it's considered an invasive species.
The sandbox tree can grow up to 60 meters or 200 feet tall. Its trunk is covered in long, sharp spikes that secrete a poisonous sap. Lumber workers must cover their eyes when clearing these trees as the sap can cause temporary blindness if it enters the eye. This tree also produces large, pumpkin-shaped woody fruits which are made up of 16 capsules arranged radially like the sections of an orange.
When ripe, the capsules explode, splitting into segments and launching seeds up to 250 kilometers per hour. That's 160 miles per hour. Seeds are dispersed from 14 to 30 meters away with the longest distance ever recorded being 45 meters. That's 147 and a half feet.
The seeds contain an oil that's toxic when consumed and can cause dermatitis when touched. The woody segments were once filled with sand and used by Western colonists to blot their fountain pens before the invention of blotting paper, hence the name sandbox tree. Sandbox trees are sometimes cultivated for medicinal or ornamental purposes. The sap is used as arrow poison and to treat skin diseases, rheumatism, and intestinal worms.
In the United States at one point, the sap was used to prepare tear gas. Due to its lightweight nature, the wood of this tree is sometimes used to make canoes and furniture under the name hira. The seeds can be made into biodiesel and soap. One of the most dangerous plants in Australia is Dendrocnide, the stinging tree.
This genus of tree and shrub is part of the nettle family of plants and is found in rainforest areas of Australia and Malesia, a biogeographical region which includes Indonesia, Singapore, Malaysia, Timor-Leste, Brunei, and the Philippines. There are four common species of Australian Dendrocnide, two being large rainforest trees that can grow up to 35 meters or 115 feet tall, while the other two are small shrub-like bushes. The smaller of the species have a more painful toxin and are responsible for the most stings due to their size and ease of accessibility. The plants tend to grow in sunny areas protected from the wind, along the edges of streams, walking tracks, and roadways.
All four species of Dendrocnide have a stinging mechanism. However, Dendrocnide moroides, the gimpy gimpy shrub, has the most painful sting. Contact with the leaves, stems, or fruit cause intense burning pain, which intensifies after a few minutes and includes severe swelling. In some cases, the sting can lead to anaphylaxis, a life-threatening allergic reaction that causes such severe swelling that breathing is obstructed.
It can also cause blindness if the victim is hit in the face. The pain lasts for a minimum of six to eight hours, but can persist for weeks, months, and sometimes years. Heat can ease the pain, but cold makes it much worse. Like the nettle, the leaves of the stinging trees are covered in trichomes, tiny needle-like hairs that contain a toxic chemical substance.
When a person brushes against the leaves, stems, or fruits, the fine five-millimeter trichomes become embedded in the skin and inject that substance into the tissue. North Queensland road surveyor A.C. MacMillan was among the first people to document the effects of the stinging tree in English. In 1866, he reported to his supervisor that his pack horse was stung by a plant, went mad, and died within two hours.
Indigenous folklore contains many tales of horses jumping off cliffs in agony after contact with the plant. More recently, the plant poses a danger to scientists and bushwalkers. Marina Hurley, an entomologist and ecologist, spent three years in Queensland's Atherton Tableland studying stinging trees and the herbivores that eat them. Her protective particle mask and welding gloves failed to protect her on several occasions, one of which required hospitalization.
After so many exposures, she developed a severe allergy to their sting. She said it was, quote, like being burnt with hot acid and electrocuted at the same time, end quote. The giant gimpy gimpy, or Dendrocnide excelsa, has long been a hazard for foresters, surveyors, and timber workers. Tales abound of forestry workers drinking themselves silly to dull the immense pain.
Today, they are supplied with respirators, thick gloves, and antihistamine tablets as a precaution. The only known treatment for the sting of a gimpy gimpy is applying hydrochloric acid to neutralize the trichomes' peptide coating, and then using wax strips to carefully remove the hairs. This treatment isn't always effective. Painkillers, heat, and time are the only other options.
Professor Irina Vetter and her team from the University of Queensland's Institute for Molecular Bioscience recently discovered that the gimpy gimpy toxin contains a protein never before documented by science. The primary structure of it resembles the protein toxins in cone snail venom, which can cause intense pain, paralysis, vomiting, changes in vision, and respiratory failure in humans. While the sting of the gimpy gimpy is quite painful, Professor Vetter theorizes it could reveal a lot about how pain works, which she hopes could lead to a new treatment for chronic pain. Vetter and her team have found a unique pathway between the plant's toxins, named gimpytides, and proteins associated with the human gene TMEM233, which causes pain in a way that's never been documented before.
If scientists could discover how to block this type of pain, they could develop a new type of painkillers, ones that don't come with the risk of addiction or other negative side effects. Another dangerous Australian tree is Rhyparosa corongii, which can only be found in the Daintree, a small area of tropical rainforests in Queensland. It's a smaller tree that produces flowers and fruit from tubercles, or lumps, on the lower parts of its trunk. The large leaves grow at all levels of the trunk, regardless of how much light those areas get.
The fruit has dark hairs all over its skin and one to several seeds inside, making its shape quite variable. The fruit is a favorite food of cassowaries, large black flightless birds with blue necks and large keels on their heads. They've developed a sort of mutualism, in that the seeds of the plant gain nutrients while in the cassowary's digestive tract, and are more likely to germinate once they are excreted in the cassowary's droppings. The danger of this tree comes when it's damaged.
When the tree is injured, it emits a blast of hydrogen cyanide in the richest concentrations ever recorded in nature. Hydrogen cyanide, or HCN, is a colorless or pale blue liquid or gas with a bitter almond-like scent. HCN is toxic to both animals and plants as it interferes with the body's use of oxygen. In animals, it may cause harm to the brain, the heart, blood vessels, and lungs.
It's so dangerous that during World War I, HCN was used by the French as a chemical weapon against the Central Powers in 1916, and by the United States and Italy in 1918. However, as the gas is lighter than air and rapidly disperses up into the atmosphere, it wasn't an effective battlefield weapon. To prevent poisoning themselves, the plants limit the production of HCN by storing its individual components in adjacent vacuoles or compartments of the cell. When the cell is damaged, the compartment walls are breached, the components meet, and a reaction takes place, which creates HCN.
This type of defense isn't unique to Rhyparosa corangi. Cyanogenesis, or the liberation of HCN from mixing the components within the cell when damaged, is recognized in over 3,000 species of plants. Cassava, a type of tuber, apple seeds, and the pits of cherries, peaches, and apricots all contain various cyanogenic components, which, when processed by enzymes within the plant or the human digestive system, can release hydrogen cyanide. However, if the shell of the seeds or pits is unbroken, usually they will pass through the digestive system without causing harm.
But if they're cut or chewed, poisoning can occur. If a human ate unprocessed cassava, 20 to 40 apricot pits, 30 peach pits, or 200 apple seeds, symptoms of cyanide poisoning would occur. HCN disrupts cell respiration. Without energy, cells die rapidly and on massive scales.
Symptoms include vomiting, nausea, headaches, and convulsions. All of these foods are perfectly safe to eat if processed correctly and eaten in small amounts. One of the most dangerous trees in the northern hemisphere is the manchineel tree, Hippomane mancinella, also known as manzanilla de la muerte, which means little apples of death in Spanish, is found in the Caribbean, Florida, the Bahamas, Mexico, Central America, and northern South America. It usually grows along the coast or in saltwater swamps among the mangrove trees.
In areas where the manchineel is known to grow, many trees have been marked with a warning sign, a red X or red band on the trunk, to indicate the danger. The manchineel tree has a milky white sap that contains numerous toxins and is present in every part of the tree, bark, leaves, and fruit. It has reddish-grayish bark, small greenish-yellow flowers, and shiny green leaves, and it can grow up to 15 meters or 49 feet tall. Its fruits resemble green apples, hence the Spanish name.
Any contact with any part of the tree results in skin blistering, and if it gets into the eyes, severe damage and blindness. It's so toxic that standing beneath a manchineel tree during rain is dangerous. A raindrop that lands on the tree then drips off the leaves takes with it For our merch and our mailing list, we also have an Amazon wishlist at bit.ly slash morbidwishlist, that's B-I-T dot L-Y slash morbidwishlist. Any purchases from this list are greatly appreciated.
We are eternally grateful for your support. My name is Hallie, and until next time, thanks for listening.