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Plants are always under attack from various things like insects and other animals. While they might not appear to be doing much to defend themselves, plants have several defense mechanisms to keep themselves safe. What’s more, they’re even able to send chemical signals to nearby plants to warn them of impending danger; pretty amazing, right?
Why Do Plants Have Defense Mechanisms?
Did you know that a plant can sense when it is touched? Moreover, plants are able to detect when they are being attacked and can respond appropriately. But why is this? Well, it’s mainly down to ensuring their survival as things like herbivores would otherwise wipe them out.
To Deter Insects & Herbivores
Insects like aphids or larger animals like elephants and many in between are all herbivores that feed on plants. Yes, this is part of the circle of life, but a plant isn’t just going to sit there and take it. While we might not be able to actively see what’s going on, plants will deter insects and herbivores in several ways.
For example, some plants will place themselves out of sight of potential predators. This is known as decreasing their apparency and sees plants growing in hard-to-reach places such as high above where animals can reach or on cliff edges.
What’s more, plants will use chemicals to repel insects, and they can even send out signals to other plants of the same species, urging them to do the same.
They also have a range of physical defenses, such as bark that’s difficult to chew, spines, or they may just taste bad.
Protection against Pathogens
If you have ever attempted to grow plants then you’ll know that they are susceptible to various diseases. These are caused by pathogens that can be fungal, bacterial, or viral and a disease is described as something that prevents a plant from performing to its maximum potential.
But plants are ready and able to protect themselves against these disease-causing pathogens using things like impenetrable bark. They use their surface as a first line of defense against pathogens, but if this fails, they aren’t giving up yet.
Should the pathogen get through, then the plant will attack it with toxins and enzymes to either fight or harm the pathogen.
Ways Plants Defend Themselves
It’s amazing to think that plants have so many effective methods to protect and defend themselves. From physical or mechanical features like thorns or chemical reactions to attack pathogens and even the ability to mimic other plants to deter herbivores, these are true feats of nature!
1. Mechanical Responses
Some plants have hairs known as trichomes, while others have spines or thorns. These mechanical defenses all work in different ways, but their aim is the same; to stop anything from wanting to touch the plant.
If you have ever brushed against a plant with trichomes, like the stinging nettle, you’ll be familiar with how painful and itchy the after effects can be; that’s the plant trying to defend itself from you. There are even some forms of tropical stinging nettles that inject venom that can be fatal for any insects trying to take a bite.
You may have also been brave (or silly) enough to touch a cactus with its stabby spines; yes, they serve as a warning to leave the plant alone. But spines also protect the plant in another way; they cast shade over it which is super important in the blazing heat of the desert.
Thorns and prickles protrude from a plant and stab any would-be attacker, telling them firmly that this plant isn’t good for eating!
Have you ever heard about those lizards that will drop their tails if they’re caught by a predator? Well some plants behave in the same way. They do this with their leaves, but rather than allowing them to fall off entirely, the plant will respond to touch by closing its leaves or dropping. This serves as a way to dislodge anything that may have landed on it. It’s a trait commonly seen in the mimosa pudica plant.
Some plants just might not have the defense tactics to effectively protect them but that doesn’t mean that they aren’t masters of disguise. Instead of actually being pointy, prickly or otherwise dangerous, these plants will mimic the appearance of insects to deter others from landing on them. For example, the passion flower is covered in brown spots that look like butterfly eggs. Any butterflies passing by will soon fly away and choose somewhere else to lay their own eggs. Passion flower: 1, butterfly: 0!
2. Physical Responses
One of the most notable passive physical responses of plants is the cellulose cell wall which helps to prevent pathogens from getting into the plant. This is essentially a barrier that’s different from the waxy cuticle or bark that acts as a first layer of defense.
The cell wall is triggered by pathogens that have made it through the first layer and it produces a chemical called callose which strengthens the cell wall, making it more resistant to the pathogen. This is a feature only seen in plants and never in animals.
However, the problem is that there are some pathogens that can release a chemical that softens and breaks down this cell wall, making it easier to get in. That’s where plants fall victim to diseases.
Before any pathogens would have the chance to get through to the cellulose cell wall, they’d have to make it through the waxy cuticle. This is a covering that is found on the stems and the leaves of almost all plants. Not only does it provide a barrier to pathogens but also to pests which would otherwise break through the plant’s epidermis and damage the leaves.
Note that some plant diseases are spread via water and if this accumulates on the leaves, there’s a greater chance of infection. It’s a good job then that the waxy cuticle also acts as a waterproof shield for the leaves.
Woody plants and trees have a layer called bark and this again provides external protection to the plant that can stop pathogens and prevent infections. Bark is actually a layer of dead cells and acts in a very similar way to human skin.
The job of the bark is also to protect the xylem and phloem which are vascular tissues found in plants. They are responsible for transporting nutrients made during photosynthesis and without them, the plant would not survive, so protection is essential.
3. Chemical Responses
There are two main types of chemical responses by plants. One that is designed to kill bacteria, which is known as an antimicrobial chemical, and one that will poison anything that wishes to eat the plant. Not only are these bacteria-killing defenses excellent for plants but they’re often used in human medicine and are incredibly effective.
Some plants have alkaloids that have a horrible taste and that’s enough to deter any herbivore from wanting to take a bite. Coffee is a great example of this, and while it does taste bad, it also poisons the animal by speeding up its metabolism. On the flip side, some opioids make the animal incredibly lethargic.
For any animal looking to indulge in a foxglove for dinner, it may be better left well alone. That’s because these plants contain toxins that affect the cardiac system, can cause convulsions, and may even be fatal when ingested.
The mint plant and the witch hazel plant produce antimicrobial chemicals that can kill bacteria. These same plants also release tannins which not only taste bad but are toxic to certain insects. Some plants can even detect when they’re being attacked by certain creatures and will release chemicals that serve as a warning to insects like the aphid to stay away.
Plants with Unique Defense Mechanisms
The plant world is brimming with unique species that have special defense mechanisms designed to ensure their very survival. Here are some rather interesting examples.
1. Passion Flowers (Passiflora spp.)
The passion flower is indeed a beautiful plant but it’s also very deceptive. Those little yellow spots you see on the leaves of this plant aren’t just for decoration. They’re actually pretending to be butterfly eggs in order that other butterflies don’t come and lay their eggs on the plant. After all, once they hatch, the caterpillars will wreak havoc for the plant.
It’s the heliconius butterfly that seems to be the problem, and if they’re unwise enough to lay their eggs on the plant, the caterpillars won’t survive for long. That’s because the stems and leaves contain a cyanide-based chemical that’s toxic to the butterflies. In addition, some types of passiflora produce a sticky substance from their leaves that traps insects.
What’s more, there are some species of passionflower that have adapted the shape of their leaves to mimic other species that aren’t attractive to the butterflies.
2. Shameplant (Mimosa pudica)
Isn’t it amazing to think that some plants are able to sense when they are being touched? That’s an ability of the shameplant, sometimes called the sensitive plant, for obvious reasons.
When it detects another organism on its leaves, it will fold them up and pretend to be dead. The stems droop down and the plant looks a whole lot smaller. What’s more, when it does this, its spines are exposed, acting as a deterrent to any would-be diners.
Native to Central and South America, the shameplant is not the most attractive but this unique behavior makes it very interesting. But what blows my mind the most is the speed at which the leaves fold in; this isn’t a plant that messes around!
3. White Dead Nettle (Lamium album)
The white dead nettle uses mimicry to fool anything that wants to eat it for their next meal. For all intents and purposes and to the untrained eye, the white dead nettle looks exactly the same as the stinging nettle. But this member of the mint family is actually harmless. In fact, they’re bursting with vitamin A and are a forager’s dream.
The way to tell the difference is to look at the flowers. Those of the stinging nettle are more green/brown, while the white dead nettle has little white flowers. Not that a herbivore or insect would be able to use this to identify the species.
4. Bullhorn Acacia (Vachellia cornigera)
The bullhorn acacia doesn’t protect itself on its own. It actually has a mutualistic relationship with the acacia ant. In situations like these, both species benefit from the relationship, and having the ants present within the tree means that should an animal try to take a bite, they’ll get a nasty surprise.
In return, the ants benefit from a comfortable home within the hollow thorns of this Central American tree and are even known to clean out the inside. There’s also been research that shows the mere presence of the ants lowers the number of bacteria on the tree’s leaves, therefore reducing the chances of disease.
5. Borage (Borago officinalis)
Borage has pretty little blue flowers, so it’s quite an attractive plant. However, what isn’t so attractive are the hairy leaves that can cause some nasty irritation to the skin. In fact, in humans, it’s not uncommon to suffer with contact dermatitis after touching the leaves of this plant.
But that doesn’t appear to stop humans from foraging the plant which has a light, refreshing taste similar to cucumber. However, it’s worth being mindful when handling the plant as research has shown that phytodermatitis, a condition caused by touching certain plants, could be linked to skin cancer.
6. Dumb Cane (Dieffenbachia amoena)
The dumb cane is a common houseplant, but it’s also pretty dangerous to predators. It has a very interesting defense that some might compare to a landmine!
When it is touched, the plant explodes idioblasts made from barbed calcium oxalate crystals. Any animals going to bite into the plant ends up with these in their mouths and it’s very unpleasant. For example, keeping dumb cane if you have cats is about as smart as the name of the plant. When your moggy bites it, those barbed crystals rip into the tissues inside the mouth and can cause symptoms ranging from pain to breathing difficulties.
Moreover, the toxic venom released by the plant is very similar to reptile venom and can cause more severe symptoms like paralysis and loss of speech; it’s not hard to see where the plant gets its name!
7. Barrel Cactus (Ferocactus spp.)
Most cacti have spines, but none are quite as impressive and intriguing as those that belong to the barrel cactus. This stubby, rotund desert plant is covered in long spines that are designed to protect the plant from any daring animal that might try to take the plant as a meal.
What’s more, those spines protect the plant from the hot desert sun by casting shade over its thick skin. This skin is covered in a waxy coating that prevents excess water from evaporating because, in the desert, plants need to hold onto as much moisture as possible.
8. Black Walnut Tree (Juglans nigra)
The black walnut tree is one of the most territorial trees in the world since it has a defense mechanism to prevent other plants from growing in the nearby area and taking its nutrients. How does it do this? It releases a chemical known as juglone from the roots. This is a natural herbicide and will prevent plants from growing. It’s so effective that humans often use it to manufacture herbicide products.
But it’s also effective in preventing bacterial pathogens from harming the black walnut tree. Again, humans have harvested the powerful effects of juglone for killing off harmful organisms and it’s even recommended by health practitioners.
9. Stone Plants (Lithops spp.)
Lithops, sometimes called living stones, have taken mimicry to the next level by having adapted to look like a stone. But they’re living, photosynthesizing examples of plants that animals would never try to eat because who wants to chow down on a rock?
The stone plant grows in the Southern parts of the continent of Africa and one of the ways it camouflages perfectly into its surroundings is not only to look stone-like but it’s also the same color as the substrate to better blend in.
What’s even more interesting is that the bulk of the plant remains under the ground. What we see on the surface is the very tip and on occasion, you may be lucky enough to see a flower.
10. Poison Ivy (Toxicodendron radicans)
If you’ve ever spent time in nature, you’ve probably been warned to avoid the poison ivy plant, and for good reason. This plant produces a chemical known as urushiol, which causes skin rashes that are terribly itchy when humans come into contact with it. This also serves as an effective defense mechanism against hungry herbivores.
The word urushiol comes from the Japanese meaning sticky, and that’s because the substance sticks to anything it comes into contact with. So, if you’re exploring woodlands, it’s best to be on your guard and wearing long sleeves and pants to avoid contact, especially since between 10-15% of humans are allergic to the chemical.
11. Foxglove (Digitalis purpurea)
Foxgloves are beautiful flowering plants, but looks can be deceptive. These plants use a chemical response to defend themselves by producing a toxin called digitalis. This toxin can be fatal to certain animals and can build up excess calcium in the body which affects the muscles, in turn making the heart beat faster.
Animals that are brave enough even just to have a small nibble on the top of the plant could find themselves in trouble. The smallest amount of this toxin can cause very unpleasant symptoms like hallucinations, vomiting, diarrhea, and delirium. In the worst cases, the foxglove will kill whatever tries to eat it.
12. Dogwood Trees (Cornus spp.)
Dogwood trees have leaves that contain sticky latex. One of the purposes of this substance is to hold the leaf together should it become damaged. However, this also serves as a defense against animals that try to eat the plant, as they’ll get a mouthful of latex, and that’s hardly the delicious meal they were expecting.
The dogwood tree is a woodland species that doesn’t get much bigger than 33 feet (10 meters) in height. You’ll be able to spot these trees easily, as the latex is very obvious when the leaves are pulled apart. To further deter animals, the dogwood tree has flowers that give off a very unpleasant smell.