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It’s no big news that plants can’t move. If something is attacking a philodendron, it can’t just get up and walk away, which might lead you to believe that plants are sitting ducks. But that couldn’t be further from the truth.
Because they don’t have the ability to escape their attackers, plants have developed various defense systems to protect themselves from leaf-munching insects, stem-gnawing mammals, and any number of other marauding enemies. They have an entire arsenal of tricks that, for the most part, protects them really well. But plants are subtle about their defenses. They don’t brandish big, loud weapons in the faces of their enemies. Instead, they quietly go about their business, deterring or killing them silently (*insert maniacal plant laugh here*). The whole thing is pretty damn cool.
Plants protect themselves in three primary ways:
1. Physical defenses
The most visible forms of plant protection fit into this category. These are the things we humans can see. Physical defenses come in the form of spines that keep thieving monkeys out of a tropical fruit tree, thorns that prevent foliage from getting browsed by deer, sharp needles intended to keep giraffes from grazing, fuzzy leaf surfaces that deter feeding insects and mammals, waxy leaf coatings whose surface cannot be pierced by tiny bug teeth, and many other innate physical barriers. Most physical defenses are “turned on” all the time – they are always present in, or on, the plant, constantly working to deter pests.
2. Chemical defenses
These are the defenses we can’t see. Plants have a near infinite number of ways to defend themselves through the chemical compounds they produce. Certain plants excrete a sticky substance to trap an offending insect when under attack. Or they may release an alkaloid (Can you say nicotine, caffeine, strychnine, and quinine, kids?), a terpenoid, or any number of other chemical compounds to deter, or even poison, whatever insect or mammal is feeding on the plant. There are scores of these secondary metabolites used by plants. For example, tannins make certain fruits distasteful by giving them an astringent flavor, the cardiac glycosides present in the milkweed plant make it poisonous to all but a few specialized herbivores, and the quisqualic acid in the flower petals of geraniums cause temporary paralysis in Japanese beetles (the little buggers kinda look like they’re drunk before they pass out flat on their back, legs in the air). These are just a few examples of the millions of chemical defense strategies employed by plants. Plants also use some of these compounds as an internal communication system, relaying alarm signals to surrounding cells when the plant is under attack by a bacteria, fungus, or insect pest.
3. Indirect defenses
Plants also use some very interesting techniques to protect themselves that do not involve taking direct action against offending pests. Probably the most surprising of these indirect defenses are the volatile chemicals many plants release into the air as an SOS. These compounds are intended to signal predatory and parasitoidal insects, inviting them to come to the plant’s aid and consume the pests munching on their foliage. For example, when a tomato is being attacked by a tomato hornworm, the plant may release a volatile chemical into the air to attract the cotesia wasp, a parasitoid that uses the hornworm as a host for its developing young. The communication system between the plant world and the insect world is jaw-dropping stuff, and it serves as a pretty special way for plants to protect themselves.
As you can see, plants are far from sitting ducks. They’re unique beings that have evolved some absolutely amazing ways to keep themselves alive. Pretty clever, huh?