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This is a tomato plant,

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and this is an aphid slowly killing
the tomato plant

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by sucking the juice out of its leaves.

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The tomato is putting up a fight
using both physical and chemical defenses

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to repel the attacking insects.

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But that's not all.

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The tomato is also releasing compounds
that signal nearby tomato plants

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to release their own insect repellent.

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Plants are constantly under attack.

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They face threats ranging from 
microscopic fungi and bacteria,

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small herbivores, like aphids,
caterpillars, and grasshoppers,

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up to large herbivores, like tortoises,
koalas, and elephants.

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All are looking to devour plants
to access the plentiful nutrients

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and water in their leaves, stems,
fruits, and seeds.

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But plants are ready with a whole
series of internal and external defenses

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that make them a much less 
appealing meal,

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or even a deadly one.

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Plants' defenses start at their surface.

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The bark covering tree trunks is full
of lignin,

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a rigid web of compounds that's
tough to chew

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and highly impermeable to pathogens.

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Leaves are protected by
a waxy cuticle

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that deters insects and microbes.

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Some plants go a step further
with painful structures

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to warn would-be predators.

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Thorns, spines, and prickles
discourage bigger herbivores.

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To deal with smaller pests, some plants'
leaves have sharp hair-like structures

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called trichomes.

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The kidney bean plant sports tiny
hooks to stab the feet of bed bugs

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and other insects.

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In some species, trichomes also dispense
chemical irritants.

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Stinging nettles release a mixture
of histamine and other toxins

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that cause pain and inflammation
when touched.

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For other plant species, the pain comes
after an herbivore's first bite.

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Spinach,

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kiwi fruit,

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pineapple,

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fuchsia

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and rhubarb all produce microscopic
needle-shaped crystals called raphides.

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They can cause tiny wounds in the inside
of animals' mouths,

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which create entry points for toxins.

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The mimosa plant has a strategy
designed to prevent herbivores

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from taking a bite at all.

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Specialized mechanoreceptor cells
detect touch

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and shoot an electrical signal
through the leaflet to its base

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causing cells there to release
charged particles.

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The buildup of charge draws
water out of these cells

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and they shrivel, 
pulling the leaflet closed.

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The folding movement scares insects away

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and the shrunken leaves look less
appealing to larger animals.

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If these external defenses are breached,

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the plant immune system 
springs into action.

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Plants don't have a separate immune system
like animals.

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Instead, every cell has the ability
to detect and defend against invaders.

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Specialized receptors can recognize 
molecules that signal the presence

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of dangerous microbes or insects.

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In response, the immune system initiates 
a battery of defensive maneuvers.

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To prevent more pathogens from making
their way inside,

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the waxy cuticle thickens and cell walls
get stronger.

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Guard cells seal up pores in the leaves.

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And if microbes are devouring one section
of the plant,

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those cells can self-destruct 
to quarantine the infection.

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Compounds toxic to microbes and insects
are also produced,

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often tailor-made for a specific threat.

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Many of the plant molecules that humans
have adopted

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as drugs, medicines and seasonings

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evolved as part of plants' immune systems
because they're antimicrobial,

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or insecticidal.

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An area of a plant under attack can alert
other regions using hormones,

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airborne compounds,

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or even electrical signals.

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When other parts of the plant detect
these signals,

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they ramp up production
of defensive compounds.

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And for some species, like tomatoes,

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this early warning system also
alerts their neighbors.

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Some plants can even recruit
allies to adopt a strong offense

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against their would-be attackers.

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Cotton plants under siege by caterpillars

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release a specific cocktail of ten
to twelve chemicals into the air.

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This mixture attracts parasitic wasps 
that lay eggs inside the caterpillars.

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Plants may not be able to flee the scene
of an attack,

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or fight off predators with teeth
and claws,

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but with sturdy armor,

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a well-stocked chemical arsenal,

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a neighborhood watch,

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and cross-species alliances,

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a plant isn't always an easy meal.