This is a tomato plant and this is an aphid slowly killing the tomato plant by sucking the juice out of its leaves the tomato is putting up a fight using both physical and chemical defenses to repel the attacking insects but that’s not all the tomato is also releasing compounds that signal nearby tomato plants to release their own insect repellant plants are constantly under attack they face threats ranging from microscopic fungi and bacteria small herbivores like aphids caterpillars and grasshoppers up to large herbivores like tortoises koalas and elephants all are looking to devour plants to access the plentiful nutrients and water in their leaves stems fruits and seeds but plants are ready with a whole series of internal and external defenses that make them a much less appealing meal or even a deadly one plants defenses start at their surface the bark covering tree trunks is full of lignin a rigid web of compounds that’s tough to chew and highly impermeable to pathogens leaves are protected by a waxy cuticle that deters insects and microbes some plants go a step further with painful structures to warn would-be predators thorns spines and prickles discourage bigger herbivores to deal with smaller pests some plant’s leaves have sharp hair like structures called trichomes the kidney bean plant sports tiny hooks to stab the feet of bed bugs and other insects in some species trichomes also dispense chemical  irritants stinging nettles release a mixture of histamine and other toxins that cause pain and inflammation when touched for other plant species the pain comes after an herbivores first bite spinach kiwi fruit pineapple fuchsia and a rhubarb all produce microscopic needle shaped crystals called RAF IDEs they can cause tiny wounds in the inside of animals mouths which create entry points for toxins the mimosa plant has a strategy designed to prevent herbivores from taking a bite at all specialized Meccano receptor cells detect touch and shoot an electrical signal through the leaflet to its base causing cells there to release charged particles the build-up of charge draws water out of these cells and they shrivel pulling the leaflet closed the folding movement scares insects away and the shrunken leaves look less appealing to larger animals if these external defenses are breached the plant immune system springs into action plants don’t have a separate immune system like animals instead every cell has the ability to detect and defend against invaders specialized receptors can recognize molecules that signal the presence of dangerous microbes or insects in response the immune system initiates a battery of defensive maneuvers to prevent more pathogens from making their way inside the waxy cuticle thickens and cell walls get stronger guard cells seal up pores in the leaves and if microbes are devouring one section of the plant those cells can self-destruct to quarantine the infection compounds toxic to microbes and insects are also produced often tailor-made for a specific threat many of the plant molecules that humans have adopted as drugs medicines and seasonings evolved as part of plants immune systems because they’re antimicrobial or insecticidal an area of a plant under attack can alert other regions using hormones airborne compounds or even electrical signals when other parts of the plant detect these signals they ramp up production of defensive compounds and for some species like tomatoes this early warning system also alerts their neighbors some plants can even recruit allies to adopt a strong offense against their would-be attackers cotton plants under siege by caterpillars release a specific cocktail of 10 to 12 chemicals into the air this mixture attracts parasitic wasps that lay eggs inside the caterpillars plants may not be able to flee the scene of an attack or fight off predators with teeth and claws but with sturdy armor a well stocked chemical Arsenal a neighborhood watch and cross species alliances a plant isn’t always an easy meal did you know there’s a ted-ed coloring competition once a month we take select scenes from your favorite ted-ed animations and carefully remove their original coloring we then invite the ted-ed community to recolor the illustrations in a friendly coloring competition hosted on our patreon page if you’re a participants send us your best work and you might see it featured on our social media channels in future ted-ed videos or if we really like it on our office fridge to learn more and to support our work please visit slash ted-ed thanks for watching.