Oo.. nice question! Plants have both physical and chemical defences which are really important as when a herbivore comes along they can’t just run away!
Passionflowers are cool – they make bumps on their leaves, which look like a butterfly has already lain eggs on it – so butterflies stay away. You don’t want hungry caterpillars eating all your freshly made leaves! Some also make sugar that attract ants, who also eat butterfly eggs.
I find the warning system of the Acacia tree to be highly interesting. When an animal (such as a giraffe) starts eating its leaves, the tree sends out ethylene gas. Other Acacia trees nearby detect the gas presence and start producing high amounts of Tannins (molecules that cause proteins to precipitate) in their leaves, which make the giraffes sick when they eat them.
If you have watched Wild-Isle by David Attenborough recently, they also talk about the “Wood-Wide-Web”, whereby the trees in every forest are connected via underground fungal networks. These networks allow trees to share nutrients and send out warnings of drought, disease and insect attacks, allowing the trees to respond accordingly.
If you want to check it out it is on BBC iplayer Season 1 Episode 2.
The plant immune system is really interesting, and it’s something that a lot of people don’t think about. Like us plants can get infected by diseases, and they respond to this by detecting the germs, and toughening up their cell walls to stop the germs invading and making toxic chemicals to kill the germs.
Plants don’t have the complicated system we have, called an adaptive immune system, that has white blood cells that make antibodies to kill germs. But they do have something very similar to our other immune system: the innate immune system.
What I think is really interesting is that animals and plants have evolved separately to each other, and it’s clear they are very different, but their innate immune systems seem to have evolved the same answers to the same problems, they even use very similar molecules to do the same things.
For example in animals, aspirin works by quietening down a part of the immune system that causes inflammation and pain. Aspirin (acetyl-salicylic acid) was discovered in plants, where it (or a very similar molecule: salicylic acid) has a very similar function. In plants salicylic acid quietens down a part of the immune system that looks very similar to the animal inflammatory response. Do plants feel pain though? I don’t think so.
This is true in all plants, but I’m going to say willow trees are the most interesting, because this is where aspirin was discovered- salicylic acid is named after the willow tree (the latin name is Salix).