I tend to like green growing things. I think most people do (unless it's allergy season, and then quite a lot of people stare darkly out of their puffy eyes at the happily growing trees and grass). Green has been my favorite color for a long time, mostly because I associate it with plants and Nature, and for me Nature is at times almost synonymous with Fantasy.
So, simply because I thought the subject might be interesting, I spent some time last year reading books on edible and medicinal plants. I wasn't disappointed! The books were fascinating indeed.
Have you ever thought about how much of a battlefield Nature is? Imagine you are a plant. (Yeah, ok, you're nothing even remotely resembling a plant, but bear with me a moment.) There are a lot of things out in the wild that want to eat you. But you don't want to be eaten. So what do you do?
Well maybe your strategy is a strong defense. You start producing poisonous chemicals to “discourage” your would-be munchers. Bingo! No more getting eaten.
Or maybe you think the best defense is a good offense. So you start producing pheromones to attract the natural predators of whatever is trying to eat you. So when the muncher shows up for snack time, it becomes the snack instead.
Or maybe you like subterfuge. The chemicals you produce aren't detectable while you're being eaten, but they get into the system of the muncher and block its ability to reproduce. Might not be effective now, but when there are no little baby munchers showing up in the next generation you get to have the last laugh. (Ever wonder where modern birth control originated? Look no further than these types of plants.)
Ok, you can stop being a plant now. So what does all this mean for humans? Well, some of these chemicals are poisonous to people as well as insects and tiny critters. But some of them have other effects, like thinning the blood or getting rid of bacteria or calming the nerves. So these are the precursors of a lot of our modern medicines. (And what I find really fascinating: sometimes the difference between poison and medicine is merely a matter of dosage.)
Here's the brief history of medicine: plant does something clever to prevent being eaten; human stumbles upon the plant and consumes it—trial (hey! it cured my headache!) and error (ack! I'm dead!); modern chemists look at plants that have been used for centuries all over the world to find what chemicals are causing the various reactions; chemists then make synthetic compounds based on the plants to sell in the drug stores.
Now there's this whole list of steps the army has come up with for testing whether a plant you don't recognize is safe to consume (and the whole process takes over 24 hours). I don't recommend going out into the woods and attempting this just for the sake of doing it, but it's great information to have if, for example, you're writing a story about characters who are lost in the woods without anything to eat.
And if you're looking for more information, go find your local library and check out a few books! There are plenty more interesting facts about plants that I haven't even come close to addressing.