It’s such a small word…so easy to miss.
I’m surprised that pepper isn’t at least a contender. It played a pretty significant role in the creation of the spice trades. From here:
Yes, so my answer should have been ‘domesticated maize’. I wasn’t counting the domestication process. In this case, I’m actually naming a plant that DIDN’T change history, i.e., the utility of the domesticated product may have inhibited possible changes.
Apparently the sequel will have to be named “1,342 plants that had some effect on history” 
The food crops being mentioned certainly allowed huge population increases and some historically significant famines, but when it comes to changing history I nominate the rubber tree.
Without rubber the Industrial Revolution could not have happened. Artificial rubber made some dents but natural rubber is still required for many things; I think it’s about 40% natural rubber these days. The history of rubber is full of stories about massive deforestation, slavery, political intrigue, seed stealing, etc., and the story continues to this day as some of the first rubber plantations in Laos begin to produce – the plantation contracts foisted upon poor uneducated subsistence farmers by Chinese wheeler-dealers – it’s another human and ecological tragedy in the making as hundred of thousands of acres of land is turned into a monoculture of plants the locals can’t eat while the good paying jobs go to foreign workers.
Eh, it’s only arguably a plant, but none of these even come remotely close to the impact the first blue-green algae had. We think our air pollution is bad? These things replaced a fifth of the atmosphere with one of the most reactive, poisonous, and all-around dangerous gasses known.
Tobacco.
I’d also include the lime (and other citrus), poppy, tea, and grudgingly, rice.
I think the Olive has to be on the short list.
Will grow with little cultivation.
Provides both food and fuel.
Drought tolerant and hardy.
Olives–interesting. My second string: alfalfa, allowing dairying to supplant grain farming and rejuvenating the soil; soya, quinine, corn, and the American Chestnut, whose demise screwed up the ecosystem on half of North America.
I think you could usefully distinguish between plants that changed the world because of an inherent qualities (rubber, quinine) and those that changed the world because of their social function (sugar cane, spices). Those are really different things.
See Steven Johnson’s talk on “Did Coffee Fuel the Age of Enlightenment?”. The basic argument is that, before coffee arrived in Europe, people typically had a glass of wine or beer with lunch and then basically zoned out; when coffee houses began to appear, people had coffee after lunch, woke up, and started coming up with ideas.
True.
Guess I’d lump in staple food plants all together in the “allows population to increase” category; the medicinal/psychoactive drug plants in another category; the practical plants and trees in another. I could see your social category for sugar sources and spices & herbs, but I tend to look at those as allowing improved food preservation.
What about various wood trees that enabled humans to build all kinds of things from boats to houses as well as providing wood pulp for paper. Though I guess most early paper wasn’t made from wood pulp.
I’d vote for Robert to be #1.
Spices had an enormous effect - not because they’re a big crop, but because they were highly sought after. They had a massive effect on exploration, the development of trade and shipping routes, international alliances, territory claims, etc.
I don’t know if I could narrow it down to a top 5 list, but I’d vote as the most recent would be U.S. government subsidized corn. The economic impact has affected food production for the entire world; The use of corn as an ingredient has affected how we manufacture food (ex: high fruitcose corn syrup) and our eating habits. Food Inc. has some very interesting ideas about the relationship between government and corn. I have to admit, I never before considered the idea of feeding animals corn to be strange.
One of the triad of plants – grapes and I forget which kind of grain that allowed cities (and hence civilisation) to develop around the Meditteranean.
Medicines made from willow and other salicylate-rich plants date back at least to 400 BCE.
Where would man be without these plants? Has to be one of the more important discoverys.
yew.