A kale plant has green or purple leaves 
I love kale! I've only ever seen the kind with green leaves, though.
Before the Middle Ages, kale was one of the most common green vegetables in Europe.
I didn't know that. I don't think of it as being associated with a lot of the stereotypical foods of Europe - say, French or Italian or British food.
Cereals remained the most important staple during the early Middle Ages as rice was introduced late, and the potato was only introduced in 1536
I think of potatoes when I think of European food - whenever I visit my family in Europe, we eat a ton of potatoes. But I guess they were discovered in the New World, so they aren't some ancient native plant
Potatoes were introduced to Europe late in the 16th century by the Spanish.