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
the Americas in the second half of the 16th century by the Spanish. Today they are a staple food in many parts of the world and an integral part of much of the world's food supply. As of 2014, potatoes were the world's fourth-largest food crop after maize (corn), wheat, and rice.
Following millennia of selective breeding, there are now over 1,000 different types of potatoes. Over 99% of presently cultivated potatoes worldwide descended from varieties that originated in the lowlands of south-
Kale
Kale () or leaf cabbage is one of certain cultivars of cabbage ("Brassica oleracea") grown for their edible leaves, although some are used as ornamentals. Kale plants have green or purple leaves, and the central leaves do not form a head (as with headed cabbage). Kales are considered to be closer to wild cabbage than most of the many domesticated forms of "Brassica oleracea".
Etymology.
"Kale" originates from Northern Middle English "cale" (compare Scots "kail