Because of the new trading resulting from the Columbian exchange, several plants native to the Americas have spread around the world, including potatoes, maize, tomatoes, and tobacco. Before 1500, potatoes were not grown outside of South America. By the 18th century, they were cultivated and consumed widely in Europe and had become important crops in both India and North America. Potatoes eventually became an important staple of the diet in much of Europe, contributing to an estimated 25% of the population growth in Afro-Eurasia between 1700 and 1900. Many European rulers, including Frederick the Great of Prussia and Catherine the Great of Russia, encouraged the cultivation of the potato.