Until the late 19th century, all tea in world trade was China tea. But when China began to resist Britain’s practice of paying for its expensive tea habit with opium, the British intensified tea production in their own colonies, particularly India. For warm regions they cultivated an indigenous variety, Camellia sinensis var. assamica, or Assam tea, which has more phenolic compounds and caffeine than China tea and produces a stronger, darker black tea. They planted the hardier China types in the Himalayan foothills of Darjeeling and at high elevations in the south. India is now the world’s largest tea producer.