I spotted these betel leaves (also known as betel pepper or piper betle) outside a house in Taitung City. They're cultivated for use in Taiwan's massive betel-nut industry (betel nut is the island's no. 2 cash crop, after rice). The leaves are wrapped around the betel nut (Areca catechu), a stimulant favoured by truck drivers, manual labourers and others, and sold from roadside kiosks by skimpily-dressed young women.
According to this website, "The betel leaf is ised in a number of traditional remedies for the treatment of stomach ailments, infections, and as a general tonic... Some evidence suggests that betel leaves have immune boosting properties as well as anti-cancer properties."
If the last point is true, it's just as well chewers chomp on the leaves while enjoying betel nuts, as habitual use of the nut dramatically increases your chances of suffering from oral cancer.