More than 80 percent of Madagascar's 14,883 plant species are found nowhere else in the world, including five plant families. The family ''Didiereaceae'', composed of four genera and 11 species, is limited to the spiny forests of southwestern Madagascar. Four-fifths of the world's ''Pachypodium'' species are endemic to the island. Three-fourths of Madagascar's 860 orchid species are found here alone, as are six of the world's nine baobab species. The island is home to around 170 palm species, three times as many as on all of mainland Africa; 165 of them are endemic. Many native plant species are used as herbal remedies for a variety of afflictions. The drugs vinblastine and vincristine are ''vinca'' alkaloids, used to treat Hodgkin's disease, leukemia, and other cancers, were derived from the Madagascar periwinkle. The traveler's palm, known locally as ''ravinala'' and endemic to the eastern rain forests, is highly iconic of Madagascar and is featured in the national emblem as well as the Air Madagascar logo.