On Mexico's flag, what plant is the eagle perched on?

On Mexico's flag, the eagle is perched on a prickly pear cactus.

The plant is a nopal cactus, a type of prickly pear native to Mexico and central to the emblem in the flag's white stripe. The coat of arms shows a golden eagle gripping a serpent while standing on the cactus, which grows from a rocky island in a lake.

The image refers to the Mexica or Aztec foundation legend of Tenochtitlan. According to the story, the people were instructed to establish their city where they saw an eagle on a cactus devouring a snake. They recognized the sign on an island in Lake Texcoco, and Tenochtitlan was founded there; Mexico City later developed on the same site.

The flag's vertical green, white, and red tricolor dates from the independence era, but the current national flag was adopted in 1968. The central emblem distinguishes Mexico's flag from Italy's visually similar tricolor, which has no coat of arms in its white band.