Numbers 17:2

Speak unto the children of Israel, and take of every one of them a rod according to the house of their fathers, of all their princes according to the house of their fathers twelve rods; write thou every man's name upon his rod. This was in accordance with the rule that the oldest son of a father's house, that is, of a patriarchate, within a tribe was considered a prince. For the member of each tribe that held this position one rod was to be chosen. These rods were small branches or twigs of trees that grew in the neighborhood, probably all, like that of Aaron, such as were taken from almond-trees.