3 And so the prophet is bidden to take an almond rod, because the fruit of this tree is bitter in its rind, hard in its shell, and inside it is pleasant, that after its likeness the prophet should set forth things bitter and hard, and should not fear to proclaim harsh things. Likewise also the priest; for his teaching, though for a time it may seem bitter to some, and like Aaron's rod be long laid up in the ears of dissemblers, yet after a time, when it is thought to have dried up, it blossoms.
Source: Letters (New Advent)