The arava, the willow branch, has a dual function on Sukkot. It is the last of four species that make up the mitzvah of “lulav”. Without this lowly branch, it makes little difference how beautiful the etrog may be—as without the arava, there is no mitzvah of etrog. It bears repeating that our Sages saw the four species as representing different types of Jews, with the arava representing the Jew who is neither learned nor observant. And yet, if such a person is rejected, the etrog—representing the learned, observant Jew—is rendered invalid.