“And you shall take for yourselves on the First Day the fruit of a beautiful tree [etrog], the branches of date palms [lulav], branches of the myrtle tree [hadasim] and branches of the willow tree [aravot], and you shall rejoice before HaShem, your God, for seven days” (Leviticus 23:40).
The biblical passage — http://shlk.com/166 — simply mentions three branches and an odd-looking fruit. But when these four species are bound together, something special happens: An annual ritual can be celebrated.
There are many interpretations about the significance of the components of the lulav and etrog and similarities drawn between the four species to those of the Jewish people, including those offered by Jewish Outreach Institute at www.joi.org/celebrate/sukkot/species.shtml