The four species represent the entire people of Israel: Torah scholars, performers of good deeds, and even people who possess neither quality. Nevertheless, it is only when they are all joined into a single bundle that the mitzva is fulfilled properly.

“The fruit of a pleasant tree” (Leviticus 23:40): This symbolizes Israel: Just as this etrog has both a good taste and a pleasant fragrance, so too, Israel includes people who have both Torah and good deeds.

“Branches of date palms” (Leviticus 23:40): These too are symbolic of Israel: Just as this date has a good taste but does not have a pleasant fragrance, so too, Israel includes people who have Torah but do not have good deeds.

“And a bough of a leafy tree” (Leviticus 23:40): This too is symbolic of Israel: Just as this myrtle has a pleasant fragrance but does not have a good taste, so too, Israel includes people who have good deeds but do not have Torah.

“And willows of the brook” (Leviticus 23:40): These too are symbolic of Israel: Just as this willow has neither a good taste nor a pleasant fragrance, so too, Israel includes people who have neither Torah nor good deeds.

And what does the Holy One, blessed be He, do with those lacking both Torah and good deeds? To destroy them is impossible. Rather, the Holy One, blessed be He, said: Let them all be bound in one bundle, and they will atone for each other. (Vayikra Rabba [Margaliyot] 30)