The Symbolism of the Four Species, Rav Meir Shpiegelman
The four species we take on Sukkot symbolize four different forms in which water appears in the world. The aravot, or "brook willows," symbolize the water in brooks and streams. The lulav is taken from the palm tree, which grows near fountains of water. The etrog, as Chazal describe (Sukka 35a; Kiddushin 3a), "grows on any water," even through irrigation, and does not depend on proximity to a natural water source. Finally, the hadasim depend entirely on rainwater.
In Eretz Yisrael, water serves a dual purpose. Beyond its standard function as a liquid critical for the existence of all living things, it also symbolizes God's close supervision over the Land of Israel:
"For the land that you are about to enter and possess is not like the land of Egypt from which you have come, where you sow your seed and water it by your foot, like a vegetable garden. But the land you are about to cross into and possess is a land of hills and valleys – it drinks water from the rain of the heavens. It is a land which the Lord your God looks after, on which the Lord your God always keeps His eye, from year's beginning to year's end." (Devarim 11:10-12)
The festival of Sukkot relates to the Almighty's providence over Am Yisrael when they left Egypt and then entered the land. We therefore celebrate this festival by taking the four species, which symbolize divine providence as manifest through water.
The Arba'ah Minim, Alan Lucas (Rabbinical Assembly)
In some ways, the mitzvah of the lulav and etrog presents a bit of a challenge to moderns accustomed to rituals that are transparent in their meaning and easily decipherable. Still, one of the dangers in contemporary Judaism is precisely that we have become overly cerebral in our approach to religion. After the intense High Holiday period of prayer and introspective thought, Sukkot appeals to our senses. We build the sukkah with our hands, and we smell the four species and shake them back and forth as a sign of our exuberant sense of thanksgiving to God for all that we have in this world. Judaism makes demands both on the intellect and the spirit, both on the brain and on the heart, and Sukkot is a vibrant reminder of this lesson.