The mitzvah of the lulav and esrog performed on Sukkos symbolizes the intrinsic unity of the Jewish people. The fulfillment of this mitzvah requires us to hold together either fruit or branches from four different species of trees - the date palm (lulav), the myrtle (hadas), the willow (aravos), and the citron (esrog).
These four species are noticeably different from one another.
The esrog has both a pleasant taste and a pleasant fragrance. The fruit of the tree from which the lulav is taken, the date, has a pleasant taste, but no fragrance. The myrtle has a pleasant fragrance but no taste, and the willow has neither fragrance nor pleasant taste.
Taste symbolizes Torah study, because understanding the Torah gives us concrete pleasure, similar to the sensation of experiencing a pleasing flavor.
Smell symbolizes the fulfillment of mitzvos, because the quality which motivates us to fulfill the mitzvos is an unquestioning acceptance of the yoke of heaven. Since we often do not understand the reasons for the mitzvos, their observance may be less tangibly gratifying than Torah study is, in much the same way that smelling something is less palpably gratifying than tasting it.
An extension of this symbolism enables us to see each of the four species as representing different types of individuals.
The esrog represents a person who studies Torah and fulfills the mitzvos, the lulav represents one who studies Torah but does not perform mitzvos, the myrtle represents one who fulfills mitzvos but does not study Torah, and the willow represents a Jew who neither studies Torah nor observes mitzvos.
No individual can attain fulfillment unless he is willing to go beyond himself and join together with his fellow man. Even the esrog, the species which symbolizes both the virtues of Torah study and observance of the mitzvos, cannot be used for the mitzvah on Sukkos unless it is taken in hand and held together with the humble willow.
The concept of unity is so central to this mitzvah that it is reflected not only in the requirement of taking all four species together, but also in the characteristics of the individual components of the mitzvah.
Our Sages stipulate that a lulav may be used for the mitzvah only if its leaves are bound together. The only species of myrtle that may be used for the mitzvah is that which has successive rows of three leaves each. In each row, the three leaves must be level with each other, with no leaf significantly higher or lower than another.
The species of willow used also expresses the concept of unity, since it grows in bunches.
The motif of unity is also reflected in the esrog. Indeed, because the esrog represents a category of people whose potential for achievement is greater than that of others, its emphasis on unity must be greater.
The esrog expresses the concept of unity by virtue of the fact that it grows on the tree for an entire calendar year, and is exposed to all the seasonal variations and changes of climate. Not only does the esrog withstand all these influences, it responds positively to them; each of these influences contributes to its growth.
We must learn from the esrog, and not merely tolerate people of all kinds, including those with characters and personalities very different from our own, but actually grow through contact with their divergent perspectives. As our Sages teach, "Who is wise? - One who learns from every man."