Divine flow is given only where there is unity. Each of the four species exhibits unity in a distinctive way. The leaves of the palm branch are attached to its spine, making it look as though it is one branch. Myrtle leaves overlap with each other. Willows grow close together, as though they are friends, while the citron grows through all four seasons of the year.

Divine flow from above comes only to a place where there is unity:

Elicitation of the infinite light occurs only in a place of oneness, that is, in a place where there is unification, and not in a place of division…. With regard to these four species, each one of them possesses the element of oneness and integration.

Unity expressed through the palm branch:

With regard to the palm branch, all its leaves are integrated, and they combine and ascend together as one with the spine, and they are all attached and unified with each other as though they are one entity.

Unity expressed through the myrtle:

The myrtle, too: Its leaves grow in triads, that is, three leaves emerging from one point. As is known, this is an aspect of oneness and integration. The myrtle is also called “a bough of a leafy tree” (Leviticus 23:40), because the foliage covers the tree [that is, the wooden stem of the branches], which demonstrates unification, as one leaf covers the next, such that it looks like one surface.

Unity expressed through the willow:

The “willows of the brook” (Leviticus 23:40) are called “ahvana” in the Talmud (Shabbat 20a), because they grow in fellowship [ahva], that is, they grow in such a way that they are joined together like beloved friends.

Unity expressed through the citron:

The citron is called “the fruit of a pleasant [hadar] tree” (Leviticus 23:40), because it “dwells [dar] in its tree from year to year” (Sukka 31b) [i.e., the citron can stay on the tree throughout the entire year without falling off or rotting]. This is because it, too, contains an element of oneness and integration. For all fruits do not survive from year to year, due to the changes in the seasons. The cold or the heat of summer or winter affect them, and they are unable to endure two opposites, like cold and afterward hot, and they therefore spoil and rot. But the fruit of the citron tree is not affected by these changes; that is, it is able to endure all the changes of the seasons, of cold and heat, because it possesses an element of oneness and integration. Therefore, it can endure the combination of different kinds of opposites [and in this way, it unites within it the effects of the four seasons]. (Rabbi Shneur Zalman of Liadi, Sefer HaMa’amarim 5568, p. 447)

Further reading: For more on the unity of the Jewish people, see p. 113.