The Torah's foundational command, recorded in Vayikra 23:40, is to take on the first day of Sukkot "the product of hadar trees, branches of palm trees, boughs of leafy trees, and willows of the brook" and to rejoice before God seven days — the verse from which the entire rabbinic and exegetical tradition of the four species unfolds.
The Gemara in Sukkah 34b preserves the tannaitic derivation of the precise quantities required: Rabbi Yishmael counts one etrog, one lulav, three myrtle boughs, and two willows from the language of the verse, while Rabbi Akiva argues that just as the lulav and etrog are each one, so too the myrtle and willow are each one — a dispute the halachic tradition had to resolve.
The Rambam (Mishneh Torah, Shofar, Sukkah and Lulav 7:5) rules that the four species together constitute a single mitzvah in which each one is indispensable — they are collectively called "the mitzvah of lulav" — and no species may be added, diminished, or substituted; the Rambam (Mishneh Torah, Shofar, Sukkah and Lulav 7:6) further rules that the ideal fulfillment is to bind the lulav, myrtle, and willow into a single unified bundle, held in the right hand alongside the etrog in the left.
The richest strand of meaning comes from the midrashic allegory in Vayikra Rabbah 30:12, developed and transmitted by Rabbeinu Bahya (Vayikra 23:40): each species maps onto a distinct type within Israel — the etrog, which has both taste and fragrance, represents those with Torah and good deeds; the lulav (date palm), with taste but no fragrance, represents those with Torah but without good deeds; the myrtle, with fragrance but no taste, represents those with good deeds but without Torah; and the willow, with neither, represents those lacking both — and God commands them to be bound together as one, so that each atones for the other.
רש"ר הירש, ויקרא, וראה שם עוד presses further, arguing that one should not imagine the species as a simple harvest-thanksgiving symbol, since most of them have no connection to agricultural produce; rather, the etrog unites within itself the qualities distributed among the other three species, and all positions in Israel's spiritual life must be brought near to God and made complete — just as the sages compared the species to the Patriarchs, to the righteous, and to those with Torah and good deeds who must form one bundle.
A second major symbolic reading, found in Midrash Tanchuma, Emor 19 and drawn out by Sefer HaChinukh 324 and Sefer HaChinukh 324:4, maps the four species onto the principal limbs of the human body: the lulav corresponds to the spine, the myrtle to the eyes, the willow to the lips, and the etrog to the heart — hinting that one should serve the Creator with one's intellect, straighten one's whole body to His service, guard one's eyes, and place a bridle on one's speech.
Midrash Tanchuma, Emor 20 adds a remarkable note of wonder: even Solomon, the wisest of all men, set aside all his wisdom and sat bewildered before these four species, seeing in the verse of Mishlei — "Three things are wonders to me, and four I do not know" — an allusion to the four species of the lulav whose inner meaning he sought to fathom.
מדרש הגדול, ויקרא כג מ offers yet another reading, identifying the four species as corresponding to the four standards (דגלים) under which Israel encamped in the wilderness.
The Gemara in Sukkah 37b explains the practice of waving the lulav in all directions: Rabbi Yochanan teaches that the to-and-fro motion dedicates the species to the One to Whom all four directions belong, while the up-and-down motion dedicates them to the One to Whom heaven and earth belong; a parallel tradition from Eretz Yisrael adds that the waving is intended to ward off harmful winds and harmful dews.
On the Kabbalistic plane, Pri Etz Chaim, Gate of Lulav 1:3 teaches that the four species correspond to the four letters of the divine Name and to the sefirot of Zer Anpin, with the three myrtles and two willows expressing specific aspects of their inner structure, and the etrog receiving the unified flow — while Recanati on the Torah, Emor similarly reads the lulav as the central column and axis, the myrtle as an allusion to three sefirot simultaneously, and the etrog as standing apart from the bundle because it corresponds to Shemini Atzeret, a festival unto itself.
Hasidic interpretation adds further dimensions: Sefat Emet, Devarim, Sukkot 21:5 understands the lulav as corresponding to Moses — the "friend of the King" who brought Torah down from heaven — through whose taking Israel, as it were, draws close to God Himself, since Torah and the Holy One are one; Sefat Emet, Devarim, Sukkot 2:4 connects the act of taking the species to Israel's own inner desire to cleave to God, expressing through the physical act a reception of true life (noting that the gematria of לולב equals חיים); and Zera Kodesh, On Festivals, Sukkot reads the four species as drawing down all the divine attributes and influences through the midah of Yosef-Yesod, which blends and channels all bounty to the community of Israel.
The Ramban (Vayikra 23:40) situates the exegetical tradition honestly: the rabbinic identifications of "pri eitz hadar" with the etrog are, on his view, Scriptural supports (asmakhtot) for a received tradition, while his own preferred reading is that "hadar" is actually the Hebrew name for the tree the Aramaic calls etrog — meaning "delight" or "beauty" — making the identification intrinsic to the text itself rather than merely a homiletical peg.
Arukh HaShulchan, Orach Chaim 645:1 grounds the entire mitzvah in the verse of Vayikra and rehearses the received tradition (קבלה) that identifies each phrase with its species, while detailing the physical characteristics of the lulav as the young, tightly-closed frond of the date palm before its leaves spread apart — reminding the reader that the legal requirements flow directly from careful attention to the natural form of each species.
