Gemara

(Mishnah): One may fulfill the Mitzvah of Maror with any of the following Yerakos - Chazeres, Tamcha, Charchavina, Ulshin, and Maror.

He is Yotzei whether they are moist or dry, but not if they were pickled, Shelukim or cooked.

They all join to comprise the Shi'ur of a k'Zayis. One is Yotzei with the stalks.

Chazeres is [Romaine] lettuce. Ulshin is endives.

(Rabah bar bar Chanah): Tamcha is horseradish.

(Reish Lakish): Charchavina is tendrils (an ivy that grows around date trees).

Maror is burdock.

(D'Vei Shmuel - Beraisa -R. Yehudah): One is Yotzei even with garden lettuce and wild lettuce. Anything with sap [that exudes when it is cut may be used].

R. Yochanan ben Brokah allows anything pale;

Others allow every bitter vegetable that has sap and is pale.

(R. Yochanan): From all of these [Tana'im] we find that every bitter vegetable [Kosher for Maror] has sap and is pale.

(Rav Huna): The Halachah follows Others.

Rav Acha brei d'Rava would strive to use horseradish.

Ravina: Do you do so because it is the most bitter? Our Mishnah and d'Vei Shmuel's Beraisa list Chazeres first, and R. Oshaya said that the Mitzvah [l'Chatchilah] is with Chazeres;

(Rava): Chazeres is Chasa (lettuce).

It is called Chasa because Hash-m was Chas (had mercy) on Yisrael.

(R. Shmuel bar Nachmani): A verse ["va'Ymareru Es Chayeihem"] alludes to the Mitzriyim like Maror to teach that just like Maror is initially soft and is hard in the end, also the Mitzriyim were initially soft and hard in the end.

Rav Acha: I retract.

Rishonim

Rambam (Hilchos Chametz 7:13): The Merorim of the Torah are Chazeres, Ulshin, Tamcha, Charchavina and Maror. All five kinds of these Yerakos called Maror. If one ate a k'Zayis from one or from all five of them, he was Yotzei, if they are moist. One is Yotzei with the stalks even if they are dry. He is not Yotzei if they were pickled, Shelukim or cooked.

Hagahos Maimoniyos (20): The Mitzvah is with Chazeres, for Hash-m was Chas on us. If he has no Chazeres, he tries to use Ulshin. Whatever is earlier in the Mishnah has priority. The Yerushalmi asked that Chazeres is sweet, and answered that the whole reason for Chazeres is because the Mitzriyim are compared to it.

Hagahos Maimoniyos (30): R. Tam derived that one is Yotzei with the stalk, but not with the root.

Beis Yosef (OC 473 DH v'Elu): Also Semag wrote like R. Tam.

Poskim

Shulchan Aruch (OC 473:5): One is Yotzei with the following Yerakos - Chazeres, Ulshin, Tamcha, Charchavina (kinds of bitter herbs), and Maror.

Mishnah Berurah (34): We do not know so clearly the names of these herbs in our language, therefore the Shulchan Aruch wrote that they are bitter species. The Acharonim wrote that Tamcha is horseradish, and Chazeres is Salatin (lettuce).

Shulchan Aruch (ibid.): One is Yotzei with the leaves and stalks, but not with the roots. One is Yotzei with leaves only if they are moist, and with moist or dry stalks.

Taz (5): R. Yerucham says that one is not Yotzei with the small roots that spread out, but the big root on which the Yerek stands is considered the stalk.

Magen Avraham (12): R. Yerucham says that from Martrich, one takes what is above ground, e.g. the leaves and what is close to it (the ground), but not what is below ground, for that is the root. Mas'as Binyamin and the Shlah agree. However, above R. Yerucham wrote that in Sukah, Rashi (13a DH Sheloshah) says that Kelachim are roots. Therefore, it seems that R. Tam disqualifies Sherashim, i.e. small roots that spread out, but the big root on which the Yerek stands is considered the stalk. Hagahos Minhagim says that "stalk" is until below the tail, and ears come out from it. This seems primary.

Taz (5): Many people bless Al Achilas Maror on the Yerek (leaves), and eat the stalk (with Matzah) for Korech. There is no reason for this. One may take the Yerek also for Korech. One can eat a k'Zayis of it. It is less painful than the stalk, which is more potent.

Magen Avraham (12): The custom is to bless on the Yerek, and use the stalk for Korech, for we say "one is Yotzei with the stalk." This implies that the leaves are preferable, so we take them for the Berachah. There are few leaves, so we take the stalk for Korech. One may take leaves for Korech.

Mishnah Berurah (36): The other species are not found. Therefore, we use horseradish. One must not eat it whole, for this is almost dangerous, and there is no Mitzvah. It is so sharp that it damages greatly. One must grate it and leave it exposed to blunt it sharpness. The Gra grated it only after returning from the Beis ha'Keneses, lest its taste be blunted, and then one is not Yotzei. If Yom Tov is Shabbos, he grates it before Shabbos. After grating, he covers it until the beginning of the Seder, and then spreads it on the plate. This blunts the sharpness, and he can eat it properly.

Mishnah Berurah (37): The stalk is thick, so it keeps its taste even if it is dry.

Shulchan Aruch (ibid.): He is not Yotzei if they were pickled, Shelukim or cooked.

Magen Avraham (13): This implies that one is not Yotzei with wilted leaves. The Tur and Rambam connote like this.

Magen Avraham (14): Rashi says that pickled is in vinegar. We hold that something soaked in water for 24 hours is like pickled and cooked. Therefore, if it was soaked for 24 hours, one is not Yotzei with it, for it lost its bitterness.

Mishnah Berurah (38): If one soaked horseradish stalks [but not leaves], b'Di'eved if he has nothing else he may rely on it, for it retains the taste of Maror. Why do weak people take stalks [even though it is hard for them]? Perhaps it is because leaves are often dry or wilted. If they are moist, one is Yotzei with them l'Chatchilah.

Kaf ha'Chayim (80): When Erev Pesach is on Shabbos, we detach the Maror before Shabbos. If it is not soaked, it will wilt. If it is soaked, it will be pickled! One wraps it in a cloth that was soaked in water.

Kaf ha'Chayim (81): If something was soaked for 23 hours, removed from the water, and soaked for another 23 hours, it is not considered soaked [regarding Isurim] - YD 105:1 The same applies here (Pri Megadim). A minority disagree. One may be lenient, for some say that soaking in water never disqualifies Maror.

Mishnah Berurah (39): Shaluk is overly cooked. All of these lose the taste of Maror.

Shulchan Aruch (ibid.): They all join to comprise the Shi'ur of a k'Zayis. The primary Mitzvah is with Chazeres.

Beis Yosef (DH u'Mah she'Chasav v'Ikar): The Tur says that the primary Mitzvah is with Chazeres. Why did the Rambam omit this? Perhaps he explains that it is also a Mitzvah with Chazeres, even though it is sweet and not bitter. R. Yerucham says "[there is a] Mitzvah with Chazeres. One need not pursue Maror more than this." This is like I wrote. Semak says "the custom is with Chasa, for Hash-m was Chas on us. Where there is no Chasa, the custom is to use Porfoi. It is better to pursue Chasa for the reason given, even if it is expensive. This is like the Tur.

Shulchan Aruch (ibid.): If he has no Chazeres, the highest priority is the first species he can use on the list.

Gra (DH v'Im): "The Mishnah says Chazeres" shows that we are particular about the order.

Beis Yosef (DH u'Mah she'Chasav v'Im): The Kolbo (50 13b) says that if he has no Chazeres, he uses Yerakos such as Ulshin, Charchavina, Gargira, and in some texts Kusbarta. I know no reason for the last two. The Agur says that Mahari Molin says that one who has no Chazeres uses Maror, i.e. wormwood. The Agudah says that he takes Tamcha, the stalks and the leaves, but not the root. Some say that Maror is forest Chazeres. Even so, Chasa is the ideal Mitzvah.

Chacham Tzvi (119): Lettuce has all the signs. It has sap, whitish leaves, and at the beginning it is soft and sweet, and at the end it is hard and bitter like wormwood. Ashkenazim who did not recognize the species relied on horseradish, and ignoramuses do not eat enough or endanger themselves.

Note: The Chacham Tzvi was an Ashkenazi, but he was the Rav of some Sefardi communities.

Kaf ha'Chayim (72): The Beis Yosef says that Chazeres is not bitter at all. Bar Hedya told Rava "your business will be bitter like Chasa" (Berachos 56a). We must say that it has some bitterness.

Mishnah Berurah (42): Chazeres is best, but it often has small worms that are very hard to see. If there are not Yir'ei Shamayim to check it, it is better to take horseradish.

Kaf ha'Chayim (75): If there is concern lest it not be checked well, one should use the stalks, and not the leaves.

Mishnah Berurah (43): A sick or finicky person may take whichever he wants and eat it slowly, within the time to eat a half-loaf. If this will harm his health, he eats a little or [only] chews in his mouth [without swallowing] to commemorate the bitterness, but does not bless.

Rema: If one has none of these Yerakos, he takes wormwood or another bitter Yerek.

Magen Avraham (15): The Shach says "the Gemara clearly says that one is Yotzei only with the species in the Mishnah. The Agur holds that Maror is wormwood. If so, one who takes other bitter Yerek does not bless. The Rema requires investigation." The Tashbatz holds that "Maror" in the Mishnah includes all bitter vegetables. Rashi (Shemos 12:8 DH Al) says so. The Gemara connotes unlike this. One is not Yotzei with swamp Maror, because it has an accompanying name (Sukah 13a). It seems that Rashi means that all [bitter herbs] are called Maror, but one is Yotzei only with what was called Maror at the time of Matan Torah. Therefore, one does not bless on other Yerakos.

Gra (DH v'Im): This is like Rav Huna, who rules like Others.

Mishnah Berurah (44): The other species must have the Simanim mentioned in the Gemara, i.e. it is edible, has sap (when one cuts it, a fluid exudes that is white like milk) and its leaves are not very green, rather, have a whitish tint. In any case one does not bless, for we are not so expert about these Simanim.

Bi'ur Halachah (DH Yikach): Several Poskim mention sap and whitish leaves, but the Rif, Rambam and Rosh did not. Perhaps this is because we are not so expert about them.

Chidushei Torah of Avroham Shlema Adler (Taste and See... II.): If Romaine lettuce is left to grow about a month after it is usually harvested, it sends up a tall stalk with new, bitter leaves. A white milky juice (sap) comes out of these leaves. Farmers and botanists call it "bolted lettuce."