The Mishna in Masechet Pesahim lists the various vegetables that may be used to fulfill the Misva of Marror on the night of the Seder. The Rabbis teach us that these vegetables are listed in descending order of preference, and thus the first vegetable mentioned – Hazeret – is the most preferred vegetable to use for Marror. The Gemara identifies Hazeret as "Hasa," which alludes to the fact that the Almighty had mercy ("Has") on the Jewish People on the night of the Exodus. "Hasa" is generally understood as referring to lettuce. Strictly speaking, even iceberg lettuce may be used for Marror, though it is customary to use Romaine lettuce (perhaps because it is generally cleaner). The Bet Yosef (commentary to the Tur by Maran, Rav Yosef Karo, author of Shulhan Aruch) writes that although iceberg lettuce does not have a bitter taste, it is nevertheless suitable for Marror because it becomes bitter when it is left in the ground. The vegetable used for Marror does not have to actually taste bitter, but rather must be a vegetable that in general has a bitter taste. Thus, although the iceberg lettuce that is generally eaten has been removed from the ground before turning bitter, it may be used for Marror, and, as mentioned, it is in fact the preferred vegetable.
Some Ashkenazim have the custom to use horseradish ("Chraine" in Yiddish), which is the third vegetable mentioned by the Mishna – "Tamcha." It appears that lettuce was not available in Ashkenazic lands, and so they had to resort to horseradish, and this then became their custom.
We follow the custom of the Arizal (Rav Yishak Luria of Safed, 1534-1572) to place "Ulshin" – endives – at the center of the Seder plate, and then place lettuce on the bottom. It should be noted that one does not have to eat the endives, neither for Marror not for Korech (when we eat Masa and Marror together), though some have the custom to add some endives for Korech.
One may, if necessary, combine different vegetables to reach the required quantity of Ke’zayit. Thus, one may combine some lettuce and some endives to reach a Ke’zayit. Likewise, an Ashkenazi who uses horseradish but finds it difficult to eat a Ke’zayit may add some lettuce or endives.
The Rama (Rav Moshe Isserles of Cracow, 1525-1572) writes that if one cannot obtain any of the vegetables mentioned by the Mishna, then he may use any vegetable that has a bitter taste. The final item listed by the Mishna is "Marror," which could be understood to mean that if none of the other items are available, one may use any bitter-tasting vegetable. The Halachic authorities write that in such a case, one should not recite the Beracha of "Al Achilat Marror."
Of course, one who uses lettuce for Marror must ensure that the lettuce is clean and free of insects, as otherwise he would be violating a prohibition by eating the Marror, as opposed to fulfilling a Misva. Nowadays one can purchase lettuce grown hydroponically or through other means that avoid infestation, and Hacham Ovadia Yosef strongly encouraged the use of this lettuce.
Summary: According to Sephardic custom, Romaine lettuce should be used for Marror. One must ensure that the lettuce is free of insects. Although it is customary to use Romaine lettuce, one may, strictly speaking, use iceberg lettuce, as well. Our custom is to place endives in the center of the Seder plate, and place the lettuce at the bottom. One does not have to eat the endives, though some people add some endives to the Korech. If necessary, one may comine different forms of Marror to reach the required quantity of "Ke’zayit." If one cannot obtain any of these vegetables, he may use for Marror any bitter-tasting vegetable, but the Beracha of "Al Achilat Marror" should not be recited in such a case.
