I found this article on Torah.org which discusses this:
http://www.torah.org/learning/halacha/classes/class118-3.htmlChapter 118:3
Preparations for the Seder
3. For "marror" ("bitter herbs") (1), it is customary to use horseradish ("Tamcha") (2). Since it is very strong, one may grate it, however, one should be careful that it does not completely lose its strength. [For this reason,] one should grate it when one returns from the synagogue [after the evening service]. (See Chapter 98, Law 3, which states that one should grate it in an abnormal manner). [When Pesach falls] on Shabbos, and thus, it is forbidden to grate it [after nightfall], one should grate it during the day and cover it until the evening (3).
It is, however, preferable to use "chazeret" ("romaine lettuce"), which is also called "chasah," because it is easy to eat, and can be defined as "marror" ("bitter herbs"), because when it is left in the ground for a long time, its stem becomes bitter (4). One may also fulfill one's obligation with "la'ana," which is called wormwood. ([One may also use] endives (5) ("Alushin") or date ivy ("charchavinah"). These, however, are uncommon in our lands (Hungary in the late 19thC).
All the [five species mentioned above which] may be used to fulfill one's obligation of "morror," may be combined [to make up] the size of an olive ("k'zayis") [ that one is obligated to eat at the Seder]. One may fulfill one's obligation by eating either the leaves or the stem [of these vegetables]. The obligation cannot be fulfilled by eating their roots.
The exclusion of the roots applies to the small roots that branch off in either direction, [but not to] the main root, from which the leaves grow. Although it grows within the ground, it is considered part of the stem. Nevertheless, if possible, it is preferable to use the leaves and the portion of the stem that protrudes from the ground, because some authorities maintain that any portion that grows in the ground is considered a root.
A person can fulfill his obligation with leaves only when they are fresh. In contrast, one may fulfill his obligation with stems whether they are dry or fresh, but one may not use them (or leaves) if they are cooked or pickled.
FOOTNOTES:
(1) Exodus 12:8 states: "...and they must eat [the meat of the Pesach sacrifice] along with matzah and bitter herbs." Although nowadays, without a Temple in Jerusalem, there is no Pesach sacrifice, the Sages instituted the eating of bitter herbs at the Pesach seder as an allusion to the fact that the Egyptians embittered the life of our forefathers in Egypt.
(2) The Mishna lists 5 species of plants with which one can fulfill the obligation of bitter herbs on Pesach: a) Chazeret (Romaine lettuce) b) Alushin (endives or escarole) c) Tamcha (horseraddish) d) Charchavinah e) Marror (many authorities feel that these last two species are no longer known to us through tradition).
(3) So that it doesn't lose its strength.
(4) According to the Jerusalem Talmud, the development of the species "chazeret" parallels the development of the bondage in Egypt. Just as the "chazeret" is at first sweet, but then becomes bitter after being left in the ground for a long time, so too the Jews in Egypt were at first treated royally and lived luxuriously, and then gradually descended into slavery.
Also, the other name for "chazeret" is "chasah," which means "mercy" and alludes to the fact that Hashem had mercy on the Jews and saved them from the bondage. Actually, the word, "Pesach," which some say comes from the root which means " to pass over," is translated in Targum Unkelus as "Eichos," which comes from the root meaning "mercy" or "love" (See Targum on Exodus 12:13 and Rashi). It would have been interesting had this second meaning been accepted as the primary one - we would then be celebrating the festival of "Love" rather than "Passover."
Although many authorities use the word "salatim" to refer to "chazeret", which could include all kinds of lettuce (crisp-head and iceberg lettuce), nevertheless, the common practice is to use Romaine lettuce. Whatever lettuce one uses, it must be checked thoroughly for bugs before using it.
The reason is was customary in many communities to use horseradish was either because lettuce was hard to find, or because it was hard to check it for bugs.
(5) The endives sold in the U.S are apparently not the true endive or escarole, but rather what is known as French (Witloof) or Belgium endive, which are grown on the chicory root. Since there is a question as to whether or not our endives are included in the species called "alushin" listed in the Mishna, it is recommended that they not be used for the mitzvah of marror ("Laws of Pesach" by Rav Avrohom Blumenkrantz).
Topic: The Seder plate. (Read 3120 times)