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This symbolizes the bitterness of slavory in Egypt. "And they embittered their lives with hard labor, with mortar and with bricks and with all manner of labor in the field; any labor that they made them do was with had labor" (Exodus 1:14)
What is Maror (horseradish or romaine lettuce are used for this. Other suitable vegetables include endive, green onions or curly parsley in some Sephardic circles.)