This pungently aromatic, sprightly flavored herb (not to be confused with the coriander seeds from the same plant) is an essential ingredient for many tacos, sauces, and ceviches. Cilantro is a fragile herb, so choose bunches that look healthy, not yellowing, wilted or feathery. All but the coarsest stems can be used. Stored in a glass of water, loosely covered with a plastic bag and refrigerated, fresh coriander will last for a week or more; bunches with roots attached will keep longer. Its aromatic flavor is diminished by cooking or drying. Pureed with vegetable oil, packed into jars, topped with a little oil and refrigerated, it can be kept for months though a certain freshness will be lost.