Water cress : A perennial cruciferous herb (Nasturtium officinale) growing usually in clear running or spring water. The leaves are pungent, and used for salad and as an antiscorbutic.

Example:


Taxonomy
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Defination from the web:
watercress is well-suited to hydroponic cultivation, thriving best in water that is slightly alkaline. ... This is due in part to the fact that cress leaves are unsuitable for distribution in dried form and can only be stored for a short period. ...