Human beings are 60% water; it is the most common molecule in our bodies. Perhaps no surprise, then, that early humans were drawn to watercress as one of the first wild greens to be consumed. After all, it is 95% water; it grows in water; and its deep green leaves undulate like seaweed in the current. The crop straddles the line between wild and cultivated: irrigated well, it can be grown in the garden; transplanted along a stream, it will set roots into the watery soil and return each spring.