From The Guardian:

It contains more iron than spinach, more vitamin C than oranges and more calcium than milk. Watercress may be better known as a decorative garnish, but a study published yesterday said the salad leaf could significantly cut the risk of cancer.

The study suggested that eating 85g of watercress a day could inhibit the growth of cancer cells and even kill them. Scientists at Ulster University found that the watercress reduced the damage caused by cancer cells to white blood cells by 22.9%. Watercress also raised levels of antioxidants which absorb so-called "free radicals", molecules which some experts believe damage the body's tissues.