Agricultural production includes cotton, linseed oil, maize, melons (such as the honeydew melon, known locally as the Bailan melon or "Wallace" due to its introduction by US vice president Henry A. Wallace), millet, and wheat. Gansu is known as a source for wild medicinal herbs which are used in Chinese medicine. However, pollution by heavy metals, such as cadmium in irrigation water, has resulted in the poisoning of many acres of agricultural land. The extent and nature of the heavy metal pollution is considered a state secret.