Foxgloves are the source of digitalis, a substance with powerful effects on the heart, used to treat congestive heart failure and cardiac arrhythmias. Nature's Medicine: Plants That Heal by Joel L. Swerdlow with photographs by Lynn Johnson (National Geographic, $35, £20), from which this picture is taken, traces man's search for healing plants through the ages, from the ancient civilizations of China and India through to the birth of modern pharmaceutical science.