Although the green parts of plants have been used in the treatment of various diseases for many centuries, it was not until the scientific investigations of Willstaetter and Stoll in 1913 that actual proof of the worth of such remedies began to be found. The close similarity of the chemical character of the chlorophylls found in the chloroplasts of green plants to that of the hemoglobin in human blood led to the theory that the chlorophylls might be the factors in green plants which give them therapeutic value, so a great deal of scientific investigation has been undertaken in laboratories, hospitals and in the private practices of many physicians to determine the efficacy of chlorophyll therapy.