Bentham and Hooker proposed natural system of classification in his book Genera plantarum. In this system all important characters of plants are taken into consideration. Oswald Tippo (1942) divided the plant kingdom into two subkingdoms i.e., thallophyta and Embryophyta. Carolus Linnaeus (father of taxonomy) published a sexual system of classification, details of which were published in Genera plantarum. John Hutchinson published a book Families of flowering plants in two volumes.