In cauliflower most of the flowers are abortive. The young leaves are cooked and eaten directly. An ancient crop. 2000 B.C. In Europe only about 12th century. Turnips are often eaten in Europe in the winter as that was about all that was available.
Rutabagas have a stronger flavor.
They may be derived from hybridization between cabbage and turnips. Rape or canola seed oil is also from other cultivars of this plant. In the U.S., they are eaten as a garnish, but in other places, e.g., the orient they actually are important food items. 4000 years ago in Egypt. Very important in Japan.