The Sugar Prune and Its Parents
Mr. Burbank's Sugar Prune was developed by selection from a cross between the French prune and the Hungarian prune. From the former it inherited sweetness and flavor and from the latter size. It improved on each parent, however, manifesting the vigor that is not unusual with hybrids. The French prune is still largely grown in California, but its improved descendants must ultimately displace it.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 8