Another View of the Apple Plum
In noting the very peculiar apple-like character of this fruit, it is interesting, by way of comparison and contrast, to consult the earlier pictures, showing the wide variation of plum forms, including the inverted pear shape of the Kelsey, the even oval of the Splendor prune, and the almost spherical form of other types. With such material to combine, almost any results may be expected."
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 5 Chapter 7