Black Walnuts Cross Section
This color print shows the nut of the California black walnut sawed in half. The heavy wooden formation of the shell is characteristic of this species as opposed to the thin paper shell of the English walnut. Mr. Burbank's quick-growing Paradox takes back to its black walnut parentage in thickness of shell, although it resembles the English walnut in outward appearance.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 5