The Paper Shell on the Tree
In the course of his experiments, Mr. Burbank produced walnuts that were almost devoid of shell, but this proved a disadvantage as the birds soon learned the secret. It was necessary, therefore, to select specimens with thin shells, instead of those with no shells, to continue the experiment. The ones here shown have shells of ideal thickness.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 11 Chapter 1