Thornless Blackberry and the Recreant Seedling
At the left, a typical stem of Mr. Burbank's thornless blackberry; at the right, a thorny seedling. The thornless blackberry almost invariably breeds true to thornlessness; so it is to be surmised that this seedling is the result of a chance fertilization with the pollen of a thorny variety.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 6 Chapter 1