Vestigial Cactus Leaves
The spine-like projections here shown on the slab of the cactus are vestigial leaves. An account of them, with reference to their evolutionary meaning, is given in Volume 1. They are all that remain of the leaves that the cactus once bore; and these reminiscent spikes drop off shortly after coming out.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 8 Chapter 6