The Cactus Still Bears Leaves
In the days when the cactus slab was a round stalk, and before the animals had began their work of devastation, the plant had leaves as other plants. That this is so is evidenced by the fact that cactus slabs even now put forth these leaves in rudimentary form, the evidence of an old instinct which has not been entirely obliterated. Shortly after the tiny leaves come out, as shown in this color photograph print, they fall away to be followed by the spines which push out behind them. The likelihood is that the original leaves of the cactus were as large as a man's head.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 1 Chapter 1