I'm quite fascinated by the way these Agave leaves render a negative image
on succeeding leaves as the outlines are exposed to the sun during growth.
Here, I've put the negative and positive of my scan side by side, to confirm the M C Escher-like effect.

It is hard to see here that these leaves are smooth (relatively), with no spikes in the surface.

I guess this is just Photosynthesis at work. I found a webpage where the artist uses
leaves etc. to create images. Nature's Darkroom. Organic Photography !

Interesting reading here

The 'real' Agave images are HERE