This is not really a cactus.

Here is a great New York Times article on the unique challenge of preserving the Blaschka collection of glass flowers at Harvard University. These were commissioned in the late 1800's to use as teaching aids for botany students. The collection features hand-blown glass replicas that look exactly like the real thing. Some of the 847 species in the collection are very rare or even extinct so it is important for these models to be preserved for future scholars.
I highly recommend a stop here if you are ever in the Boston area!
"The curators cringe at the thought of decades of schoolchildren's feet vibrating like jackhammers on the fragile models, truncating an iris leaf and shaking green chips off a shrubby trefoil."