Awhile back Chemical & Engineering News published a fascinating article called "The Secret Life of Plant Crystals" with some wonderful photos of calcium oxalate crystals. Special cells (called "idioblasts") produce these crystals, with shapes that are unique to each type of plant.
Even though 75% of flowering plants make these crystals, no one knows why they make them and in fact, their functions may be as diverse as their shapes. Some crystals look sharp and dangerous, like thistles or thorns, suggesting that they function to defend a plant from hungry herbivores. Other crystals are thought to bind heavy metals or other harmful substances and sequester them away in some kind of a molecular toxic waste containment center.
Calcium oxalate crystals (raphides) have even implicated in a case of a mysterious illness that occured after they had eaten a vegetable dish (2). It's every child's nightmare; the vegetables were fighting back.
The best feature of this story, though, is the gallery of photos. The Botanic Crystal Fashion Show is composed of excellent pictures, all taken by Harry T. (Jack) Horner at Iowa State University. It's well worth taking a look.
1. Chemical and Engineering News (2006, 26-27)
2. Clinical Toxicology (2005, 1, 17)