A druse is a group of crystals of calcium oxalate, silicates, or carbonates present in plants, and are thought to be a defense against herbivory due to their toxicity. Calcium oxalate (Ca(COO)2, CaOx) crystals are found in algae, angiosperms and gymnosperms in a total of more than 215 families. These plants accumulate oxalate in the range of 3–80% (w/w) of their dry weight through a biomineralization process in a variety of shapes. Araceae have numerous druses, multi-crystal druses and needle-shaped raphide crystals of CaOx present in the tissue. Druses are also found in leaves and bud scales of Prunus, Rosa, Allium, Vitis, Morus and Phaseolus.
Druses in onion scales (100x magnification)
Raphides are needle-shaped crystals of calcium oxalate monohydrate or calcium carbonate as aragonite, found in more than 200 families of plants.
Both ends are needle-like, but raphides tend to be blunt at one end and sharp at the other.
Raphides in Epipremnum Devil's ivy (600x magnification)
Raphides in Hypoestes phyllostachya, the polka dot plant