Calcium oxalate in plants Vincent Franceschi  Trends in Plant Science  Volume 6, Issue 7, (July 2001) DOI: 10.1016/S1360-1385(01)02014-3

Fig. 1 Cross-section of a mature Claoxylon sandwicense leaf. A large calcium oxalate crystal idioblast has grown by intrusive growth in between palisade and spongy mesophyll cells and now spans the entire cross section of the leaf. The idioblast has a single large styloid crystal with sharply pointed ends. A role in defense of the plant is obvious because any animal trying to chew on this leaf will have the equivalent of a 2–3-mm long, double-pointed needle to contend with. Many plants have specialized cells, called crystal idioblasts, that contain single or multiple needle-like crystals that appear to serve a primary function in bulk regulation of calcium in tissues and a secondary function in defense against grazing animals. The needles of calcium oxalate can even have grooves and barbs, which are thought to be responsible for channeling toxins or anchoring the needles in the wound. Scale bar=400 μm. Trends in Plant Science 2001 6, DOI: (10.1016/S1360-1385(01)02014-3)

Fig. 2 Peperomia leaf. (a) Scanning electron micrograph of a cross section in the region of the palisade layer. Each columnar palisade cell, most of which are cleaved open in this sample, has a spherical-looking druse crystal near the top of the cell. Scale bar=20 μm. (b) High-magnification image of druse crystals. The crystal is a conglomerate of multiple facets that radiate out from a central core. Scale bar=5 μm. Trends in Plant Science 2001 6, DOI: (10.1016/S1360-1385(01)02014-3)