The Arum family is a tropical plant family which is a very common in houses and offices around the world. These attractive plants include Anthurium or Arrowhead, Caladium, Diffenbachia or Dumbcane and Mother-in-Law's Tongue, and Philodendron; all these however contain chemicals called "Oxalates"; salts derived from the toxic Oxalic Acid, present as Calcium Oxalate ( Ca[C2O4] ) crystals. These needle-shaped crystals are held in "bundles" called Raphides, inside the leaves (predominantly) in "idioblasts". They are small specialized cells which are capable of expelling the double-pointed and sharp raphides when damaged, for example when eaten. The raphides lodge very easily into the lining of the mouth, throat and gastro-intestinal tract causing an intense burning irritation, swelling, and often violent choking even in small doses. Occasionally, the raphides pass through to the stomach with little effect. However, these oxalate crystals are converted back to Calcium and Oxalic Acid. This is potentially even more of an irritant, corrosive and is potentially very damaging internally. In larger doses, other symptoms may include severe stomach upset and breathing difficulties. Suffocation may even occur if the oesophagus is blocked by swelling of the tissue around the base of the tongue and throat. Too high a dose (by excessive comsumption) is lethal. Here is a picture of an Arum.
The raphides are useful protection against herbivores and for removing of excess oxalic acid as the insoluble salt. In fact the only way to remove them from the mouth (dizzolving) is by a vinegar rinse (without ingesting!!).
In Dumbcane (Dieffenbachia picta) in addition, recently discovered is an enzyme similar to those in snake and scorpion venom. It has been known to cause "local" paralysis, (of the mouth and throat), hence its was given rather an appropriate name, "Mother-in-Law's Tongue". Contact with the skin should also be avoided especially from the sap, as the raphides cause irritation, rashes and may cause dermatitis.
Wild arum varieties include Jack-in-the-Pulpit (Arisaema triphyllum) and Skunk Cabbage (very pungent!) (Symplocarpus foetidus), often found in marshes and ditches. Both of these share the properties of the more cultivated varieties.
The plant that is possibly most surprising is that of Rheum rhaponticum or the common edible Rhubarb! Raphides are present throughout the plant, albeit in very low concentrations in the stems, fortunately, and very high in the leaves. Predominantly Potassium Oxalates are present ( K2[C2O4] ). The effects are the same as above.