Take a bite out of a dumbcane (Dieffenbachia spp.) or a pothos (Philodendron spp.) and it won’t be long before your mouth and throat start to burn (please don’t actually do that). Eat enough of it and your symptoms may also include intense numbing, oral irritation, excessive drooling, localized swelling, and possibly even kidney and liver failure (again, please don’t). What you are experiencing is a brutal form of plant defense caused by tiny crystals called raphides.
Raphides are tiny, needle-shaped crystals made up of calcium oxalate. A lot of plants accumulate calcium oxalate. Research has shown that in doing so, plants are able to sequester excess calcium in their cells. Many plant lineages then use that calcium oxalate to make raphides. Not all raphides come in the form of needle-like crystals. Often they are ‘H’ shaped or even twinned. Others are blunt, kind of like tiny crystalline cigars.
How raphides form within the plant is rather fascinating. As far as we can discern, raphide crystals form in vacuoles of specialized cells called “idioblasts.” It is thought that an exquisitely controlled scaffolding or matrix shapes the biomineralization process. To the best of my knowledge, no one has been able to reproduce this process in a laboratory setting. For now, plants are the undeniable masters of raphide manufacturing.
Within the cells, raphides are often associated with acrid and toxic proteins. Together, they comprise one hell of a defense against herbivory. Raphides are only the first part of the defensive equation. When plant tissues containing raphides are damaged, usually by chewing, the raphides shoot out of the idioblasts and into the oral cavity of the herbivore. This is where their needle shape comes in.
Raphides wreak havoc on sensitive tissues. They literally act like tiny needles, cutting into and tearing the lining of the mouth, esophagus, and gut. This is only half of the story though. As mentioned, raphides are often packed in with acrid and toxic proteins. The laceration caused by the raphides allows these compounds to enter into the wounds. This is where things can get especially nasty. If the proteins are toxic enough, the herbivore now has far more to worry about than simply the burning sensation.
Raphides are not produced in equal amounts in all tissues. Stems tend to have more than leaves, but raphide content in leaves has also shown to be a function of leaf size. Raphides also differ from species to species. Not all plants that produce raphides produce them in the same shape and quantity. Still, more than 200 plant families contain species that have evolved this form of defense and many of our most prized houseplants fall into this category. However, this should not scare you away from these plants. Provided you or your loved ones don’t go nibbling on the leaves or stems, all will be fine. If anything, this remarkable form of plant defense should earn these plants even more respect than they already get.