Just as the Victorians held a ghoulish fascination with Gothic horror stories of man-eating plants, current audiences for films such as “Little Shop of Horrors,” Tentacula in the Harry Potter book series, and even Pokémon cartoon characters based on carnivorous plants demonstrate that we still are drawn to the idea of plants that incorporate insects in their diets. And with the publication by members of the Botanical Journal of the Linnean Society of London (led by Professor Mark Chase, Keeper of the Jodrell Laboratory at the Royal Botanic Gardens, Kew, in conjunction with co-authors from the RBG, Kew and the Natural History Museum) that espouses the idea that even the common petunia can be considered a “meat-eater,” botanists must now take a wider view of the exact definition of the order Carnivora.
In 1875, Charles Darwin wrote the seminal treatise on carnivorous plants; ironically, botanists, including Linnaeus, had previously rejected the idea. Darwin recognized that carnivorous plants adapted to growth in places where the soil was thin or poor in nutrients, especially nitrogen, by trapping and consuming insects.
But petunias? While they have sticky hairs that can capture insects, they probably don’t have the ability to digest the trapped insects, or to absorb the breakdown products. Tomato and potato plants also have fine hairs that catch and kill some small insects, which then fall to the ground, degrading into nourishment for the plant. This is known as “passive carnivory” and is one part of a sliding scale of carnivorous activity in the plant world.
The title of Professor Chase et al.’s paper, Murderous plants: Victorian Gothic, Darwin and Modern Insights into Vegetable Carnivory, would have us look beyond the commonly accepted carnivorous plants, which employ five basic trapping mechanisms. These are pitfall traps that trap prey in a rolled leaf containing digestive enzymes; flypaper traps that use a sticky mucilage; snap traps that utilize rapid leaf movements to capture their prey; bladder traps which generate an internal “vacuum” that sucks in prey; and lobster-pot traps that have inward-pointing hairs forcing prey to move towards a digestive organ. The degree of activity or passivity depends on whether the plant incorporates movement to aid in the capture of prey.
Plants commonly considered to be carnivorous are North American pitcher plants which belong to the genus Sarracenia and which form upright, tubular leaves; the sundew plant (Drosera capensis) which is covered with dense, sticky hairs that trap the prey which will then be enzymatically digested and absorbed by the plant; and its relative, the Venus flytrap (Dionaea muscipula, Droseraceae) which employs a snap trap formed by hinged leaves fringed with stiff hairs. As the leaf blade closes, it traps the insect behind the intermeshed hairs.
For gardeners, it’s fun to delve deeper into the science of botany. But, the next time you’re planting six-packs of petunias in your window boxes, best to not turn your back on them.