Everyone knows that carnivorous plants exist. Even if their sheer weirdness and amazing capabilities hadn't already brought them into classrooms, Little Shop of Horrors would have cemented them firmly in the popular consciousness. One thing that's rarely discussed, though, is exactly why a plant would resort to carnivory. Why bother catching and eating animals when you're a plant and can already make all your own food?
The answer is simple: minerals. Carnivorous plants are as good at photosynthesis as any other species, but photosynthesis only makes sugar. They still need soil nutrients -- nitrogen, phosphorus, potassium, and a host of other trace minerals -- for the same reason that people need our vitamins. Most plants get their nutrients from the soil, where the stuff is abundant; farmers and gardeners add minerals, in the form of fertilizer, to help their crops thrive.
On its own, though, a plant can't very well pop down to Home Depot and buy itself a bag of fertilizer. When the soil lacks important minerals, what's a plant to do? Some species have found ways to conserve or reuse scarce nutrients, making a little go a long way. Others have found ways of making their own: legumes can pull nitrogen right out of the air (and that's a subject for another post). Carnivorous plants do neither. Instead, they just let their prey bring the goods right to them.
When a plant traps and digests, say, a fly, it gets to suck up all the nutrients in that bug's body. There's nitrogen in the protein of its muscles, potassium in its nerves to let them fire, phosphorus down the very backbone of its DNA, and much more. A single fly can save the plant from a startling amount of malnutrition. Carnivorous plants aren't just being vicious: like any carnivore, they're only trying to survive.
This week, look for posts on the wonders of four different carnivorous plants: the literal hair-triggers of the Venus flytrap, the booby traps set by pitcher plants, the gluey hairs on sundews, and the vacuum suction of the bladderwort.