Carnivorous plants

turning the tables on the usual story

Carnivorous plants have fascinated people for over a hundred years or more. We are so familiar with the idea that animals eat plants, that the notion of plants eating animals seems quite anomalous. Yet there are hundreds of species of animal-eating plants around the world.

Carnivory by plants came to popular culture in the form of a show called Little Shop of Horrors, featuring an entirely fanciful plant that grew big enough to eat humans. In reality, carnivorous plants usually eat nothing larger than a tadpole. They concentrate on insects or, in some cases, on tiny aquatic organisms such as mites, nematode worms, and micro-crustaceans.

All carnivorous plants usually grow in nutrient-poor, open habitats, although they can sometimes occur in richer conditions when competition from other plants is low. Carnivory is considered to be an adaptation that provides supplemental nutrients to these plants—nitrogen and perhaps especially phosphorus. When Darwin fed insects to carnivorous plants, he observed that they grew bigger and reproduced better, and most experiments since then have found similar results. Thus, in most cases, captured prey is directly beneficial to the plant, although to varying degrees, depending on many other factors.

In Southeast Alaska, and Alaska at large, we have three basic kinds of carnivorous plants. Sundews (two species) grow in muskegs, along with Sphagnum moss. Butterworts (two species) seem to grow on sandy soils and in muskegs. And bladderworts (three species) are aquatic, living in freshwater ponds, sending up aerial shoots with flowers. All of these are potentially insect-pollinated; the flowers are white or blue-violet or yellow, respectively, but their pollination has been studied much less than carnivory.

Long-leaf sundew with prey. Photo by Bob Armstrong

Sundew leaves bear numerous hairs with sweet, sticky secretions at the tips. Insects are attracted by the sweetness, and perhaps also by colors on the leaf. When an insect touches a sticky hair, other hairs on the same leaf lean toward the unhappy bug, so that it is then caught securely by several hairs. Digestion takes place in part by the sticky hairs and in part by glands at the bases of the hairs. The sticky stuff is carbon-based, so its production is dependent on photosynthesis, and thus the sundews are limited to open habitats with solar radiation.

Butterwort. Photo by Bob Armstrong

Butterworts have sticky hairs on the leaves too, but they don’t move. The leaf margins are often slightly curled up, to help corral the hapless insect. Again, digestion is a two-step process, partly by the hairs and partly by glands in the leaf surface. Insect juices enter the leaf through holes in the surface, but these holes also let water escape, making the plant vulnerably to drying out.

Bladderwort. Photo by Bob Armstrong

Bladderworts work in an entirely different way. They capture their prey in small traps. In the resting position, the trap is closed and flat. When trigger-hairs near the opening are touched by a passing invertebrate, the trap suddenly expands, and the inrushing water takes the prey in with it. If the prey happens to be too large for the trap, it can be ingested bit by bit. Re-setting the trap after digestion can happen in less than thirty minutes but requires energy.

The traps of bladderworts often contain rich and abundant communities of bacteria, algae, and other tiny organisms. Some researchers suggest that these organisms actually live there and contribute to prey digestion. Others say that they eventually become prey themselves. The jury is still out on that.

These three kinds of carnivorous plants are well-known and fairly well studied. I suspect, however, that others remain to be discovered. In my former life as a professor, one of my students found that nitrogen markers from insects placed on sticky patches outside the flowering heads of a certain thistle or on the hairs of the inflorescences of penstemon were taken up and circulated through the plant. This suggests that previously unsuspected carnivores may be out there, waiting to be discovered.

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