Insectivorous Plants

Pitcher plant

Carnivorous plants seem to attract their own breed of plant lover. Browsing through the many Web sites posted on this subject one is struck by the difference in tone between a site dedicated plants that eat insects versus, say, one devoted to roses. People devoted to roses emphasize their pedigree and elegance while those interested in carnivorous plants focus on their exotic behavior and the apparent ingeniousness of the insect-snaring mechanisms.

Although there seem to be many outlets for the sale of carnivorous plants, there are fewer sites that go into any detail about how to care for them. A sort of central clearinghouse for this kind of information is to be found at the site of the International Carnivorous Plant Society. One of the more detailed sites to be found is that of Michael Zenner, although it has not been updated since 1998.

One of the more complete references for carnivorous plant systematics can be found at the Web site of the Botanical Society of America. The strategy of carnivory (more strictly, insectivory) has evolved independently and differently in several phyletic groups (five different orders are listed here) within the kingdom Plantae.

Pitcher plant

The insectivorous plants are frequently divided into active and passive eaters. Members of the latter group simply have architecture that traps hapless trespassing insects, while members of the former group have moving parts that close on insects that trip triggers (usually hairs) on the plant.

Classic examples of the passive type are the “pitcher plants,” which also illustrate the polyphyletic development of similar forms. The pitchers of the eastern United States are members of the genus Sarracenia in the family Sarraceniaceae. Those of the western United States are members of the genus Darlingtonia in the same family. In the tropics the family Nepenthaceae includes the genus Nepenthes. All of these plants have reservoirs lined with downward pointing hairs that allow insects to crawl in, but not out. The insects are attracted by an odor and when unable to get out, slip into a pool of digestive enzymes at the base of the reservoir.

Venus fly-trap

The best-known active insect eater is the Venus’ flytrap (Dionaea muscipula) a member of the family Droseraceae, which include the passive-strategy member, the sundew. The Venus’ flytrap has a series of sensitive hairs that must be either tripped successively or triggered more than once in order for the plant to react. The trap portion of the plant consists of semi-circular plates lined with tooth-like structures that are held open until triggered, whereupon the close with surprising rapidity. The mechanism is still not completely understood. The flytrap makes an airtight seal around its prey and secretes digestive enzymes. It takes five to 12 days to digest an insect.

All of the carnivorous plants have developed their insect “hunting” abilities in order to supplement a nitrogen-poor environment. Many live in bog environments, which are highly acidic. The acidity is due to the accumulation of minerals in the ambient water, which is a result of water leaving the ecosystem only by evaporation rather than run-off. Reactions necessary for nitrification – conversion of ammonia into nitrites and nitrates, nitrogen compounds usable by plants – do not go forward in low pH conditions. It is therefore necessary for the plants to harvest the nitrites and nitrates directly from the insect’s bodies.
Because they are adapted to somewhat extreme, marginal environments, these plants are often restricted to such places.

Wild pitchers

Unfortunately wetlands have been traditionally viewed with distaste. In the pre-“germ theory” era it was thought that the “vapors” rising out of wetlands brought sickness and they were filled or drained in the interest of public health. Because they are mosquito breeding grounds, even in the germ theory era they are viewed as sources of pestilence.

The eastern pitcher plants (Sarracenia purpurea et al.) are broadly distributed – found across Canada from the Rockies to the Maritime provinces and down the East Coast – but still largely confined to bogs and other marginal environments. By contrast, the natural habitat of the (much cultivated) Venus’ flytrap is a small region of coastal North and South Carolina.

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