The Science of Botanical Art

Carnivorous Plants

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 15, Issue 1


Carnivorous plants hold a special, if somewhat bizarre niche in the world of plants. The idea of a plant getting nourishment from a member of another kingdom appeals to our sense of the macabre. Many visitors who traipse through the galleries of the Haupt Conservatory at the New York Botanical Garden who are only minimally affected by displays of floral splendor or vegetative diversity gape in awe at the case of Venus flytraps and pitcher plants. What’s going on here?

Although carnivorous plants were known from very early in the study of botany, no one fully understood the process. When Linnaeus was first presented with examples of the phenomenon he knew of the ability of plants to trap insects. It was when this trapping appeared to lead to death and digestion that his deepseated religious beliefs were so appalled that he refused to accept the possibility, and further pursuit of the study was delayed almost 100 years. In 1875 Darwin, after close observation and a carefully designed series of experiments published the definitive paper on insectivorous plants. Even Darwin’s irrefutable research was questioned at the time by scientists who remained repelled by the idea. Ever since, the exploration of this fascinating group has proceeded apace.

According to the most recent studies we believe there are upwards of 630 different species of carnivorous plant, including a couple of carnivorous liverworts. The need for carnivory arises when plants grow in hostile habitats like bogs and other highlyacid nutrient-poor habitats. Animal life, mostly arthropods and insects, supply much needed nitrogen where normal nutrition is difficult if not impossible.

To accomplish this, carnivorous plants attract, trap, kill and digest critters and the way they go about the process is constantly intriguing. Attracting and sometimes trapping insects is nothing new, plants do this all the time for pollination purposes. Think about the cyathium of the figs, or the closing blossom of some water lilies. On the other hand think about the leaf arrangement in certain epiphytic plants that is designed to catch rainwater. Sometimes small animal life drowns in these pools and provides additional food for the plant.

The stretch to a plant actively assisting in the process with digestive enzymes is not all that big a jump. Glandular hairs that are present on some plants, often as an attractant but sometimes as repellents, are logical forerunners of the mucilaginous hairs used as entrapment devices. The development of insect eating plants is just another example of the evolutionary process, building on small shifts in morphology or physiognomy to allow a plant to thrive in conditions that would otherwise defeat them.

There are five trapping mechanisms that characterize these plants. The pitfall trap, where a leaf forms a funnel able to hold liquids is one. Pitcher plants, Nepenthes, Sarracenia and Darlingtonia are examples of this trap, and what happens inside is amazing. First of all the pitchers are designed to look like flowers. There is an attractive lid covered with nectar, and sometimes enticing smells to lure the prey to the opening of the pitcher. The rim of this opening is also coated with nectar, and the inner surface of the pitcher, right below the rim is very slippery leading to an area of downward pointing hairs to pull the insect down once inside. Some species add the diabolical inducement of translucent patches on areas of the funnel to convince the insect that they are preceding to the light as they plunge to their death in the digestive juices at the bottom of the trap.

Adhesive traps are the mechanism of choice in a number of genera. Sundews and butterworts use actively motile glandular hairs loaded with sticky mucilage to snag their prey. Small insects, attracted by a sweet scent and what appear to be droplets of nectar land on these adhesive surfaces. Death in this case is indirect, the mucilage blocks the respiratory openings of the insect and suffocation follows, so as the hairs roll the leaves around the victim to digest them, they are already dead.

The snap trap, the most dramatic of all the devices is limited to a single genus, Dionaea the one we know as Venus flytrap and a fascinating aquatic plant named Aldrovanda that bears the descriptive moniker of waterwheel plant. In Venus flytrap the leaf develops an adaptation at the apex that looks like an open book bordered with bristle like teeth at each open page. On the inner surface of the pages are usually three trigger hairs. If a bit of flying debris should hit a single hair nothing happens, but if an insect, attracted to the juicy vibrant color or appealing odor should happen to trigger two or more hairs his name is mud. Faster than a split second the arms pop closed interlacing the barbs, and the insect is caught in a jail-like space when the enzymes get to work.

In Aldrovanda underwater leaves form whorls of narrow foliage topped by a set of rounded leaves triggered to snap shut on any unwary victims that happen by. Although this is aquatic and probably difficult to obtain, the architecture of this plant strikes me as a graphic challenge for the artist that is willing to go an extra step for material. It is in its own way an exquisite possibility.

There is a single genus Genlisea that sports what is defined as a snare trap. This is probably not one we would normally come across as artists, but the phenomenon is amazing and only recently have the scientists figured out how the trap works and more important that the prey are one-celled protozoans and ciliates. In this species the trap is a non-chloropyllous underground leaf that forms what looks like a “Y”. There are slits in the arms of the Y and the protozoans are chemically enticed to enter the slits. Once inside unidirectional hairs propel the one-celled creatures to their destination in a digestive bladder to become dinner for the Genlisea.

The suction trap is employed by the largest family of carnivorous plants, the bladderworts. Here again, some aquatic, but mostly terrestrial plants have underwater or underground leaves adapted into bladders with inward opening trap doors. The bladders are only partially expanded creating something of a vacuum inside. Enticing bristles grow around the door and when a victim gets too close the trap springs in a speed that has been timed at one five-hundredth of a second, the fastest recorded time for any plant movement. The prey is inside of the bladder trapped by a door that closes one way, and he’s dinner.

Sadly from a point of view of the artistic merit of this group of plants, while the individual flowers can be rather charming, the business of carnivory is carried out by very small mechanisms that are hidden from sight. The mystery of this insect-eating plant is more intriguing scientifically than visually.

However, as artists a number of this group of plants provides extraordinary graphic challenges. The most important thing to remember is the fact that all the organs of carnivors are leaf adaptations. The magnificent vases of the pitcher plant with their amazing range of color, intricate surface patterning and varied forms are leaves. The vivid, almost erotic traps of the Venus flytrap are leaves. The small glistening paddles of the sundew are leaves.

Of all the types of insect eating plants my choice for the most graphically exciting has to be the pitchers. Displays of Sarracenia at the flower show I was at in England were spectacular. They looked like multicolor rows of organ pipes. The range of form, depending on the particular genus of pitcher including the Cobra lily goes from squat to tall and slender. Not only greens color these leaves but variations on purples, reds and yellows extend the spectrum, as do the intricate veining patterns. The flowers of many of these plants often play second fiddle, but some are equally intriguing. Think of what a narrow line these plants have to tread- needing insects for pollination as well as for nutrition, and what they have to go through (flowers on long peduncles far away from the adapted leaves for example) to achieve this delicate balance. In the end, as always it is the visual inspiration that drives our work, no matter what extremes the plants go to in order to survive.

  • Sarracenia, watercolor, © Lisa Pompelli 2007