Nepenthes – not your “cup” of tea

If you think about a cup in nature, what comes to your mind? The monkey cup is a good call, as a carnivorous plant, scientifically referred to as Nepenthes sp., it is unique because of its modified cup-shaped leaf that looks like a small vase with a lid. Some people use the cup as a container to cook sticky rice added with coconut. Travelers often use older pitchers as water scoops or to catch rainwater. Primarily grown in Southeast Asia, most pitcher plants are climbing or scrambling vines, but some species are epiphytes. Pitcher plants are not typical jungle plants as many people thought they are, actually, they prefer open and sunny ridges, slopes, or stunted forests. About thirty percent are lowland species where the temperature is hot during the day and warm at night. The rest are highland species or commonly known as mountain plants that prefer warm days with cool nights. They are usually found in areas that have a high level of humidity and acidic soil containing low or no minerals or nutrients throughout the year.

Pitcher plants are rosette at the young stage and grow into vines when reaching maturation. The pitcher, in a shape of a cup, erects at the end of leaves. Connecting with the leaf through a tendril loop extended from the center of the leaf called midrib. The major function of the tendril loop is to grab and hold on nearby branches or twigs, so as to extend the flowers higher in the air and facilitate seed dispersal, the seeds are very thin and filiform that can be dispersed by wind easily. Unlike other carnivorous plants, pitcher plants have an extensive root system that is brittle and hair-like roots as branching taproot.

Pitcher plants are dioecious, meaning male and female plants are separated. Seeds can be produced via cross-pollination by pollinating the pollens of the male plants onto the stigma of the female. Naturally, the ratio of male to female plants is about 7:3. Collectors and hobbyists often grow the plants separately and produce hybrids in a greenhouse. However, sexual reproduction is searching for new phenotypes and the production is low. The practical propagation methods are through vegetative multiplication with identical phenotypes, such as cuttings, multiple basal shoots can be induced. Air-layering and ground layering are other alternative approaches to propagate pitcher plants. Live or dead sphagnum moss is the best growth medium because of its ability to maintain both moisture and airflow. Home growers should water pitcher plants with rainwater, reverse osmosis (RO) water or distilled water as these types of water have low mineral contents, measured in part per million (PPM). Tap water is not a good choice as pitcher plants do not tolerate well with the impurities and may be killed by roots rot if being watered too much.

The maturation process of the pitcher is unique and interesting. Initially, the leaf tip grows like a swelling balloon and eventually develops into a hollow and sealed pitcher. Subsequently, the lid is separated from the pitcher and the trap reaches its full development. The top edge of the cup is unfolded into the shape of the lip or peristome. Under high light intensity or bright sun, the pitcher colors up itself quickly and starts to produce sugary nectar that attracts insects that like sugar such as ants, flies, moths, butterflies, etc. The small bodies of liquid contained within the pitcher traps are called phytotelmata. Native Southeast Asian people use this sterile solution as an eyewash, asthma reliever, and painkiller during childbirth. This plant also has several medicinal properties and is used to treat indigestion issues, heartburn, stomach ailments, and dysentery.

The upper zone of the pitcher is lined with slippery and waxy scales that prevent the prey from escaping and make sure to be immersed in the “liquid bath” within the trap. Whereas, in the lower walls of the pitcher, secretive glands that secrete the digestive fluid are put in the mode of action. The fluid contains enzymes that capable of breaking down the prey, which pitcher plants use to cook their “yummy soup”. The luring mechanism is pretty straightforward where preys were attracted by the color and shape of the cup as well as the intoxicating property of the nectar. Other than ants and insects, some small size animals such as frogs and rats may fall into the trap as well, by accident. The preys eventually get “drunk” after nectar consumption and slip into the trap especially on a rainy day. Some insects who are seeking refuge under the lid can be catapulted into the trap by the force of raindrops hitting the lid. After the prey is dissolved, the nutrients were reabsorbed by the glands from the “soup”. The leftover or debris such as carcass or exoskeleton sinks to the growing graveyard of corpses at the bottom of the pitcher. Unlike the Venus flytrap, the lid does not close once the prey gets in. The lid will be closed only when the humidity is too low, the temperature is too high, or when it is acclimating to new conditions so that the digestive fluids don’t evaporate. The lids will open back up when it is acclimated.  

Interestingly, Nepenthes is called a monkey cup for a reason. Monkeys drink the pitcher solution when they are having digestion problems. The digestive fluid is often thick and almost syrupy. The pH of the fluid is neutral but it will become acidic when the struggling movement of the prey signals the pitcher. However, if you are planning to keep Nepenthes at your home, you have to beware of the breeding of mosquito and fly larvae. Mosquito and fly larvae can complete their life cycle even when the juice is as acidic as pH 3. Some pitchers had evolved in such a way that they do not obtain the nutrients through luring and preying but attracting small animals such as rats and frogs. The lip evolved into the shape that acts as a “toilet bowl” for them to settle their “business”. The rat and frog will use their front limbs or hands to grab the lip while pooping their feces into the pitcher. The species obtained nutrients through the feces of the small animals.

Further readings:

Bauer, U., & Federle, W. (2009). The insect-trapping rim of Nepenthes pitchers: surface structure and function. Plant signaling & behavior, 4(11), 1019-1023.

Bohn, H. F., & Federle, W. (2004). Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface. Proceedings of the National Academy of Sciences, 101(39), 14138-14143.

Rahaman, S. T., & Sai, P. R. (2018). A short review on carnivorous plants and recent developments in the field of cancer research. iJP5, 205-12.

Temple, P. (1988). Carnivorous plants.

Pietropaolo, J., Pietropaolo, P. A., & James. Pietropaolo. (1999). Carnivorous plants of the world. Timber Press.

Meyers-Rice, B. (2006). Growing carnivorous plants. Timber Press.

D’amato, P. (2013). The savage garden, revised: Cultivating carnivorous plants. Ten Speed Press.