IntroductionWe often think of plants as the primary producers, occupying the bottom of every food chain. What if, plants had a taste for meat?
Meat-eating plants
How do they get their food?
Tropical pitcher plants photosynthesize like other plants, but because they live in places where the soil has very little nutrients, they need extra adaptations to help them grow well. Tropical pitcher plants do this by catching insects and other arthropods, like spiders and even small mammals! These plants have "cups" called pitchers filled with their own special digestive juices that hang from the tips of their leaf blades to help them attract, trap, and digest they prey! Their prey are lured to the these death traps by sweet nectar produced on the underside of the pitcher lid, and if they are unlucky, slip into their watery grave. The inside of these pitcher are also slippery and keeps prey from climbing out (Tan et al., 1997). After a while, the prey drowns and is digested by the plant. Nutrients from the prey is absorbed through special glands on the inside of the pitcher (Owen, 1999).
Video 1. Pitcher plants have many adaptations to help them be more effective at catching prey. The peristome of the pitcher becomes more slippery when it is wet, making it a more effective trap. This adaptation appears to be consistent among the tropical pitcher plants. Watch the video for a quick overview of the pitcher plant trapping mechanism! Source: https://www.youtube.com/watch?v=Ytjpi7r18eo
Vegetarian or meat-eating?
Like most tropical pitcher plants, the narrow-lidded pitcher plant catches insect prey, but this member of the tropical pitcher plants also relies on plant matter for food (Pavlovic et al., 2011). The narrower lid of this tropical pitcher plant exposes the mouth of the pitcher more so that leaf litter can fall into the pitchers more easily (Clarke, 2001). The nitrogen from the leaf litter is then released as it decomposes and taken up by the narrow-lidded pitcher plant through the glands on their inner walls. Many other species of tropical pitcher plants also have their special ways of getting more food! Click on the links to find out more about the adaptations of other species of tropical pitcher plants: Nepenthes bicalcarata, Nepenthes rajah, Nepenthes lowi, Nepenthes hemsleyana (formally known as Nepenthe rafflesiana var. elongata).
Cheating death
Although the pitchers of tropical pitcher plants are death traps to the vast majority of insects, at least 59 species of animals are thought to live in the pitchers of the narrow-lidded pitcher plant for all or some part of their lives (Adlassig et al., 2011). These organisms are known as inquilines. These inquilines not only live in the pitchers, they often feed on the prey that the plant catches for itself! However, by "sharing" their food with the inquilines inside them, pitcher plants get additional help in digesting their food and help obtain the nutrients from the prey faster (Clarke et al., 1995; Lam et al., 2017). The inquilines also prevent putrefaction caused by the rotting of excess prey. Figures 1 to 4 show a few of the inquilines that are associated with the narrow-lidded pitcher plant.
Reproduction
Each narrow-lidded pitcher plant produces only either male or female flowers (Fig. 5) (Clarke, 2001). The flowers are arranged along stalk, with the flowers closer to the base blooming first. The entire stalk, along with the flowers is known as an inflorescence. Female flowers have a gynoecium, which refers to the part of the flower that eventually becomes the fruit and seeds, making them look swollen compared to the male flowers which do not have a gynoecium (Tan et al., 1997). Instead, male flowers have a stamen. These flowers produce nectar that attract insects to pollinate them. The fruits of the narrow-lidded pitcher plant are green and turn brown when ripe. After which they split open, releasing their seeds so that the wind can carry their seeds away from the parent plant (Tan et al.,1997).
How are tropical pitcher plants related to humans?
Apart drawing interest for its unusual taste for meat, there is a considerable market for tropical pitcher plants in the horticulture trade throughout the world (Clarke, 2001). However, it has been noted that cultivating a large variety of them in one place can be challenging, even for botanic gardens (Clarke, 2001). Additionally, tropical pitcher plants are important to scientific research because their unusual meat-eating habit is considered rare among plants (Clarke, 2001), and the many inquilines they house pose several important questions to science as demonstrated by the unusual interactions between pitcher plants and animals (Bazile et al., 2012). For example, do the inquilines benefit or exploit the plant? Do these inquilines specialise in living in the traps of tropical pitcher plants? Furthermore, studying pitcher plants could facilitate better understanding of ecology (Srivastava et a., 2004).
The Species Profile - Taxonomy
Name
Scientific name: Nepenthes ampullaria (Clarke, 2001)Other scientific names (Synonymn): Nepenthes ampullaceae Low (1848) (Clarke, 2001)
Common name: Narrow-lidded pitcher plant (Tan et al., 1997)
Type
Lectotype was designated by Jebb and Cheek and resides in Singapore Botanic Gardens Herbarium (SING).Natural hybrids in Singapore
N. ampullaria X N. gracilis ( = N. trichocarpa) == common name: Hairy-fruited pitcher plantN. ampullaria X N. rafflesiana ( = N. hookeriana) == common name: Hooker's pitcher plant
Tropical pitcher plants tend to hybridize easily, but hybrids are rare in nature! They usually have in-between traits of both parent plants. They are also vulnerable to competition from their parent species because they have less specialized adaptations to the environment than their parents.
What do they look like?
The narrow-lidded pitcher plant are large vines that grow up to 15 m long (Clarke, 2001). They either rely on other plants as support to grow upwards, or grow along the ground forming extensive carpets of pitchers that may be hidden by undergrowth (Tan et al., 1997; Clarke, 2001). Refer to Figures 6 to 8 for the parts of the pitcher.Leaf blade ---- The leaf blade has a broad-rounded end. Its base wraps the stem at the point it attaches to it.
Pitchers ---- Tropical pitcher plants form two types of pitchers — upper and lower. They usually look quite different, and are differentiated by which side of the pitcher is facing the tendril. When the front of the pitcher is facing the tendril, it is a lower pitcher. When the back of the pitcher is facing the pitcher, it is an upper pitcher. In the narrow-lidded pitcher plant, the lower pitchers are urn shaped, while the upper pitchers are funnel shaped. However, unlike the most tropical pitcher plants, upper pitchers are rarely observed in the narrow-lidded pitcher plant. There may be other differences between the two types of pitchers, but these are usually unique to the species. It is not clear as to why such differences exist, but it appears to correlate with the types of prey that the pitcher catches (Gaume et al., 2016).
Differentiating the local natives ---- Three species of tropical pitcher plants naturally exists in Singapore. They are the narrow-lidded pitcher plant (Nepenthes ampullaria), slender pitcher plant (Nepenthes gracilis) (Fig. 19), and raffles' pitcher plant (Nepenthes rafflesiana) (Fig. 10). These three species look very different and are easy to tell apart.
| Species |
Key differentiation features |
| Slender pitcher plant |
1. Leaf blade is shaped like a lance head. The base fuses directly with the main stem/vine. 2. Lower pitcher ovoid shape, upper pitchers with same shape but are narrower. |
| Raffles' pitcher plant |
1. Stalk looks white. Stalk is obvious, unlike the slender pitcher plant. 2. Tip of the leaf blade is rounded. 3. Lower pitchers have rounded base and more cylindrical towards the top. Upper pitchers are funnel-shaped. 4. Leaves and pitchers are usually larger than the other two native species. |
Other resources to identify tropical pitcher plants include an incomplete dichotomous key in the book "Nepenthes of Sumatra and Peninsular Malaysia" by Charles Clarke. For information more relevant to the local context, I recommend the book "A Guide to the Carnivorous Plants of Singapore" which provides information on the three native pitcher plants as well as the inquilines that are found in them.
Where can they be found?
The narrow-lidded pitcher plant is distributed throughout Southeast Asia (Fig. 11) and can be found from altitudes from 0 to 2100 meters above sea level (. The narrow-lidded pitcher plant inhabits nutrient poor environments such as heath forests, degraded swamp forests, and fem-dominated wastelands. Unlike many tropical pitcher plants, the narrow-lidded pitcher plant can sometimes be found completely covered by ferns (Fig.14).
Phylogeny
One of the most recent work on the phylogeny of tropical pitcher plants was based on 11 species found in Peninsular Malaysia (Bunawan et al., 2017). The authors sequenced the plastid-trnL intron and the nuclear internal transcribed spacer (ITS) consisting of ITS1, ITS2 and 5.8S. The resulting sequences were aligned using ClustalX Multiple Sequence Alignment programme. The authors employed three methods to analyse evolutionary history: (1) neighbour-joining (NJ), (2) maximum parismony, and (3) Bayesian inferences. Neighbour-joining and maximum parsimony were followed by a heuristic search as a large sample set was used (>9), although branch-and-bound would also be a suitable option.The different trees were compared. Branch supports was assessed by Bootstrap analyses.
There is strong bootstrap support for most nodes on the phylogenetic tree generated (Fig. 15). Analyses revealed that more related species tended to occur in more similar altitudes, which suggests that altitude could have help drive radiation among these tropical pitcher plants (Bunawan et al., 2017). The difference in grouping of Nepenthes benstonei between the two trees might correspond to the it occupying an intermediate altitude. Bunawan et al. (2017) further suggests that highland tropical pitcher plants might have evolved from lowland species.
Literature and References
Adlassnig, W., Peroutka, M. & Lendl, T. (2011) Traps of carnivorous pitcher plants as a habitat: Composition of the fluid, biodiversity and mutualistic activities. Annals of Botany.
Bazile, V., Moran, J.A., Le Moguédec, G., Marshall, D.J. & Gaume, L. (2012) A carnivorous plant fed by its ant symbiont: A unique multi-faceted nutritional mutualism. PLoS ONE.
Bunawan, H., Yen, C.C., Yaakop, S. & Noor, N.M. (2017) Phylogenetic inferences of Nepenthes species in Peninsular Malaysia revealed by chloroplast (trnL intron) and nuclear (ITS) DNA sequences. BMC Research Notes, 10, 67.
Clarke, C., 2001. Ecology. In: Chung, R. (eds.), Nepenthes of Sumatra and Peninsular Malaysia. Natural History Publications (Borneo) Sdn. Bhd. Pp. 29–76.
Gaume, L., Bazile, V., Huguin, M. & Bonhomme, V. (2016) Different pitcher shapes and trapping syndromes explain resource partitioning in Nepenthes species. Ecology and Evolution.
Ngai Lam, W., Yan Chong, K., Anand, G.S. & Tiang Wah Tan, H. Dipteran larvae and microbes facilitate nutrient sequestration in the Nepenthes gracilis pitcher plant host.
Pavlovič, A., Slováková, Ľ. & Šantrůček, J. (2011) Nutritional benefit from leaf litter utilization in the pitcher plant Nepenthes ampullaria. Plant, Cell and Environment, 34, 1865–1873.
Srivastava, D.S., Kolasa, J., Bengtsson, J., Gonzalez, A., Lawler, S.P., Miller, T.E., Munguia, P., Romanuk, T., Schneider, D.C. & Trzcinski, M.K. (2004) Are natural microcosms useful model systems for ecology? Trends in Ecology and Evolution.
Tan, H. T. W., W. K. Tan & C. L. Wong, 1997. Native species and hybrids: Tropical pitcher plants. In: Tan, H.T.W. (ed.), A Guide to the Carnivorous Plants of Singapore.Singapore Science Centre, Singapore. Pp. 51–96.