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72 pages

Pitcher plants: A fatal attraction

By Gloria Chay - December 2006

Pitcher plants are almost incongruous to the plant kingdom for they are carnivorous, and like their prey, people have also fallen under their spell, though hopefully not with the same deadly consequences.
Pitchers are intriguing, for they come in various shapes, sizes and colours. Always jug-shaped, some are short and squat like mini-tubs; others are elongated with a narrow neck like amphoras. Mottled colours on some render no two pitchers to be exactly the same. There are 7 genera of pitcher plants, found throughout parts of the Americas, Asia and Australia:
Nepenthes (Family Nepenthaceae) is the largest genus (at least 90 recognised species), which is distributed from northern Australia, throughout South-east Asia to southern China. Outlying species of Nepenthes occur in Sri Lanka, India, the Seychelles, Madagascar and New Caledonia. The vast majority occur in Borneo and Sumatra.
The pitchers are actually leaves or extensions of leaves, which have evolved to trap and in many cases, secrete digestive enzymes to digest trapped animals for the plants' nutritional needs. Those species, which do not have digestive enzymes, may rely on bacteria or other organisms for the digestion processes.
Pitcher plants differ from other carnivorous plants like the Venus Fly-trap, in that they do not have any moving parts. Instead they rely on the slippery walls and the pools of fluid in their pitchers to trap their prey. A nectar gland located beneath the lid of the pitcher attracts insects to crawl up or land there, thereby setting the trap. The fluids in the pitchers have been found to be at least slightly acidic.
The most common pitcher plant found in Singapore nurseries, Nepenthes rafflesiana, have a very acidic fluid. The lid prevents rainwater from falling into the pitcher and diluting the fluid.
Pitcher plants, in particular Nepenthes, include the largest of all carnivorous plants, with stem lengths exceeding 15 metres. The smallest pitcher on mature plants may be less than 2 ml in volume (I teaspoon = 5 ml); the largest may exceed 2 litres! There is a great variety in the shape of pitchers, and therefore, similarly, a great variety of animals are caught in the pitchers. Ants are the most common preys but many different types of insects are also caught. Some of the larger ones trap vertebrates like lizards, and even rats have been found inside. No other carnivorous plant can boast catches of this magnitude.
It is thought that rats are attracted to the fluid as a source of drinking water, and not due to nectar. Occasionally pitchers are found with large holes bitten out on their side, presumably by thirsty animals.
Remarkably, certain animals actually co-exist within the pitcher. How they can resist being digested is still unclear. Of concern is the finding of mosquito larvae in Nepenthes pitchers, which have been identified as belonging to the genus Toxorhynchites, which are not involved in the spread of malaria or dengue fever. Most mosquitoes cannot survive the hostile conditions within the pitchers and as such are unlikely to pose a threat to the public, whether growing in the wild or in cultivation. Realistically, there arc many more suitable water-holding vessels around the average garden and beyond which can support mosquito breeding.
The Nepenthes is the most commonly found pitcher plant in cultivation. In Singapore nurseries, only a few species are found, with N. rafflesiana being the most common. The plants are from tissue-cultured stock. Nepenthes are herbaceous plants and most of them grow as vines, climbing by means of the curling tendril located at the tip of the leaf. They can be broadly divided into 2 groups based on the altitudes at which they grow: Lowland species (sea level to 1000 metres) and highland species (above 1000m).
The highland group is by far the largest, but most of this species have very limited distribution, thus making them more vulnerable to extinction. Currently, all Nepenthes are in CITES II, with 2 exceptions, N. rajah and N. khasiana are listed under CITES I. This means that the trade of Nepenthes is strictly regulated and permits and licenses are required. Tissue cultured specimens from CITES 11 species are however, exempted. Cites is the acronym for Convention on International Trade in Endangered Species.
Nepenthes are classified as tropical plants, even though some from the highlands experience temperatures that are akin to those in temperate climates. However, they do not experience fluctuations in day length, light intensity or temperature throughout the year as in a true temperate climate. Nepenthes can be found growing in a wide range of soil types: peat swamp forest, on moss or on cliff surfaces and some on ultra basic soils on limestone rocks. Fundamentally, they grow in rather poor soil, low in nutrients.
Lowland species of Nepenthes are most ideal for beginners as their growing conditions can be easily simulated in Singapore. They can grow and withstand a maximum daytime temperature of 30-35 Celsius, with a nighttime minimum of typically around 28 deg. High humidity is required and normal outdoor conditions should suffice.
Nepenthes may be grown as indoor plants, requiring at least three hours of good sunlight per day, although I haven't tried it myself. Misting may be required in this case to maintain the humidity. As much indirect light should be given to outdoor Nepenthes, whether grown under trees or in a shade house (50-80 per cent shade have been cited, depending on species).
Watch out for burnt leaves, which means too much direct sun. A plant with large dark green leaves and no pitchers can be an indication of inadequate light. A Nepenthes pitcher may take anywhere from less than a month to up to a year to develop and mature. The longer a pitcher takes to mature, the more substantial it is likely to be. Pitcher production is a good indicator of general health, although not every leaf will necessarily produce a pitcher. Some species only seem to produce pitchers in intermittent flushes.
Pitchers last one to eight months, depending on the species. They naturally brown off and wither from the top and gradually move downwards. This withering process may take several months, but the pitchers are still generating nutrients for the plant and should not be trimmed off until they are completely brown and dry.
What do they grow on?
Ask 10 Nepenthes growers and you'll get 10 different answers. The most important criteria is a well-drained potting medium. This may come as a surprise when you consider that many Nepenthes grow on peat swamps. However, even in peat swamps there is frequent movement of water underground, preventing excessive stagnation of water and allowing for oxygenation of the sod. Root rot is probably the most common ailment in cultivated Nepenthes, with the early signs showing up as a blackening of the base of the stem. There is no cure once it occurs.
Healthy Nepenthes roots are black in colour; don't be fooled into thinking they're rotting! The growing medium has to be slightly acidic and this is achieved most often with the use of peat moss. My own potting medium is a mixture of peat moss, charcoal chunks and coarse gravel or granite chips in equal amounts. Coarse coconut fibre, sphagnum. moss, coarse wood bark are other alternatives that have been used to generate the mix.
We grow them in hanging clay orchid pots, although any well draining plastic pot will do. Hanging pots or baskets display the climbing formation of Nepenthes well, and also prevent earthworms from getting to them. It is best to elevate pots from the ground to prevent access by earthworms, which eat the roots. When using clay pots, it is even more important that water used to water the plants are as pure as possible.
The chemicals from tap water can be absorbed into the clay and over time, accumulate to such levels that they will kill the sensitive roots. Rain water, distilled water. and water from a from a natural source (stream, underground etc) is best, as it is fairly pure and the former is slightly acidic. Water may be treated by making a "peat tea": Wrap a ball of peat moss in a cloth and let it seep in a container of water. Water the plants when the media is nearly dry; over watering is a bigger problem than under watering Nepenthes!
Nepenthes do not require much fertilising and they often suffer fertiliser burn even from the safest, most organic form. Whilst Nepenthes do absorb nutrients from their roots, it is possible to replace this entirely with nutrients absorbed through the leaves and pitchers. To get the fastest, fullest growth possible, a monthly application of a liquid fertiliser to the leaves and soil can be done.
Fertilisers that are high in nitrogen with a full range of micronutrients is recommended. Orchid fertilisers work well, but use it at 1/4 the recommended strength. Do this only to actively growing, healthy and mature plants. Young plants and slower growing highland species should get less feeding at lower doses, otherwise they can be killed. Always do a trial first: never do anything new to all your plants otherwise you may lose the entire collection.
We hardly ever fertilise our plants, as those growing outdoors will catch enough insects to sustain them. We use a few balls of a slow release fertiliser like Osmocotc to each young plant 1-2 times a year and use an organic liquid fertiliser like fish emulsion or seaweed extract several times a year, at very weak doses.
As the pitchers mature and get bigger, natural prey like dead flies, mealworms and crickets can be put into the pitchers directly. However, do not treat them as garbage bins, raw meat and eggs will cause the pitchers to rot and die within days. Only feed the natural way when the pitches are large enough for the intended food and never go overboard - a few insects per pitcher is sufficient. Excess will lead to bad odours and death of the particular pitcher.
Large plants can be pruned yearly by tip cutting the stems to promote basal growth and hence, formation of lower pitchers. Do not prune to soil level. Propagation is by stem cuttings, seeds or commercially, from tissue culture.
One last word about growing Nepenthes successfully do not mollycoddle them. They like to be left alone and ignored if growing well enough, and are far better at looking after themselves than other plants.
A good source of information and plants is Malesiana Tropicals, based nearby in Kuching, Sarawak. You can access their website at at www.malesiana.tropicals.com.my.
Reproduced with permission from the Singapore Gardening Society (www.gardeningsingapore.org).

The Plant
The terminus "pitcher plants" is used for a number of different carnivorous plants with pitcher-like leaves. But these features have evolved independently in several genera belonging to different families of plants. Nepenthes are the tropical pitcher plants of the Old World. It's the only genus of the Nepenthaceae family. Higher systematic categories and their relations to the New Worlds Pitcher plants (Sarraceniaceae) are still discussed. You can find Nepenthaceae either in Aristolochiales (Magnoliidae) or in it's own Order Nepenthiales (Dilleniidae, as a neighbour to Sarraceniales). There are other suggestions and more interesting questions to solve. At least there is the consensus that they are dicotyledones...
Btw: the name Nepenthes is Old Greek and means "soothing grief"

The distribution of the genus Nepenthes (green). Borneo is marked red.
Carnivory is an answer to the lack of nutrients. While plants get their energy from sunlight, air and water, they still need nitrogen and phosphor to grow. Usually these can be found in the soil but in some places they are so rare that it's hard for plants to grow. Some plants have opened up another source: living animals. With sticky leaves, quick closing traps or slippery pitchers they catch insects and digest them to get the precious nutrients.
All Nepenthes grow in extreme habitats poor of nutrients. Peat swamp and mountain rainforests or degraded, eroded areas are typical habitats for pitcher plants. Some are even pioneers growing on pure sand where the tropical forest has been destroyed. When soils are rich they are not strong in concurrence, and other plants are growing faster.
Nepenthes rafflesiana growing on white sand where both forest and secondary vegetation have been destroyed by fire and the soil was washed away.
Most Nepenthes grow as lianas, and some can climb more than ten metres up the trees. The tendril of their leaves helps them to get hold on trees and bushes they use as support. Some grow epiphytic, others usually stay as rosettes on the ground.
They usually produce two morphological and sometimes even ecological different types of pitchers. Young plants in the rosette stadium have lower or ground pitchers. Their mouth is looking towards the tendril, and they have 'wings' on the pitcher wall. When the plants begin to climb, they start to produce upper or aerial pitchers. These lack the wings, and the tendril is on the backside of the pitcher. Often these two pitcher types look completely different and are hard to recognise as parts of the same plant.
The leaf morphology is among the strangest found in the kingdom of plants. The pitcher is the lamina or leaf spade; the tendril is the stalk. What looks like the leaf is an extremely enlarged leaf base, the part of the stalk next to the stem.


The leaf anatomy of Nepenthes in comparison to a normal leaf.
Nepenthes are dioecious, that means there are male and female plants. Inflorescences are long raceme- or panicle-like. The flowers have only four leafs (sepals) which are covered with nectar glands. The pollinators of some species seem to be flies and moth, but beetles, bugs and ants are also observed to visit the flowers. The development of the fruit capsule takes about three month. It can contain 500 or more seed. Those are very light and have long wings to be carried by the wind.
A female inflorescence of Nepenthes rafflesiana and two male flowers (top)
The flowers

The pitchers are traps to catch small animals. Though not all species use every method, they attract their prey with colour, smell and nectar. Some species are even known to have UV patterns like many flowers have.
The brim of the pitcher, the peristome, produces the highest amount of nectar. When animals try to get it they have to step on the slippery, waxy surface of the peristome and most of them are not able to walk there. They fall into the pitcher and then there is no way back. While the lower half of the pitcher which is filled with the fluid has a glandular wall, the upper half is as slippery and waxy as the peristome. The animals drown in the fluid and are digested by the digestive enzymes.
The pitchers

The basic structure of Nepenthes pitchers (from "Nepenthes of Borneo" by Charles Clarke, with permission of C. Clarke and C.L. Chan)
While they catch mostly insects there where also found small vertebrates, even rats in bigger pitcher. But these are mere accidents for both sides, and the putrefaction of big prey diminishes the pitchers live span. Carnivorous plants should be rather called insect eating plants.
Some Nepenthes species are highly specialised in their prey. While ants are the most common prey for the lowland species, e.g. N. mirabilis seems to be a real ant specialist. It's extremely attractive for ants, and some pitchers are filled with them. On the other hand, N. albomarginata feeds mostly on termites. How it attracts them is still not known.
Not all Nepenthes species feed only on animals. N. ampullaria seems to catch a good amount of dead plant parts and faeces falling from the forests canopy, and N. lowii possibly feeds on birds excrements. These plants produce a huge amount of nectar on the underside of it's lid. Birds like nectar, and the peristome of the pitcher is an ideal place to sit on and feed, while the wide pitcher opening collects the birds excrements.

The prey
The pitchers are designed as traps for small animals but some have learned not only to survive but to live there. The fluid is a habitat for a variety of specialised larvae of mosquitoes which can be found only in Nepenthes pitchers. There are also flies and midges, a specialised crab spider (Misumenops) hunts in the pitchers, some frogs lay their eggs there, and even a terrestrial crab can sometimes be found in Nepenthes.
Nepenthes bicalcarata is inhabited by the most fascinating community. Though carnivorous it's a real ant-plant with a mutualistic relationship between the two parners. The ant Camponotus schmitzi are nesting in the hollow tendrils. They rest under the peristome and dive and hunt in the pitcher fluid.
A strange habitat

The Species
On Borneo there can be found over 30 of the (up to now) 82 species known world wide. We can show only a selection of them, those species we have seen and photographed. In lack of any better systematical order the plants are grouped in lowland and highland species. Lowland species usually can be found in wide areas and are only missing on high mountains whereas highland species have a mostly patchy distribution in highlands.
The Ants

Pitcher plants produce sweet nectar to lure insects in their deadly traps - this is what can be read about Nepenthes. But there is much more in it than a green robber catching animals. It seems to be true for the solitary insects, for moth, flies or beetles. But the relationship between ants and pitcher plants is a very special and for more interesting one.
As other insects ants are attracted by the nectar produced by the plants. Searching for the nectar they climb the plant, and if successful they go back to the nest to alarm their sister workers. (Having a perfect working trap would be of a massive disadvantage for the plant. It would catch one ant when it could attract lots of them.) But ants do not only collect nectar, and here begins the more interesting part. While adult animals need the sugar as "fuel" the larvae need protein to grow. Depending on the species this can be e.g. fungae or other insects. Many other plants use this to get protection against herbivores. They produce nectar, and the ants who collect it also collect other prey on the plant, cleaning it while doing so. This phenomenon is called myrmecophily. On Nepenthes ants do the same, and this seems to be the reason why the pitcher plants offern nectar not only on the peristome but also on other parts, especially the young, develloping leafs and pitchers.
Nepenthes bicalcarata goes one step beyond this. It is usually living with its own ants, and the ant species Camponotus schmitzi can only be found nesting in this pitcher plant species. The plant offers "her" ant not only nectar but also nesting space. The tendrils of the pitchers are hollow and swollen. Many other plants produce similar domatia in leaves, stems or other parts. Most of them have their specialized ant species, and most of them can not survive without this protection. N. bicalcarata is not depending on its ant.


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