Pitcher plants, renowned for their captivating carnivorous nature, have evolved ingenious methods for acquiring nutrients in environments where soil conditions are poor. These plants employ specialized leaves modified into pitfall traps to lure, capture, and digest prey, primarily insects. This article delves into the fascinating dietary habits and nutritional strategies of pitcher plants, exploring the mechanisms they use to obtain essential elements for survival and growth.
Pitcher Plant Traps: An Overview
Pitcher plants are known as pitfall traps, with a deep cavity filled with digestive liquid. The traps of pitcher plants are considered to be "true" pitcher plants and are formed by specialized leaves. The term "pitcher plant" generally refers to members of the Nepenthaceae and Sarraceniaceae families, but similar pitfall traps are employed by the monotypic Cephalotaceae and some members of the Bromeliaceae.
The evolution of pitfall traps is widely assumed to have occurred through epiascidiation, which involves the infolding of the leaf, with the adaxial or upper surface becoming the inside of the pitcher. Over evolutionary time, selection pressure favored more deeply cupped leaves.
Luring Mechanisms
Pitcher plants attract foraging, flying, or crawling insects such as flies to a cavity formed by the cupped leaf. They use visual lures such as anthocyanin pigments and nectar. Olfactory cues can also play a role in attraction. Most have tall, narrow pitchers which attract insects with bright colours and inviting scents. Visitors are forced into perilous positions by waxy surfaces or tricked by transparent leaves, while nectar laced with poison intoxicates them.
Capture Mechanisms
The rim of the pitcher (peristome) is slippery when moistened by condensation or nectar, causing insects to fall into the trap. The walls of the pitfall may be covered with waxy scales, protruding aldehyde crystals, cuticular folds, downward-pointing hairs, or guard-cell-originating lunate cells, to help prevent escape.
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The Digestive Process
The small bodies of liquid contained within the pitcher traps are called phytotelmata. They drown the insect, whose body is gradually dissolved. This may occur by bacterial action (the bacteria being washed into the pitcher by rainfall), or by digestive enzymes secreted by the plant itself. Pitcher trap fluids largely vary in their viscoelasticity and acidity, which then dictates which type of prey they can target. Whatever the mechanism of digestion, the prey items are converted into a solution of amino acids, peptides, phosphates, ammonium and urea, from which the plant obtains its mineral nutrition (particularly nitrogen and phosphorus).
Digestive Fluids
The digestive fluids of carnivorous plants can be highly acidic, often reaching pH 2-3, which is more acidic on average than the gastric acids of insect-eating animals. This acidic environment is primarily generated by inorganic acids, mainly hydrochloric acid.
Digestive Enzymes
Carnivorous plants use enzymes similar to animal pepsin to breakdown animal proteins. Carnivory-active proteolytic enzymes were isolated from Nepenthes (tropical pitcher plants), Cephalotus, and Sarracenia (North American pitcher plants) and were found to be aspartic proteases. Dionaea (Venus flytrap) also secretes aspartic proteases, cysteine proteases are likely the most abundant proteolytic enzymes in its digestive fluid.
Many carnivorous plants possess several additional enzyme classes that degrade various high-molecular weight compounds found in an insect’s body. Examples include chitinases, which breakdown chitin, a component of the arthropod exoskeleton; ribonucleases, which breakdown nucleic acids; and other enzymes, such as amylases, esterases, and phosphatases.
Nutrient Absorption
Digested food in the human stomach is transported to the intestine, where degraded products are absorbed. Numerous transporter proteins in animal intestines participate in the uptake of a variety of nutrients such as ions, sugars, amino acids, and peptides. Several transporter proteins involved in nutrient absorption have been identified in Dionaea, whose repertoire may be distinct from its human counterparts. Mammalian intestines can encapsulate extracellular macromolecules in vesicles and absorb them into cells intact. This process, endocytosis, enables relatively nonselective nutrient uptake. This combination of membrane protein action and endocytosis is also found in carnivorous plant leaves.
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Glandular Structures and Functions
Glands are not unique to carnivorous plants, as many vascular plants possess glands for secreting various materials, including nectar, mucilage, resin, salts, aromatic compounds, and physiological residues. Digestive glands secrete mucilage, ions, and proteins including digestive enzymes. The same or morphologically distinct glands then absorb the degraded compounds via the activities of membrane transport proteins and endocytosis.
The occurrence of more than one type of gland is common in carnivorous plant groups, but their functional differentiation is not clearly understood in many species. It has been assumed that morphological differences come with functional differences in terms of digestive and absorptive capabilities, but more recent evidence points toward at least partial overlap in functions between different types of glands in different lineages.
Nutritional Adaptations
Like all carnivorous plants, pitcher plants all grow in locations where the soil is too poor in minerals and/or too acidic for most plants to survive. Their carnivorous nature is an evolutionary strategy to compensate for the lack of nutrients in their environment. Producing and maintaining pitchers consumes significant energy, so once they fulfill their role, the plant will naturally shed them. Older leaves (towards the bottom) turning yellow is no cause for concern.
Carnivorous plants thrive better when they feed on prey. In their habitats of nutrient-poor soils, they derive valuable nutrients (in particular Nitrogen and Potassium) from the digestion of prey, and respond with vigorous growth. For the hobbyist this means that your plants will grow better when they have a chance to feed or be fed.
Symbiotic Relationships
Mature plants of Nepenthes lowii attract tree shrews (Tupaia montana), which feed on nectar that the plant produces but also defecate into the pitcher, providing nitrates and other nutrients. The plant and tree shrew have a mutualistic relationship. The rim of N. lowii is not slippery so that tree shrews can easily get in and out; it provides more nectar than other pitcher plants.
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Nepenthes rafflesiana var. elongata has a similar relationship with Hardwicke's woolly bats (Kerivoula hardwickii). The bats roost inside the pitchers and the plants derive much of their foliar nitrogen from the feces of the bats. Compared to other varieties of Nepenthes rafflesiana that do not exhibit this form of mutualism, N. rafflesiana var. elongata has elongated pitchers that can accommodate both single bats and mother-juvenile pairs.
Cultivating Pitcher Plants: Feeding and Fertilizing
Outdoor growing will naturally provide sufficient food, but indoor growing may need to be supplemented by occasional feedings. Catching live flies and bugs for food can be great fun. Consider also using freeze-dried bloodworms, crickets, flake fish food, etc. Crush or grind the food into small pieces or powder for better absorption on Sundews, Butterworts, Byblis. Carnivorous plants do not need to feed everyday. In fact two or three feedings a year may well be enough for most of them. If they have the chance to catch more, they will, and benefit from this natural "fertilizer." They do have the ability to attract and capture food on their own, even indoors where they will catch an occasional insect on their own. They do not feed during winter dormancy either.
Most beginners are best off not using any chemical fertilizers on their plants. It is very easy to over fertilize, cause nitrogen burn on the trap, and kill the plant. Since carnivorous plant traps are modified leaves, typical leaf or foliar fertilizers are useful, but at very weak dosages. If the pitchers are dry, squirt water in them with a pipette or eyedropper after feeding, otherwise they won’t be able to absorb the nutrients.
Feeding Specific Genera
Butterworts do a pretty good job of catching food on their own. If they seem to be having trouble though, sprinkle some fish food or bloodworms on a sticky leaf or two every 2-3 weeks. Some butterworts will enter a succulent phase during certain times of the year. They will stop producing sticky leaves and the plant will usually shrink in size.
Pitcher Plants are probably the easiest carnivorous plants to feed. During their active growing season, drop bugs, fish food, or fertilizer pellets in a few of the pitchers every 2-3 weeks.
Like butterworts, sundews do a pretty good job of catching food on their own. If they seem to be struggling though, feed a few dewy leaves dry fish food or bloodworms every 2-3 weeks.
Venus flytraps are some of the most fun carnivorous plants to feed! Using tweezers, gently brush a bug, damp fish food, or bloodworms against the trigger hairs inside one of the traps. The trap will snap shut after a couple of strokes, getting a mouthful of food. For food that isn’t fed live, gently massage the trap after it has snapped shut. This mimics a bug moving inside and stimulates the trap to seal more tightly and produce extra digestive enzymes.
Sarracenia: The North American Pitcher Plant
Sarracenia pitcher plants are mostly found on the southeastern coastal plains of North America, and grow in sunny, open wetlands. The greatest diversity can be found in Alabama, Georgia, and South Carolina, although one species extends north along the east coast into the Upper Midwest and Canada. Sarracenia populations have declined rapidly in recent years due to drainage of their habitats for housing developments.
Species and Hybrids
There are currently eight recognised species of Sarracenia. North American pitcher plants are unusual in that all species can be hybridised, and that these crosses result in fertile offspring. Many hybrids occur naturally in the wild and were once given names as if they were species. The vigour often exhibited by hybrids is another reason why hybrids - along with the super-hardy Sarracenia purpurea! - are frequently recommended to first-time growers.
Cultivation of Sarracenia
Sarracenia grow best in full direct sun. As such, they are candidates for only the brightest of windowsills - a lack of direct sunlight will cause your plants to become weak and poorly-coloured. Terrariums are generally unsuitable. While seedlings can thrive in a terrarium, you will struggle to provide the light intensity required by adult plants. The tall erect species of Sarracenia, often known as trumpet pitchers, grow best in full sun. Generally, Sarracenia grow best in unheated greenhouses and conservatories. They are quite temperature tolerant, with conditions in their natural habitats frequently reaching 32°C (90°F) during summer.
Like many other carnivorous plants, Sarracenia require rainwater, distilled water, or deionised water, and lots of it! This is because they grow in low-nutrient waterlogged soil, and evolved to draw minerals from prey instead. As such, you must avoid tap water, bottled water and filtered water - all will result in a build-up of minerals and cause your plant’s health to deteriorate. You should not use fertiliser for similar reasons. During the growing season, you should stand your plants’ pots in about 2cm of water (just under an inch) and avoid watering from the top. The traditional compost mixture for Sarracenia is sphagnum peat moss mixed with either lime-free horticultural sand or perlite, to a ratio of about 2:1.
All North American pitcher plants require a cold winter dormancy between November and February. If you keep your plants indoors during the growing season, you will need to move them somewhere colder - sit them next to a window in your shed or garage, for example. Plants growing in an unheated greenhouse can remain there over winter. As the days shorten and the temperature drops, pitchers will turn brown and your plant will start to die back. This is normal, and you can safely trim off any dead growth. The end of the winter dormancy period is a good opportunity to repot and divide your plants, if they require it. Sarracenia grow from a rhizome, which is a thick underground stem that sprouts roots and leaves. Large adult plants will often have multiple growth points on their rhizome, and by snapping the rhizome and potting the pieces separately, you can propagate multiple (genetically identical) plants!
Kept outside, Sarracenia will catch more than enough food for themselves. If you keep your plants indoors, you can hand feed them with dried insects every few weeks.
Nepenthes: Tropical Pitcher Plants
Tropical pitcher plants, or Nepenthes, are surprisingly easy to care for. Nepenthes pitcher plants can be categorized into highland, intermediate, and lowland varieties.
Care for Nepenthes
Nepenthes pitcher plants need a photoperiod of 12-14 hours with bright, indirect light. Generally, daytime temperatures of 70-90°F and nighttime temperatures of 50-70°F are safe for Nepenthes. While high humidity is ideal, Nepenthes can adapt to household conditions. However, increased humidity promotes better growth. You can use a humidifier, but ensure there's adequate airflow. For most intermediate Nepenthes, ambient room humidity suffices, especially after bag acclimation.
Nepenthes require pure water from distilled, rain, or reverse osmosis water. Top watering involves pouring water directly onto the soil surface around the base of the plant, allowing it to seep down to the roots. Though not recommended, Nepenthes can be bottom watered by placing the plant pot in a tray or saucer filled with water. Dump any water that remains in the tray after an hour.
A chunky potting mix is recommended to allow the roots to dry out enough to remain healthy. We recommend a mix of 1:1 sphagnum moss and perlite. Make sure that all ingredients lack added fertilizers or nutrients, and that coco chips are leached below 40 ppm.
Nepenthes plants generally dislike repotting and can thrive in the same pot for several years. Nepenthes plants require occasional feeding to sustain growth. You can feed them biweekly with cricket powder, bugs (live or dead), or fish food. Alternatively, foliar feeding with Maxsea at 300 ppm once a month is effective. Foliar feeding is a method of applying fertilizer by mixing water-soluble fertilizer with water before spraying the plant with the solution. In the right environment, Nepenthes pitcher plants can be grown outside.
Addressing Common Issues
A pitcher plant not producing pitchers is often due to insufficient light, although low humidity can also play a role. If your Nepenthes arrives with black roots, there is no cause for concern. Nepenthes roots tend to look different than typical roots. Producing and maintaining pitchers consumes significant energy, so once they fulfill their role, the plant will naturally shed them. If the pitcher is entirely brown or black, it can be trimmed off. Nepenthes are really good at recycling their nutrients. Older leaves (towards the bottom) turning yellow is no cause for concern.