Above: - illustration of a Nepenthese pitcher. The pitchers are modified leaves and are beautiful structures and sometimes quite large!
Pitcher plants are extraordinary carnivorous plants. They have modified leaves that form special receptacles that contain fluid. Insects and other small animals (and also plant debris) that fall into the pitcher are broken down and absorbed. In particular, the nitrogen obtained in this way is a valuable nutrient as nitrogen often limits plant growth and carnivorous plants often occur where nitrogen is in short supply. The pitchers can be quite large and dead rats are occasionally found inside the very largest. Some pitchers secrete enzymes and acid to digest their prey, whilst others rely on bacterial decomposition or both. The enzymes that may be secreted include proteases (digest proteins) and carbohydrases (digest carbohydrates like starch). Digestion is typically slow and to stop the animals escaping various tactics are employed. Downward pointing hairs or spines inside the pitcher, combined with a slippery, waxy surface make it hard for insects to crawl back out!
Darlingtonia, the California pitcher or cobra plant has particularly fascinating pitchers that look like snakes, with their forked protruding appendage and curved over hoods. A spiral passage connects the digestion chamber to the outside world and even if trapped insects can navigate this, the hood has a special trick to deceive them - it contains fenestrae or translucent windows which allow light through. The insect makes a line for these windows, believing them to be exits, flies up, bumps into the hood and falls back down the spiral shoot into the trap! This compensates for the fact that Darlingtonia does not secrete digestive enzymes, but relies on the slower process of bacterial decay.
Chance encounter might not be sufficient and pitchers have ways of luring their prey! Sarracenia purpurea, a pitcher from NE America has a green pitcher which is red at the top and has nectaries around the aperture and releases a violet odour. The colour, promise of food and odour all serve to attract insects. Most of these insects are actually not flies but largely crawling insects and Sarracenia has a single ventral ridge the ala ventralis (ventral wing) which is a causeway along which insects can crawl up the pitcher to the slick, waxy surface of the rim (peristome) - and down they fall! Rows of downward pointing hairs, the slippery inside surface and the viscosity of the fluid in the trap all make it hard for the insects to get back out. The nectaries secrete an alkaloid narcotic, in addition to sugar, to anaesthetise the insect, again making it hard for the animal to avoid slipping in! Nepenthes has nectaries around the waxy rim and large downward-pointing teeth inside the tube and secretes digestive enzymes. Nepenthes is a tree-climbing vine.
Nepenthes (diagram based upon the type description and figures of Nepenthes pantaronensis in Gronemeyer et al.
2014. Plants 3: 284-303). Species of Nepenthes are extremely variable in
pitcher form. (External link: http://specialflowersintheworld.blogspot.com/2015/04/nepenthes-flower.html).
Nepenthes species use a variety of
trapping mechanisms to catch prey. In some the peristome (the coloured rim of the
pitcher) is a wettable surface which easily becomes coated with a
film of rain water or nectar, forming a slippery surface. Insects
alighting on the rim easily slip into the pitcher (wet capture). In
others, the main mechanism is the secretion of slippery wax crystals
on the inside of the pitcher tube. These plate-like waxy crystals
easily detach, sticking to the adhesive pads of insect feet,
contaminating them and making them more slippy dry capture).
Additionally, the wax seems to neutralise the sticky secretion of
the insect's feet. Insect feet (tarsi) have two primary adhesive
mechanisms: tarsal claws and adhesive pads (see insect locomotion). Both mechanisms must be
overcome for an insect to slip. Downward-pointing hairs (and the
downward-pointing epidermal cells) on the inside pitcher wall make
it harder for the insect/arthropod to crawl back up and escape the
has different varieties, with different pitcher forms, one relying
on peristome capture, the other on a slippery inner pitcher wall
Nepenthes gracilis relies primarily on the lid, the undersurface of which secretes most of the nectar and is coated in semi-slippery crystals of wax. An arthropod, such as an ant, stretching across the opening to reach the nectar may be flicked into the trap by falling rain! Further, the digestive fluid of the trap may be highly viscous and sticky and may contain alkaloids which anaesthetise the prey. (8)
Nepenthes aristolochioides has a pitcher with a translucent dome which acts as a light trap. Experiments have shown that light transmitted through the pitcher dome is important in attracting and trapping small flies, such as fruit flies (Drosophila). (9)
Finally, many pitchers are detritivores rather than insectivorous, collecting and digesting falling plant debris.
Not all carnivorous plants use mechanisms as passive as those of pitcher plants. The next example we look at are the aquatic bladderworts, or utricularias. These have extremely elaborate trap mechanisms!
a bladder of Utricularia. Utricularia is a freshwater aquatic
plant which lacks roots and floats in the water. It gives off
stalk-like stolons which bear small bladders, which contain fluid
under reduced pressure, each no more than 5 mm long.
These bladders are sophisticated traps, primed and ready to catch prey, which is typically small crustaceans, like water fleas and copepods, and insect larvae. These creatures try to hide amongst the hairs (trichomes) that project from the bladder (borne on wings (W) in the example on the left) but these hairs are primed triggers and when the prey brushes against them the trap springs open! Water rushes into the bladder, sweeping along the prey with it. The trap then seals shut and enzymes digest the unfortunate victim! R, rostrum; S, stalk; W, wing.
Below: a longitudinal section through one of the bladders. The trap is primed and a flap-like valve (V) closes the entrance. This valve is wedged shut. Pulling on the trichomes (T) springs the trap, the valve opens and water rushes in. Inside the bladder lining possesses small glandular hairs or internal glands (IG), some of these have four cells at their tips (4 end-branches) and are called quadrifid glands, whilst some have two terminal cells and are called bifid glands. The outside of the bladder contains equidistantly spaced small spherical or nipple-like glands (external glands). The functioning of these glands are described below.
The bifid glands are generally found immediately below and inside the trap entrance. Above: transverse sections of Utricularia bladders. Leftmost - when primed the bladders side-walls cave in (they are concave) as the fluid inside the bladder is at lower pressure than the water outside. Rightmost - an activated trap - water has rushed in and stretched the walls of the bladder which are now convex (curve outwards).
e: external glands, i: internal glands: b: bifid, q: quadrifid; s:
stalk; T: trichomes; V: valve.
Below: structure of a quadrifid or bifid gland, shown in section. (Based on the model of Fineran, 1985).
internal glands consist of an elevated basal cell, which is part of
the epidermis (cell layer covering the wall of the bladder). Above
this is a pedestal cell and above this are 2 (bifid gland) or 4
(quadrifid gland) capital or terminal cells. Black and dark shading
indicate water-impervious regions of the cuticle and cell walls
respectively. These ensure that water can only enter or leave the
gland through the terminal cells (which lack any substantial
cuticle) and also block the apoplastic pathway between the basal and
These glands pump out water from the inside of the trap when it is being primed, lowering the pressure in the trap and creating the suction when the trap springs open. This process is rapid and so probably occurs through the apoplast (cell walls) as shown by the blue arrow. The movement of this water is actively driven. Pumps in the cell-surface membrane of the pedestal cell use cellular energy to pump water from the apoplast across the membrane into the cytoplasm of the pedestal cell. The pedestal cell is a type of transfer cell and has tubular wall-ingrowths which increases the surface area of its cell-surface membrane to accommodate more pumps. These pumps are membrane-spanning proteins that pump chloride ions. (Chloride ions are negatively charged and their charges attract and drag positively charged sodium ions with them). This increases the salt concentration inside the cytoplasm of the pedestal cell, lowering its water potential. Water then follows by osmosis (drawn out by the salt) as it moves from higher to lower water potential. This water is replaced by water from the apoplast and a stream of water movement through the apoplast is set-up. Water then moves from the pedestal cell into the basal cell and surrounding tissues through the symplast (cell cytoplasms) moving from cell to cell through pores called plasmodesmata.
The terminal cells possibly also secretes the enzymes to digest the prey when the trap is activated. The products of digestion are absorbed by the terminal cells and are thought to follow the symplast route (movement in the cytoplasm) moving from cell to cell through the plasmodesmata.
Once the trap has done its job, it is reset and can be used repeatedly. The water that is pumped out every time the trap is reset has to be removed from the bladder and this is the function of the external glands.
external glands/trichomes consist of two cells - the pedestal cell
and the terminal cell. Their function is to excrete excess water
taken up from inside the trap when it is reset. Water absorbed by
the external glands enters the tissues of the bladder wall and is
transported from cell to cell through the plasmodesmata until it
reaches the external glands where it enters the pedestal cell
through plasmodesmata. The pedestal cell has tubular wall ingrowths
(though not as extensive as in the internal glands). These ingrowths
increase the surface area of the cell-surface membrane follows them
passively by osmosis. The cuticle of the terminal cell is highly
porous (especially when stretched by water entering the terminal
cell) and the water passes out through these pores.
The relative lengths of and angles between the arms of the quadrifid glands are important taxonomic characters in identifying species of Utricularia.
Flytrap (Dionaea muscipula)
Similar to the bladderwort, the flytrap is a spring trap. Dionaea produces modified leaves as traps, with each trap comprising a pair of valves. Each valve is fringed with hairs called cilia and has three trigger hairs, arranged in a triangle, on its inside surface. Each plant produces 5-7 traps which are each 3-7 cm long.
Charles Darwin in his 'Insectivorous plants' (1875, (17)) carried out thorough observations and experiments on feeding behaviour in Dionaea muscipula. Some of his key observations are useful to plant growers and botanists alike and are listed below:
Above: note the triangle of sensory bristles on each valve of the trap. Usually there are 3 bristles per valve (but sometimes 2 or 4). The redness inside the trap is due to the production of anthocyanin pigment. Anthocyanin synthesis is generally considered a stress response to high light intensity in flowering plants. This does not harm the plant, however. This individual plant was originally very green when bought and still very healthy, but the inside of the traps flushed red on exposure to direct sunlight. If a flytrap is too shaded then its leaves will etiolate and develop thin elongated leaves (21).
Some varieties naturally produce more anthocyanins than others, even in lower light intensities. In those forms that do produce the anthocyanin in response to light, plants illuminated enough to produce red traps are reported to grow better than those under-illuminated plants that stay green. Some varieties however remain yellow-green, being unable to synthesize the shielding anthocyanins, and when these cross with the red-trap varieties they produce individuals of various redness, including varieties like this one in which the center of the trap lobes flush red but never become deeply red.
The flower of Venus's Flytrap. Flytraps flower in April, shortly after leaving winter dormancy, but may continue flowering into July (21). The main inflorescence stalk or scape (peduncle arising from a subterranean stem) may reach 40 cm in length and terminates in a series of bracts, each bract subtending a pedicel bearing a single flower. Each inflorescence may consist of up to 40 flowers which open one at a time at intervals of 24 to 48 hours. The flowers are protandrous: the male organs ripen first with pollen being shed shortly after the flower opens. The stigma then becomes receptive 24 to 48 hours later. This reduces the likelihood of self-pollination. The flower then withers as the next flower opens.
Each of the five petals is up to 15 mm long and there are 5 green sepals. There are up to 15 stamens and a style terminating in a stigma fringed with hairs. The superior ovary is green and encloses a single chamber. The fruit is a capsule containing up to 40 shiny black seeds. The capsule lid shrivels away, sometimes explosively, to expose the seeds for dispersal. (21)
It is often said that flowering may weaken a Dionaea plant, especially in cultivation. I don't think there is much well documented evidence for this, mostly anecdotes and this seems to be a controversial topic. Plants often alternate their allocation of resources between flowering and vegetative growth and so it is reasonable to expect a reduction in leaf growth while flowering occurs: plants allocate their resources intelligently. Plants may also abort some of the flower buds if resources are limiting. This particular individual has so far flowered the past two springs after leaving winter dormancy. I think carefully controlled cultivation experiments are needed to test the effects of flowering on healthy plants.
Genlisea - mouths, stomachs and all!
a rhizophyll of Genlisea, showing the vesicle
(stomach) and the spiral arms with their slit-like openings. The
main mouth is in the junction of the two arms and hidden from
Many plants are not classed as fully-carnivorous but still 'eat' insects and make use of the nitrogen as a nitrogen supplement. One such example of a semi-carnivorous plant is the teasel. To be considered a fully carnivorous plant, three criteria must be satisfied (21):
The teasel certainly has specialized leaves that trap animal prey and
has been shown to derive considerable nutritive benefit. However, it is
not clear whether the traps lure prey. Many plants are able to derive
nutriment from trapped animals.
Sarracenia Pitcher Plants (Trumpet pitchers)
Above and below: Sarracenia psittacina, the Parrot Pitcherplant (Chelsea Physic Garden). The pitchers of this plant resemble those of Sarracenia minor except they are prostrate, forming a prostrate rosette whereas those of S. minor are erect. The hoods of S. psittacina also tend to form beak-like protuberances and look more parrot-like.
The traps of S. minor have hoods that hang down over the
openings and translucent windows at the back. insects attracted to the
nectar secreted by the trap rim and the edge of the wing (ala) which
leads up to the rim, are attracted to the light passing through the
windows so when they attempt to leave and fly away into the sky they aim
for the windows instead and hit the back of the hood and slide down into
the trap, which gets narrower towards the bottom, making it increasingly
difficult for insects to reverse. Eventually they become trapped in the
digestive fluid at the base of the pitcher.
Sarracenia psittacina has small openings and windows and backward-facing hairs make it hard for an insect that has entered the trap to escape. The prostrate traps are frequently submerged by water and catch arthropods and tadpoles and works more like a 'lobster pot' trap whereas the upright pitchers of Sarracenia minor operate as pitfall traps. The Parrot Pitcherplant sometimes also produces semi-erect pitchers with reduced wings (alae).
There are 8 to 11 recognised species of Sarracenia and these plants span the subtropical, temperate and Arctic zones of North America. Most occur in the USA, but the Northern Pitcherplant, Sarracenia purpurea, extends into Canada. Sarracenia minor is much more southern and grows in bogs in openings in pine forests in North Carolina, South Carolina, Georgia and Florida (regions that are chiefly subtropical). (External link: Botanical Society of America).A giant form of S. minor, the Okefenokee Giant variety, var. okefenokeensis also occurs. Sarracenia psittacina occurs from Georgia to Louisiana again in subtropical pine forests. Similarly, a gigantic form of S. psittacina, var. okefenokeensis has pitchers reaching over 30 cm in length.
The plant below was purchased from Chelsea Physic Garden, London (without reading the labels carefully). It is clearly a hybrid and after careful consideration I have decided that is most probably Sarracenia x Swaniana. Sarracenias hybridise easily and hybrids may consist of 2 or more parent species in differing proportions. The hybrid Swaniana is a cross between Sarracenia purpurea and S. minor (Sarracenia purpurea x S. minor).
Note the drops of nectar on the edge of the ala and the rim of the trap and the tracts of downward-pointing hairs on the inside of the hood. The shape of the hood and the tracts of hairs are very suggestive of Sarracenia purpurea as is the wavy ala. Note also the translucent windows, suggestive of S. minor. (The transparent trays were eventually replaced with opaque ones since they encouraged the growth of cyanobacteria and it was not sure what effect nitrogen fixation and secretion of other materials by these organisms into the water would have on the pitcherplant, although there are lower numbers of cyanobacteria in the surface soil of course).
The pitchers slowly reddened at their top ends whilst sitting on the windowsill, beginning with the veins. This red coloration is due to the pigment anthocyanin. The amount of anthocyanin produced depends on the species and variety and also on exposure to light and perhaps heat. The anthocyanins are produced when the plant is stressed by excessive radiation and help to protect the chlorophyll against bleaching. This does not necessarily mean that a red plant is unhealthy since plants that often grow in the open must shield themselves from UV light and in some plants, those anticipating very bright sunlit conditions, anthocyanin synthesis is often constitutively switched on so these plants always tend to be reddish. By the same token, reddness is not necessarily an indication that the plant is getting 'enough' light since this plant was mostly green when purchased but evidently very healthy.
This plant is growing near to open windows in the summer and so has caught some of its own food, though this has been supplemented by rehydrated mealworm pieces. the large tray is constantly filled with deionised water to about an inch in depth. This generates a moist enough atmosphere to prevent the tops of the pitchers from drying out (a problem that occurred early on and was quickly remedied). Not all deionised waters appear to be suitable, however, and when a good supply of water is found I think it's advisable to stick with it. Although this water has neutral pH, it has been claimed that the plants may prefer slightly acidic water as would be found in their natural bog/fen/seepage marsh habitats. this particular hybrid appears to grow well indoors and once the conditions are right then it will grow vigorously, but if anything is not quite right then the pitchers and roots will quickly begin to die back. Sarracenias do have particular requirements but grow vigorously once conditions are suitable.
Note that the developing leaves are mostly blade (ala) with no
traps to begin with no trap, then the trap cavity develops and
eventually the trap opens and the hood expands. The smaller traps are
not developing traps but mature small traps and are a deliberate part of
the plant's growth strategy. However, larger traps generally catch more
prey, though studies in Sarracenia pitchers have shown that in
nature as many as 50% of pitchers may fail to catch anything. The newer
pitchers began to produce copious nectar, which may reflect the ample
light for photosynthesis or the age of the traps or perhaps the plant
was not catching enough insects. The large frilly openings in the larger
pitchers is a character inherited from S. purpurea. Some of the
smaller traps appear more covered as are the traps in S. minor.
Sarracenia purpurea forms large sac-like traps with large opening that are not covered. These traps will catch falling debris in addition to insects and other small invertebrates and micro-organisms. There is some controversy over whether or not this species produces digestive enzymes or relies on digestion carried out by the microbial and invertebrate communities that accumulate in the open traps and simply absorb what is produced by them. However, research has clearly shown that although individual traps may persist for two years in this species, they catch most of their prey within the first 50 days after opening. Within the first two weeks liquid within the traps contained digestive enzymes (proteases, nucleases and phosphatases) even though the microbial count was very low. This suggests that traps of Sarracenia purpurea do indeed synthesise some of their own enzymes, especially early on. Although this enzyme secretion switched off later on, the necessary genes would switch back on if arthropods (or raw nutrients) were introduced into the traps. However, older traps do indeed have communities of microbes and support the development of the larvae of specialised species of insect, such as the Mosquito Wyeomyia smithii and the moth Endothenia daekeana. They also house the copepod Paracyclops canadensis (which is rare elsewhere). These organisms grow and develop inside the liquid of the pitchers without being digested, suggesting that older traps either eventually lose their ability to produce digestive enzymes or that the inhabitants send some signal to switch them off or that the enzymes are fended off by living organisms, e.g. by lipid or mucus secretion. Parts of plants that trap water that may house unique ecosystems are called phytotelmata (sing. phytotelma).
Sarracenias are perennials, overwintering largely as an underground rhizome, though the leaves in Sarracenia purpurea are evergreen and last for over a year. In most species the pitchers die back over winter. The amount of short days and cold nights that a pitcherplant needs to induce its overwintering dormant state obviously depends on the species and its natural latitude. The Swaniana hybrid here has one parent from the subtropics and one from the arctic, so it will be interesting to see how it manages. Sarracenia purpurea and S. flava have been planted out in several wild locations in Britain and are apparently doing well.
The flower of Sarracenia hangs upside-down on a long pedicel. There are three bracts, five sepals and five petals. the style expands out into an umbrella which traps pollen that falls from the numerous stamens. Insects entering the flower, perhaps seeking refuge, may pick up some of this pollen. The umbrella is pentamerous (with five-fold radial symmetry) and bears stomata and is photosynthetic and bears five radial veins. Near the tip of each vein is a small projection inside the umbrella which is the stigmatic surface. The petals are arranged such that insects can only enter the flower by passing past a stigma and they may deposit any pollen they are carrying here. The five veins contain pollen-tube conducting tissue as well as vascular tissue. the conducting tissue forms a hollow tube for the pollen tube to grow through, guiding it to the ovules inside the ovary, a distance of about 4 cm. On this long journey the umbrella must nourish the growing pollen tube, which is presumably assisted by the fact it carries out its own photosynthesis (14).
In S. minor and S. oreophila, flowering occurs at the same time as trap production, but generally in Sarracenia, flowering in spring occurs for several weeks before trap production to prevent pollinators from becoming prey! (20)
Below: phyllodia (sometimes phyllodiae is used for the plural and phyllodium for the singular) produced by the Sarracenia purpurea X S. minor hybrid discussed above. These leaves have proportionately better developed blades and smaller (generally nonfunctional) trap structures. Sarracenia produces phyllodia under two conditions: 1) low light levels (these two were the first two leaves to emerge after winter dormancy on this specimen but similarly they may also appear in autumn) and 2) when nitrogen levels are sufficient (15, 16). In other words they help the plant increase photosynthesis and the plant will not waste resources on producing traps until traps are needed. These phyllodia grew vertically initially, but since the trap structure does not develop and thicken they curved over to become prostrate, which is normal for large phyllodia. Traps normally pass through a small phyllodium stage as they develop, but in the case of phyllodia the trap development is inhibited and the mature leaf retain juvenile features.
Another factor affecting leaf morphology in Sarracenia is light. McPherson and Schnell (2011, Sarracenia of North America, Redfern Natural History Productions) make a distinction between true phyllodia and pseudophyllodia and suggest there is much confusion between the two. In this scheme, true phyllodia, e.g. of Sarracenia flava, completely lack a pitcher cavity and are sword or sickle-shaped and usually upright. In contrast pseudophyllodia are pitchers formed under stress conditions, such as insufficient illumination. These 'etiolated pitchers' have reduced pitcher tubes and domes and expanded wings. The phyllodia shown here look more like pseudo-phyllodia: note the small partially formed opening at the tip.
This plant was recently moved to a new location and was indeed showing signs of stress and has since been moved to a third location with a bit more light to see if this helps. I have no ideal place for it (outdoors or a window with more hours of sunlight would be better) and so I am doubtful this plant will thrive. It has at least gone through two winter dormancies and is still alive, however, so we shall see. When placed in brighter conditions this plant produced a number of true upright phyllodia and a normal trap.
The production of true phyllodia at certain times of the year, in addition to traps of different sizes, is part of the normal annual growth cycle for some species. For example, S. minor may sporadically produce a small number of phyllodia, whereas S. psittacina and S. purpurea normally produce no phyllodia at all. In contrast, S. leucophylla and S. rubra first produce small spindly traps in spring, sword-shaped phyllodia over the summer and larger and sturdier traps in late summer and autumn. (20)
There is perhaps a spectrum of leaf morphologies from phyllodium to pitcher, with true phyllodia being the most leaf-like and pseudo-phyllodia intermediate. There is also a possible difference in function, however. McPherson and Schnell describe true phyllodia as being normally produced by some species in addition to normal pitchers with the phyllodia persisting overwinter. A group of upright leaves which could be phyllodia or pseudo-phyllodia (hard to tell) persisted on the plant shown here over winter, whilst the prostrate leaf shown died away (its prostrate habit does suggest etiolation - the base become too thin to support the leaf).
It should be noted that it has been shown that the roots of Sarracenia purpurea can absorb amino acids directly (18). The presence of inorganic nitrogen increases the proportion of leaves that develop as phyllodes, so the roots still play an important part in N acquisition. under some conditions, carnivory may supply as little as 10% of the plant's nitrogen (15). The relative importance of the roots versus the traps in the mineral nutrition of the carnivorous plant varies considerable with species.
In many carnivorous plants the roots are either poorly developed (accounting for an unusally small proportion of the plant's biomass) or absent in aquatic forms (19). These roots are usually short, weakly branched or unbranched but regenrate easily. Despite being in waterlogged bog-soil these roots generally have a low air space volume without extensive development of aerenchyma seen in the roots of many aquatic plants or plants of waterlogged soils (such as in Salix (willow), Phragmites (reeds) and Carex (sedges)). Some, such as Darlingtonia, do have an extensive system of small air-spaces in the root cortex, however, and the rates of respiration in the roots of carnivorous plants is generally high regardless. Subterranean parts are important in surviving fires (that may clear away competitors) in Darlingtonia californica, some Drosera and Dionaea muscipula. Utricularia lacks roots but has specialised shoots (colourless shoots and rhizoids) that take over the functions of roots.
17 Dec 2015
22 Sep 2019
17 Apr 2020
04 Apr 2021