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Keywords:

  • carnivorous plants;
  • insectivorous plants;
  • phylogenetics;
  • proto-carnivory

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES

Darwin's interest in carnivorous plants was in keeping with the Victorian fascination with Gothic horrors, and his experiments on them were many and varied, ranging from what appears to be idle curiosity (e.g. what will happen if I place a human hair on a Drosera leaf?) to detailed investigations of mechanisms. Mechanisms for capture and digestion of prey vary greatly among the six (or more) lineages of flowering plants that have well-developed carnivory, and some are much more active than others. Passive carnivory is common in some groups, and one, Roridula (Roridulaceae) from southern Africa, is so passively carnivorous that it requires the presence of an insect intermediate to derive any benefit from prey trapped on its leaves. Other groups not generally considered to be carnivores, such as Stylidium (Stylidiaceae), some species of Potentilla (Rosaceae), Proboscidea (Martyniaceae) and Geranium (Geraniaceae), that have been demonstrated to both produce digestive enzymes on their epidermal surfaces and be capable of absorbing the products, are putatively just as ‘carnivorous’ as Roridula. There is no clear way to discriminate between cases of passive and active carnivory and between non-carnivorous and carnivorous plants – all intermediates exist. Here, we document the various angiosperm clades in which carnivory has evolved and the degree to which these plants have become ‘complete carnivores’. We also discuss the problems with definition of the terms used to describe carnivorous plants. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 161, 329–356.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES

Carnivorous or insectivorous plants have long induced fascination in men, and they are among the most popular plants in cultivation; they are often offered for sale in garden centres and over the Internet. There are many amateur botanical societies that focus upon them. The first living specimen of Dionaea muscipula Ellis ex L. came to the attention of the populace of London in 1768, an event that ‘caused a sensation throughout Europe’ (Magee, 2007: 49). Indeed, Linnaeus is reported to have declared ‘miraculum naturae’ (Magee, 2007) upon seeing D. muscipula. Prior to this event, John Bartram had sent Patrick Collinson, a London botanical collector, several plant parts, after the specimen sent by Governor Dobbs of North Carolina had failed to arrive (Magee, 2007). Bartram used a popular name for D. muscipula, tipitiwitchet, a somewhat ribald Elizabethan term for vulva (McKinley postscript to Nelson, 1990). This connection between female sexuality and carnivorous plants continued into 19th century England and may have had something to do with their popularity and continued public fascination.

Insectivorous plants epitomize Victorian England's cultural interest in the Gothic form in literature, architecture and art and, in terms of natural history, bizarre spectacles. These ‘queer flowers’, as Grant Allen described insectivorous plants in 1884, reached a zenith of popular and artistic attention during the mid to late 19th century. Allen's essay demonstrated the lure of the insectivorous plant as a floral femme fatale and in richly descriptive language described its ‘murderous propensities’ (Allen in Smith, 2003). Smith (2003) considered Swinburne's poem ‘The Sundew’ in relation to Allen's essay and Darwin's (1875) study on insectivorous plants. He noted that ‘both Swinburne's poem and Darwin's book were prominent elements in a cultural fascination with the sundew that extended from the 1860s well into the 1880s’, and ‘in the aftermath of Insectivorous Plants the potentially subversive moral and cultural implications of “The Sundew” become more difficult to ignore’ (Smith, 2003: 130–131).

In one of the most fanciful of Victorian stories, the German explorer Carl Liche and members of the cave-dwelling Mkodo tribe were described as making a trip through the Madagascan jungle. At one point, they come upon an amazing sight: a large plant with a bulbous trunk resembling a 2.5-m pineapple with eight elongate leaves, 3–4 m long, studded with hook-like thorns surrounding a depression filled with honey-sweet liquid. At the top of the tree are a set of long, hairy green tendrils and tentacles, ‘constantly and vigorously in motion, with … a subtle, sinuous, silent throbbing against the air.’ The story goes on to say that one of their women is forced at javelin point to climb the trunk. Then ‘the atrocious cannibal tree, that had been so inert and dead, came to sudden savage life. The slender delicate palpi, with the fury of starved serpents, quivered a moment over her head, then as if instinct with demoniac intelligence fastened upon her in sudden coils round and round her neck and arms; then while her awful screams and yet more awful laughter rose wildly to be instantly strangled down again into a gurgling moan, the tendrils one after another, like great green serpents, with brutal energy and infernal rapidity, rose, retracted themselves, and wrapped her about in fold after fold, ever tightening with cruel swiftness and savage tenacity of anacondas fastening upon their prey.’‘The great leaves slowly rose and stiffly, like the arms of a derrick, erected themselves in the air, approached one another and closed about the dead and hampered victim with the silent force of a hydraulic press and the ruthless purpose of a thumbscrew.’[‘Liche, 1881’ (almost certainly a fictitious author; see below) cited by Osborn (1924)].

Some readers took this account seriously. Travellers had been returning from the jungles of the world with astonishing stories: ferocious man-like apes, vine-shrouded lost cities. Gorillas and the Mayan ruins turned out to be real. Why not the man-eating tree in far away Madagascar, when at home in England there was a vegetable carnivore, the sundew, known to everyone? In Victorian times, Gothic stories combining horror and romance prevailed, and the story of a man-eating tree or any carnivorous plant was utterly exciting. Vegetable man-eaters matched the characters used in the supernatural tales about ghosts, haunted mansions, werewolves etc.

Combined with the fantastic findings in natural history by travellers, these man-eating trees tickled the fancy of writers. Buel (1887), for example, in his book Sea and Land included a section on carnivorous plants, in which, following a description of the action of Dionaea Ellis and Drosera L., he then wrote about a plant from Central Africa (possibly Liche's Madagascan plant) and tropical America, where it is known as Ya-te-veo[I-see-you.] (Fig. 1), ‘that is not contented with the myriad of large insects which it catches and consumes, but its voracity extends to making even humans its prey’. What fate awaited the ‘unfortunate traveller’? ‘The body is crushed until every drop of blood is squeezed out of it and becomes absorbed by the gore-loving plant, when the dry carcass is thrown out and the horrid trap set again.’ Due to the spines reported to pierce the body of the victim, Buel made an analogy to the maiden, a torture instrument ‘of the dark ages’ (with inward pointing spikes) which ‘was made, somewhat crudely, to represent a woman, hence the name applied to it’. Following more gory details about ulcers resulting from puncture wounds inflicted by the plant and mention of the ‘hundreds of responsible travelers [who] declare they have frequently seen it’, he concluded this section as follows: ‘All of which, however, I am inclined to doubt; not that there is no foundation for such statements as travelers sometimes make about this astonishing growth, but that the facts are greatly exaggerated’. Thus even writers aiming to provide a vivid, but true, account of the wonders of nature managed to confuse real and fictional carnivorous plants.

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Figure 1. The man-eating tree Ya-te-veo reported to occur in Central America by Buel (1887).

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Liche's story was adopted by Chase Osborn (1924) in his book Madagascar, Land of the Man-Eating Tree. Osborn said missionaries had vouched for the existence of the tree. No one has ever laid eyes again on this carnivorous horror, or on the Mkodo tribe for that matter, and Ley (1955) wrote that the Madagascan man-eating tree, the Mkodo tribe and even Carl Liche himself were all fabrications. It is nevertheless a gruesomely good story that may have its origins in older works, such as the True History of Lucianus Samosatensis, written in the 2nd century AD, in which female grapevines consumed sailors who tried to mate with them (note again the references to carnivorous plants being alluring and female).

Darwin himself was fascinated by carnivorous plants. He came to believe, after much experimentation, that the movement-sensing organ in sundews (Drosera) is far more sensitive than any nerve in the human body (Darwin, 1875). American naturalist Mary Treat, who studied many of these plants in her garden and understood their functioning in detail, discovered the function of bladderwort (Utricularia L.) traps (Fig. 2). She communicated regularly with Darwin about carnivorous plants and helped him understand their habits better (Sanders & Gianquitto, 2009). Another female correspondent and friend, Lady Ellen Lubbock (the wife of Darwin's great supporter Sir John Lubbock, later Lord Avebury) wrote a poem on reading Darwin's Insectivorous Plants and sent it to him (see Milner, 2009). It included the verse:

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Figure 2. Utricularia minor L., cultivated at RBG Kew. Photograph taken by Maarten J. M. Christenhusz.

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I never trusted Drosera

Since I went there with a friend,

And saw its horrid tentacles

Beginning all to bend.

We are accustomed to think of plants as being immobile and harmless, and there is something deeply unnerving about the thought of carnivorous plants. The ongoing fascination with these stories, exaggerating the traits of real-life carnivorous plants, indicates the deep horror we feel towards the idea of being devoured by a plant. No wonder the myths of the man-eating tree have stayed with us for centuries.

In 20th century Anglo-American culture, we saw a shift from the Gothic form to kitsch with the show Little Shop of Horrors and the character Audrey II– an extraterrestrial carnivorous plant that constantly cries ‘feed me’ (Fig. 3). The Life of Pi (Martel, 2001) featured a floating mat of carnivorous algae. Vegetable carnivores have also been adopted in science fiction stories such as Parasite Planet (Weinbaum, 1935), in which a plant on Venus eats humans. In The Fellowship of the Ring (Tolkien, 1954), hobbits fall asleep and find themselves eaten by a plant. Huge carnivorous trees are also dangers of the forests in Beyond the Deepwoods (Steward & Riddell, 1998).

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Figure 3. The man-eating plant Audrey II from the film Little Shop of Horrors (Film Group Inc.; Griffith, 1960).

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In Comet in Moominland (Jansson, 1973) Moomintroll saves the Snork Maiden from the twining arms of a poisonous bush of the ‘dangerous Angostura family’ with his penknife.

A carnivorous plant, Tentacula, is also featured in the Harry Potter series (Rowling, 1998), building upon the romantic but horrific idea of plants devouring people. Even some Pokémon characters (e.g. Bellsprout, Weepinbell and Victreebell; Pokémon, 2009) are based on carnivorous plants, bringing them into popular modern culture. More recently, the discovery of Nepenthes attenboroughii A.S.Robinson, S.McPherson & V.B.Henrich (Robinson et al., 2009) has stimulated a flurry of internet-dispersed rumours of ‘rat-eating plants’ (e.g. Instructables, 2009) and so a fascination with the ‘murderous propensities’ of carnivorous plants continues to capture the public imagination.

Definition of vegetable carnivory

Turning to a more serious side of the subject, several definitions of botanical carnivory have been proposed, but most researchers still consider at least some species lacking some aspects to nonetheless be fully carnivorous. The basic definition includes at least the ability to absorb the products of decomposition, either directly on the leaves or through roots in the soil, thereby increasing their fitness, ultimately leading to increased seed production. Including absorption by roots of nutrients released through one method of decay or another means that nearly all plants are capable of a degree of carnivory, and indeed this minimal definition is the one we follow in this paper (at least in terms of the taxa we treat). However, researchers who follow the ‘six lineages of carnivore’ argument (see below; Ellison & Gotelli, 2009) would add as well the provisos that these plants should also exhibit some means of (1) attracting prey to their traps (glistening glandular hairs being a minimal common attractant), (2) capture of prey and (3) their digestion. Digestion in many species generally held to be carnivores is not necessarily through enzymes secreted by the plant itself, but this can be achieved via enzymes secreted by bacteria, fungi or even the stomachs of other animals that eat trapped prey with the subsequent absorption of nutrients released by deposition of their faeces on nearby soil. The minimal glandular apparatus that secretes mucilage or other compounds needed to trap animals is widespread in angiosperms, almost to the point of being universal, at least in eudicots. Croizat (1961) thought that carnivory was an ancestral angiosperm attribute, a concept not far removed from the sentiment of the previous sentence; extant fully developed carnivores are merely more refined cases of underlying capacities present in all flowering plants. Formation of tanks into which prey and vegetable detritus fall and decay is also frequent, particularly in the commelinid monocots (e.g. bromeliads and others; see below). Proving that there is a net energetic benefit to carnivory when its occurrence is frequent and subtle makes it more difficult (what would one use as the background case for angiosperms?). Studies have shown that construction costs and scaling relationships for leaf traits in ‘true’ carnivores are substantially greater when traps are active (Ellison & Gotelli, 2009), but relatively few taxa have developed such complex traps; active traps occur in only two families, Droseraceae and Lentibulariaceae. In the great majority of the taxa we discuss below, carnivory is simpler, and in terms of leaf construction the costs are less and scaling relationships much like those of ‘normal’ plants. Overall, angiosperms of many types may be involved in a degree of carnivory and be ‘proto-carnivorous’; perhaps we should be more curious about why more plant species have not developed a ‘taste’ for animal-derived nutrients.

Development of a phylogenetic framework for carnivorous plants

Phylogenetic studies of DNA sequences have greatly aided efforts to assemble a complete tree of life for all angiosperms; based on this understanding of flowering plant phylogeny, full-blown carnivory has evolved in the angiosperms at least six times (Albert, Williams & Chase, 1992; Chase et al., 1993). These phylogenetic studies have also provided insights into morphological evolution of the varied mechanisms employed by plants to attract, retain, kill and digest animals and finally absorb the released nutrients. Darwin (1875) himself was certain that there had been several independent origins of carnivory, and this assumption has been broadly accepted by plant taxonomists since the time of Darwin. However, acceptance of polyphyly of carnivorous plants has been against a backdrop of major disputes over which groups of carnivores were closely vs. distantly related and to which other groups of angiosperms they are related (e.g. Hutchinson, 1973; Cronquist, 1981; Thorne, 1992; Takhtajan, 1997). Suggestions of relationships between groups of plants that we now know to be spurious were put forward by some of the most prominent botanists of their time: Hooker (1874; based on an idea from Linnaeus), for example, believed that species of Sarracenia L. were not only related to but had also evolved from waterlilies as a result of the move onto dry land. Takhtajan (1969) placed Sarraceniales near the ranalean complex, which included Nymphaeales, Papaverales and Ranunculales. It is equally clear that the origins of carnivory have been clouded by both convergent and divergent evolution and, before the advent of molecular phylogenetics, this subject was at an impasse and could no longer progress.

The first phylogenetic assessment of carnivore relationships was that of Albert et al. (1992), and this paper set the stage for more complete subsequent investigations by other authors. Albert et al. (1992) were particularly interested in the degree to which the carnivorous syndrome could provide evidence about general macroevolutionary patterns and process in flowering plants. Indeed, this must have also been the reason why Darwin studied these plants and wrote Insectivorous Plants (Darwin, 1875). Extreme specializations were thought to provide important insights into more general phenomena, such as the functions of sticky glands. Sticky, mucilage secreting, glandular hairs occur commonly throughout the angiosperms, and their exact functions are not always easily determined. Many angiosperms are undoubtedly passive carnivores; once an insect has been trapped by sticky glands, even if prevention of herbivory is the actual role of these hairs, its subsequent decay can provide nutrients, taken up by roots of the trapping plants. This sort of passive carnivory is clearly taking place in the southern African Roridula Burm. ex L. (Roridulaceae, Ericales), aided by the action of a Nemo-like bug that is unharmed by its host and lives its entire life on these plants, depositing on the soil around the plants its faeces, to the benefit of its host.

The ability to produce water-storage structures, whether through epiascidiation (inrolling of the abaxial leaf lamina with subsequent fusion of the margins as in Nepenthes L. and Sarracenia) or tank formation (as in the bromeliads Brocchinia Schult.f. and Catopsis Griseb.), provides the possibility of subsequent absorption of nutrients released by decay of any matter, whether plant or animal, that ends up falling into these structures. Other plants appear to be mining these same sort of veins, but are not considered to be carnivores, although they certainly could benefit from the decay of animals that happen to be among the items they collect in either ‘trash basket’ systems of roots (as in some orchids, including species of Ansellia Lindl., Catasetum Rich. ex Kunth and Grammatophyllum Blume) or sterile leaves (as in the ferns Platycerium Desv. and Drynaria (Bory) J.Sm.; Polypodiaceae).

The carnivorous syndrome in angiosperms has largely been focused on the plants that are clearly trapping, digesting and absorbing the resulting products, such as Drosera, Utricularia and Genlisea A.St.-Hil., but many authors have noted that other species may be going about the same business in a much less obvious way. Darwin (1875) noted that many plants have developed sticky glands that trap and kill insects, such as Erica tetralix L., Mirabilis longiflora L., Pelargonium zonale L'Hér., Primula sinensis Lour. and Saxifraga umbrosa L., but no one has further investigated these species to determine if something more sinister is in fact taking place. Some of these, such Erica tetralix, grow in nutrient-poor soils and are members of families related to those of carnivores (in this case, Sarraceniaceae), so they are likely candidates for investigation. Earlier in the 20th century, several Italian researchers (Mameli, 1916; Mameli & Aschieri, 1920; Zambelli, 1929; see discussion of these papers by Simons, 1981) suggested that yet other plants (Martynia L., Lychnis L. and Petunia Juss., respectively) with sticky leaves were in fact digesting prey, although the experiments to support these contentions were not particularly sophisticated and today would be considered inconclusive.

In this part of our review, we focus upon (1) the phylogenetic arrangement of the families that most authors have thought to be carnivorous, (2) the degree of modifications made to enable carnivory (by describing the mechanisms by which prey are trapped, killed and digested) and (3) the other genera of plants that have been hypothesized by at least some authors to be carnivorous and discuss the evidence for this.

Phylogenetic relationships and modern insights

As stated above, Darwin assumed that some parallelisms were likely among trapping types. Tank-forming plants such as the bromeliads Brocchinia and Catopsis aside (because they inherited the trapping structure from their ancestors and merely developed greater absorptive capacities in order to become carnivorous), pitcher plants evolved three times in the angiosperms, in Nepenthaceae (Nepenthes) in Caryophyllales, Cephalotaceae (Cephalotus Labill.) in Oxalidales (a rosid order) and Sarraceniaceae (Darlingtonia DC., Heliamphora Benth. and Sarracenia) in Ericales (an asterid order), whereas flypaper leaves may have evolved at least five times: Droseraceae (Drosera), Drosophyllaceae (Drosophyllum Link), Dioncophyllaceae (Triphyophyllum Airy Shaw), all in Caryophyllales, and Byblidaceae (Byblis Salisb.) and Lentibulariaceae (Pinguicula L.), both in Lamiales (an asterid order). Most authors would add Roridulaceae (Roridula) to this list, but, although the two species of this genus clearly trap insects, they neither digest nor absorb the released nutrients with specialized cells on their leaves (see below). There is an equally parsimonious alternative to this because three of these families are closely related; flypaper traps may have evolved once in the common ancestor of Droseraceae, Drosophyllaceae and Dioncophyllaceae, and then been lost in Ancistrocladus Wall. (the sole genus of Ancistrocladaceae), Habropetalum Airy Shaw and Dioncophyllum Baill. (the other two genera of Dioncophyllaceae). Many other species exhibit similar capacities to trap insects and other arthropods, so classifying Roridula as a carnivore is arbitrary. Snap traps probably evolved only once, in Droseraceae in the common ancestor of Aldrovanda L. and Dionaea Ellis (Cameron, Wurdack & Jobson, 2002). In Utricularia and Genlisea (Lentibulariaceae), we find two novel types of traps, bladders under pressure (formed on stolons, which are modified stems) and spiralled tubular traps (formed by subterranean leaves), respectively; Pinguicula in the same family has active flypaper traps.

Among angiosperms as a whole, flypaper traps are by far the most numerous, and it is easiest to consider a flypaper trap as the logical evolutionary antecedent for the development of more specialized traps. Once trapping/digesting of prey occurs, then further structural modifications/physiological developments to improve its function or specialize in particular types of prey are easier to envisage. In nearly all cases of other specializations, flypaper traps occur among the related genera, except in the case of Cephalotus (Cephalotaceae, Oxalidales), which is the most distantly related species relative to the others known. Other families in Oxalidales, for example, Cunoniaceae and Elaeocarpaceae, have mucilage secreting cells in their epidermis, and a few genera (e.g. Eucryphia Cav., Cunoniaceae) are glandular. In several other cases, stalked secretory glands are common among other families in orders with carnivorous taxa, for example, Caryophyllales (Lledóet al., 1998) and Lamiales.

OVERVIEW OF MURDEROUS PLANTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES

Below we provide an overview of known carnivorous plants and their phylogenetic relationships, presented in the order from APG III (2009). Considering that there are varying degrees of carnivory exhibited by many angiosperms, this list will not be exhaustive, and good evidence for carnivory will most likely be found in other plant groups in the future. For each carnivorous genus, we provide habitat preferences, mechanisms of trapping insects, glandular modifications to aid digestion and absorption of digestion products and an assessment of their degree of active carnivory.

MONOCOTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES

Poales

Bromeliaceae

Rosette-forming leaves of many bromeliads form tanks, in which they catch rainwater, allowing them to survive dry periods in the canopy of rainforests. Roots of most bromeliads are non-absorptive and function only to attach these plants to their substrate or host plant (in the case of the epiphytic species). Absorption by roots is replaced by absorption of minerals and potentially nutrients from their tanks via specialized scale glands, and these have been documented to absorb labelled amino acids (reviewed in Givnish et al., 1984). In these ‘pitchers’, whole ecosystems can develop, and they are an important breeding ground for tree-dwelling frogs in tropical America, as well as a source of nutrients for the epiphytic species in particular.

Brocchinia reducta Baker, a terrestrial species from southern Venezuela and Guyana, grows in sunny, wet, nutrient-poor habitats alongside other carnivores such as Heliamphora. This species has a specialized ‘pitcher’ formed by the leaf rosette in which water accumulates. The insides of the leaves are covered in waxy scales that reflect ultraviolet light, and many insects are attracted by this combination and lured into the trap. Brocchinia reducta was originally thought not to produce enzymes, and digestion was then considered to be performed by a community of inquilines (Givnish et al., 1984). The pH of the liquid in the pitcher of B. reducta measured 2.8–3.0, so this too would aid in breaking down material that fell into the liquid. It is known to contain large numbers of drowned insects and has recently been demonstrated to produce phosphatases, albeit weakly (Płachno et al., 2006); thus, it can be considered a carnivorous plant in the strict sense. The trapped insects are broken down, and the nutrients released are absorbed by the glandular leaf scales. A closely related species, B. hechtioides Mez, has also been considered to be similarly carnivorous, but this species has not been studied so far in detail (Givnish et al., 1997).

Similarly, Catopsis berteroniana (Schult. & Schult.f.) Mez, an epiphytic species that can be found from Florida to Brazil, appears to trap more insects than other bromeliads of a similar size (Frank & O'Meara, 1984). However, Givnish et al. (1984) performed a cost–benefit analysis, which showed that only plants growing in wet, bright conditions would be likely to make full-blown carnivory a successful strategy. Epiphytic species should not be carnivorous, but it is clear that some species growing in dry areas can be carnivores (e.g. Drosophyllum, Drosophyllaceae, and some Drosera spp., Droseraceae; Caryophyllales, see below). Some Nepenthes spp. growing in humid environments can effectively be epiphytes, and many grow in a high degree of shade.

Most species of Puya Molina have extremely sharp, outwardly pointing spines, probably to deter herbivores. In contrast, the spines of P. raimondii Harms point inward towards the rosette. This tree-like Andean bromeliad is bird pollinated, and several bird species also use the rosette as a nest site, with the inward pointing spines providing stability for the nest. Some birds are also trapped by the spines and killed. Rees & Roe (1978) counted a total of 44 dead birds in only 17 plants. Because the leaves are curved into a trough shape, it may allow P. raimondii to benefit from the added nutrients provided by run-off of accumulated nest debris and faeces, not to mention dead birds.

Eriocaulaceae

Paepalanthus bromelioides Silv. shares the habitat of Brocchinia reducta, and its leaves also form a tank that may function in a similar fashion as that of Brocchinia (Jolivet, 1998). This is, however, highly speculative, and no study has been carried out on possible trapping, digesting or absorbing mechanisms of this species.

EUDICOTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES

Saxifragales

Saxifragaceae

Darwin (1875) reported that peduncles and petioles of Saxifraga umbrosa L. are clothed in pink glands that secrete a yellowish viscid fluid, which can rarely entrap minute Diptera. He proceeded to experiment on flower stems in a manner similar to, although less extensive than, his studies on Drosera. Dipping them into infusions of raw meat, he noted movements in the protoplasm ‘exactly like those described in the case of Drosera.’ With exposure to solutions of ammonium carbonate, he observed that ‘there is the closest resemblance to what takes place when a tentacle of Drosera is immersed in a weak solution of the same salt. The glands, however, absorb very much more slowly than those of Drosera.’ He concluded that more evidence was necessary ‘before we can fully admit that the glands of this saxifrage can absorb … animal matter from the minute insects which they occasionally and accidentally capture.’ For Saxifraga rotundifolia L., he observed that the glands were able to absorb ammonium carbonate much more quickly than those of S. umbrosa. Darwin stated that ‘The most interesting case for us [among the plants with glandular hairs that he studied] is that of the two species of Saxifraga, as this genus is distantly allied to Drosera’. Although, molecular phylogenetic studies have refuted this last statement, it appears that at least some species of Saxifraga have glands that are capable of trapping insects and that these same glands can absorb a range of substances. The exudates from the glands have not been demonstrated to have digestive properties, however, and it is unlikely that these plants are true carnivores. Their habitat preferences also do not appear to be those associated with the classic cases of carnivory.

Oxalidales

Cephalotaceae

Cephalotaceae are monotypic, the sole species being Cephalotus follicularis Labill. (Fig. 4A), which is only found in southwestern Western Australia. Generally known as the Albany pitcher plant, it is also sometimes referred to as the moccasin plant or Western Australian or (historically) New Holland pitcher plant. Described by Labillardière on the basis of material that has apparently been lost (Willis, 1965), further details of the taxonomic history of this species are given by McPherson (2009). Although described in 1806, it appears to have been unknown to Darwin and is not mentioned in his book, although he did visit Western Australia, which left him famously unimpressed:

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Figure 4. A, Cephalotus follicularis Labill., RBG Kew. B, Passiflora foetida L., showing flower with glandular bracts, Punaauia, Tahiti. C, Dionaea muscipula Ellis ex L., active traps, Carolina Beach State Park, North Carolina. D, Drosera pulchella Lehm., with winged petioles, cultivated at RBG Kew. E, Drosera regia Stephens, regular circination of unfolding leaf, cultivated at RBG Kew. F, Drosophyllum lusitanicum (L.) Link, reverse circination of unfolding leaf, RBG Kew. All photographs taken by Maarten J. M. Christenhusz.

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FEBRUARY 17TH. – The Beagle sailed from Tasmania, and, on the 6th of the ensuing month, reached King George's Sound, situated near the S.W. corner of Australia. We staid there eight days; and I do not remember, since leaving England, having passed a more dull, uninteresting time. (Darwin, 1839)

Although the pitchers are superficially similar to those of Nepenthes, Brown (1832) stated that the two genera ‘differentiate in so many other important characters that they cannot be considered as nearly related.’ Among other characters, Cephalotus Labill. differs from Nepenthes in its hermaphrodite flowers. Like Nepenthes, however, Cephalotus produces its own digestive enzymes and is thus an active carnivore (see McPherson, 2009 for further details). It also grows in wet, sunny sites where other carnivorous taxa are common, particularly Drosera spp., including the enormous D. gigantea Lindl. Phylogenetic studies indicate that Cephalotaceae belong to Oxalidales, within which they are related to Brunellia Ruiz & Pav. (Brunelliaceae) and then to Eleaocarpaceae and Cunoniaceae; Cephalotus folicullaris is thus only distantly related to any other carnivorous taxa (APG III, 2009) and unusual in belonging to a clade in which glands are not always present. Mucilage-producing epidermal cells or hairs (some genera of Elaeocarpaceae and Eucryphia, Cunoniaceae) do occur in the families of Oxalidales and evidently can serve as antecedents for development of the digestive and absorptive glands in Cephalotus.

Malpighiales

Passifloraceae

Passiflora L. is known for its glands that mimic insect eggs to fool especially butterflies that do not lay eggs if there are already some present on leaves. Similarly, the peculiar, pantropical, weedy passion flower, Passiflora foetida L., has long glandular hairs on the bracts subtending its white–green flowers (Fig. 4B). These multicellular, stalked glands that are remarkably similar to those of taxa in Caryophyllales have been proven to produce digestive enzymes (Radhamani, Sudarshana & Krishnan, 1995), suggesting insects get trapped in the fine netting of the bracts and are digested. However, there is no mention of sterilizing the bract surface or otherwise demonstrating that production was endogenous, nor was absorption definitively demonstrated. This may simply be another case of a sticky floral defence against herbivory. It does not fit the syndrome of habitat features observed in other carnivorous taxa (Givnish et al., 1984); P. foetida inhabits wet tropical areas in which carnivores are otherwise absent. It is also an invasive in tropical ruderal zones worldwide.

Rosales

Rosaceae

Many members of Rosaceae are covered with glandular, often sticky, processes. Insects often become entrapped, and it has widely been assumed that these glands have a defensive capability. However, members of the Potentilla arguta Pursh complex (sometimes placed in Drymocallis Fourr. ex Rydb., including also P. glandulosa Lindl. and P. rupestris L.) appear capable of not only trapping insects, but also digesting them and absorbing the released nutrients (Spomer, 1999). These species occur in habitats that can support other carnivores, especially Drosera, and fit the syndrome of conditions associated with carnivory, but Spomer (1999) admitted that he could not rule out surface microbes as the source of the enzymes he observed to be digesting insects trapped by the hairs on these species of Potentilla.

Geraniales

Geraniaceae

As in the case for Potentilla (Rosaceae; see above), Spomer demonstrated that Geranium viscosissimum Fisch. ex C.A.May has the potential to be carnivorous. It appears to secrete digestive enzymes and be capable of absorbing amino acids. As for P. arguta, it cannot be ruled out that surface microbes are responsible for digestion, but, in the same series of experiments, leaves of Petunia did not exhibit this activity, so either the microbes responsible do not occur on Petunia or the enzymes are released only by G. viscossisimum. The habitat preferences for this species (moist roadsides and creek banks, sometimes in shade) do not seem to be clearly those associated with the carnivorous syndrome, and no other carnivorous taxa grow with it. Darwin (1875) also mentioned possible carnivory associated with the glandular hairs on Pelargonium zonale L'Hér., but no one has ever investigated this further. As for G. viscossisimum, this seems an unlikely carnivore, given its habitat preferences.

Brassicales

Brassicaceae

Carnivorous seeds are perhaps a strange concept, but tests have been carried out on the seeds of shepherd's purse, Capsella bursa-pastoris Medik. (Barber, 1978). It was shown that the mucilage layer of seeds upon imbibition contains chemicals that attract soil nematodes, protozoa and soil bacteria and a toxin that kills these organisms. The mucilage also produces proteinases and labelled amino acids were found to be taken up by the seeds. It is easy to imagine that seedlings would benefit nutritionally from the breakdown of these organisms.

Caryophyllales

Caryophyllaceae

Various members of the genera Lychnis and Silene L. have sticky hairs and are known by the common name of ‘catchfly’. Despite this, most modern authors have given little credence to them being carnivorous. Barthlott et al. (2007), for example, reported that Lychnis viscaria L. is one of the non-carnivorous plants that ‘have developed sticky leaves as a defense against feeding insects’ (along with Nicotiana tabacum L. and Passiflora foetida), but continued that the insects thus trapped ‘are not digested because these plants have no digestive enzymes’. However, Mameli & Aschieri (1920) demonstrated that L. viscaria‘without doubt contains a proteolytic enzyme, that renders the proteinous substances slowly soluble’ and that ‘the viscous zones of the stalks of the plants are permeable’. Spomer (1999) demonstrated that the sticky exudates of some other members of Caryophyllaceae (Cerastium arvense L. and Stellaria americana (Porter) Stanl. and S. jamesiana Torr.) showed protease activity, but he did not investigate uptake of the products of digestion. Further research would be necessary to demonstrate full carnivory in this family.

Dioncophyllaceae

Dioncophyllaceae consist of three monotypic genera, Dioncophyllum Baillon, Habropetalum Airy Shaw and Triphyophyllum Airy Shaw, all lianas or shrubs endemic to tropical West Africa. Molecular studies (e.g. Fay et al., 1997; Cameron et al., 2002; Cuénoud et al., 2002; Heubl, Bringmann & Meimberg, 2006) have shown them to be most closely related to Ancistrocladaceae, with a single genus Ancistrocladus Wallich consisting of 12 species of lianas in tropical Africa and Indomalaysia. This pair of families is most closely related to Drosophyllaceae and then more distantly to Droseraceae, Nepenthaceae and other caryophyllalean families.

Green, Green & Heslop-Harrison (1979) demonstrated that Triphyophyllum peltatum (Hutch. & Dalziel) Airy Shaw is carnivorous. The other two genera of Dioncophyllaceae and Ancistrocladus lack the carnivorous habit. Dioncophyllum appears to be eglandular, and the glands in Habropetalum are simple and present only on young stems. This lack of carnivory probably represents losses in these three genera, given the relationship to Drosophyllaceae and other carnivorous plant families (e.g. Heubl et al., 2006).

Triphyophyllum is heterophyllous, producing three types of leaves, two generally found on non-climbing, juvenile shoots and the third on adult climbing shoots. Short shoots firstly produce mostly eglandular oblanceolate leaves (Fig. 5A), but, just before the height of the rainy season, glandular filiform leaves (Fig. 5B) are produced, which are relatively short-lived, surviving for only a few weeks. As in Drosophyllum, these show reverse circinnation. The glandular leaves bear stalked and sessile glands (Fig. 5C) and are efficient in trapping prey (Fig. 5B; mostly insects, with Coleoptera being the most common). Large numbers of prey are trapped: Green et al. (1979) found c. 21 identifiable carcases per leaf plus other chitinous remains. Occasional leaves intermediate in form between these two types are found on the juvenile shoots (see for example McPherson, 2008). The leaves of the mature axis are oblanceolate and have apical hooks (Fig. 5D), enabling the liana to climb into the canopy. The account of Triphyophyllum by McPherson (2008) included many more photographs illustrating the different leaf types and other features, including the prominently winged pink seeds.

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Figure 5. Triphyophyllum peltatum (Hutch. & Dalziel) Airy Shaw. A, eglandular leaves on juvenile shoots. B, glandular leaves on a juvenile shoot. C, close-up of glandular leaf showing two types of glands. D, mature axis with grappling hooks. Photographs taken by Stewart McPherson (2008).

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The glandular leaves possess stalked and sessile glands. The stalked glands fall into two size classes (2–3 mm and c. 1 mm) and bear secretion droplets that trap prey. The sessile glands are dry until stimulated. The gland stalks are vascularized, as in Drosera and Drosophyllum, and also in Passifloraceae (including Turneraceae), but this condition is otherwise unknown in the angiosperms. Green et al. (1979) stated that the stalked glands are ‘the most anatomically elaborate known in the plant kingdom’. They also demonstrated that the secretions of these glands contain proteases and other enzymes.

Juniper, Robins & Joel (1989) referred to Triphyophyllum as the ‘part-time carnivorous plant’. It is unusual among carnivores in being well rooted and has only a transient carnivorous phase. Green et al. (1979) reported that there is evidence that Triphyophyllum leaves are relatively rich in potassium, which is deficient in the surrounding soil, and they wrote ‘It seems possible that the transition from the juvenile phase requires that some threshold level should be reached in nutrient reserves, and in the absence of adequate soil resources carnivory could be the means for gaining this threshold more rapidly.’ They continued ‘The limited growth of the short shoot is accompanied by an alternation of groups of long-lasting photosynthetic leaves and shorter-lived insect-capturing leaves. This growth pattern might be expected to permit the hoarding of nutrients until the moment was reached when a switch could be made to the development of the rapidly growing, efficiently climbing liane.’ More recent studies in the field by McPherson (2008), however, indicated that plants are able to switch between primary and secondary foliage types more than once in their life cycle.

Carnivory appears to be well established in the case of Triphyophyllum, although it grows in rather atypical shady environments, not generally associated with carnivorous plants (Juniper et al., 1989). Green et al. (1979) suggested that a parallel with Triphyophyllum may be seen in epiphytic and climbing Nepenthes, although the ability of Triphyophyllum to grow in shade and the lack of carnivory shown by the leaves of the mature liana clearly distinguish it from members of that genus.

Droseraceae

Droseraceae comprise three genera, Aldrovanda, Dionaea and Drosera. Aldrovanda (one species) is a widespread aquatic in the Old World, and Dionaea (also a single species) is endemic to a small swampy area in North and South Carolina. Drosera (more than 180 species), in contrast, is extremely widespread, occurring on all continents except Antarctica. Notable radiations occur in Australia and South Africa. Although Darwin (1875) considered Droseraceae to include six genera (Aldrovanda, Byblis, Dionaea, Drosera, Drosophyllum and Roridula), phylogenetic studies have demonstrated that Byblis, Drosophyllum and Roridula should be excluded, with Drosophyllum being more closely related to the other caryophyllalean families (Ancistrocladaceae, Dioncophyllaceae and Nepenthaceae) and Byblis and Roridula being members of Lamiales and Ericales, respectively. In spite of this narrowing of their circumscription, Droseraceae nevertheless demonstrate remarkable diversity in morphology, specifically in their mode of carnivory, with two major forms of traps: flypaper traps in Drosera and snap traps in Aldrovanda (Fig. 6) and Dionaea (Fig. 4C). Cameron et al. (2002) showed that Aldrovanda is sister to Dionaea and this pair is sister to Drosera (Fig. 4D, E), but with relatively weak support for this pattern of relationships.

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Figure 6. Aldrovanda vesiculosa L. (from Insectivorous Plants, Darwin, 1875: fig. 13).

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In analyses using rbcL sequence data alone, Rivadavia et al. (2003) with greater species sampling placed Aldrovanda as sister to D. regia (Fig. 4E), but with no support for these relationships. In their analyses of rbcL in combination with sequences of 18S nuclear ribosomal DNA, however, they recovered a weakly supported monophyletic Drosera, and Aldrovanda was moderately supported as sister to Dionaea. Heslop-Harrison (1976) referred to the traps of Drosera as being passive and those of the other two genera as active, although the power of movement demonstrated by Darwin (1875) in Drosera leaves indicates that these are also active, if somewhat slower in their response.

Darwin (1875) devoted the great part of his book on carnivorous plants to a series of experiments on Drosera, the first ten chapters (out of 18) dealing almost exclusively with Drosera rotundifolia L. (Fig. 7), the common sundew, the eleventh summarizing the previous ten chapters and the twelfth detailing observations on other species of Drosera. Chapters 13 and 14 cover his observations on Dionaea muscipula and Aldrovanda vesiculosa L., respectively.

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Figure 7. Drosera rotundifolia L. (from Insectivorous Plants, Darwin, 1875: fig. 1).

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Through his work on D. rotundifolia, Darwin demonstrated that the ‘tentacles’ are capable of rapid movement in response to stimuli (beginning after as little as 10 seconds), the glands are absorptive and that stimulating one tentacle caused movement in the surrounding tentacles (Chapter 1). In Chapter 2, he showed that animal substances caused a much quicker and stronger response than inorganic substances or mechanical irritation and that movement only occurred if a tentacle was touched briefly three or more times. In Chapter 3, he described the process of aggregation of cellular contents in the glands after excitation. Chapter 4 deals with the effects of immersion in water at different temperatures and Chapter 5 with the effect of nitrogenous and non-nitrogenous organic fluids (comparing, for example, the effects of decoctions of green peas or cabbage with infusions of raw meat). In Chapter 6, Darwin described the experiments by which he demonstrated that ‘the leaves are capable of true digestion, and that the glands absorb the digested matter’, stating that ‘These are, perhaps, the most interesting of all my observations on Drosera, as no such power was before distinctly known to exist in the vegetable kingdom’. Through these experiments he demonstrated that the enzymatic action was not active in untouched leaves and that ‘the ferment of Drosera is closely analogous to, or identical with, the pepsin of animals’. Observations on the effects of ammonium salts, other salts and acids and ‘alkoid poisons, other substances and vapours’ are presented in Chapters 7–9, and the nature of the sensitiveness of leaves and transmission of the motor impulse are discussed in Chapter 10.

Although the trapping mechanism of all Drosera spp. is basically the same, vegetative morphology of the plants varies greatly between species. Many are rosette forming, but an extreme variation is found in D. gigantea Lindl. in southwestern Australia, which is c. 1 m tall and produces annual growths with many flypaper traps on a highly branched, erect, self-supporting stem.

Dionaea muscipula is one of the best-known carnivorous plants, and it has been demonstrated to show all parts of the carnivorous syndrome. The trap mechanism (with its trigger hairs) and enzyme secretion have been studied in detail (e.g. Darwin, 1875; Heslop-Harrison, 1976; Cameron et al., 2002; Forterre et al., 2005; Płachno et al., 2006). When the trigger hairs are stimulated, a current is generated by calcium ions, the change in acidity makes the midrib cells change shape as a result of osmosis and the trap closes. After several days the prey is reduced to its chitin skeleton, and the trap reopens for reuse.

The popularity of Venus' fly traps in cultivation and the limited area of distribution in the Carolina swamps has resulted in a great threat to the native populations. Currently wild plants are protected, and wild collections are illegal. Nevertheless, the sites are under threat from urban development, and the species is therefore classified as ‘vulnerable’ (IUCN, 2009). Dionaea muscipula has been introduced to swamps in Florida, New Jersey, California and Jamaica, where populations have become well established.

Despite the difference in their habitat, the traps of Aldrovanda bear clear similarity to those of Dionaea (e.g. Juniper et al., 1989), albeit somewhat miniaturized, and function in a similar way. Darwin (1875) described Aldrovanda as ‘a miniature, aquatic Dionaea’, and Cameron et al. (2002) stated that ‘it is not difficult to envision a Dionaea-like, terrestrial ancestor of Aldrovanda vesiculosa becoming adapted to a permanently aquatic lifestyle’.

Darwin's fascination with carnivorous plants carried forward into his studies of the power of movement in plants (Darwin, 1880), with observations on Dionaea, Drosera and Sarracenia. With reference to altered behaviour because of carnivory, he stated that:

Heliotropism prevails so extensively among the higher plants, that there are extremely few, of which some part, either the stem, flower-peduncle, petiole, or leaf, does not bend towards a lateral light. Drosera rotundifolia is one of the few plants the leaves of which exhibit no trace of heliotropism. Nor could we see any in Dionaea, though the plants were not so carefully observed. Sir J. Hooker exposed the pitchers of Sarracenia for some time to a lateral light, but they did not bend towards it. We can understand the reason why these insectivorous plants should not be heliotropic, as they do not live chiefly by decomposing carbonic acid; and it is much more important to them that their leaves should occupy the best position for capturing insects, than that they should be fully exposed to the light.

Drosophyllaceae

Drosophyllaceae include one species, Drosophyllum lusitanicum (L.) Link (Fig. 4F), native to the Iberian Peninsula (Atlantic coast of Portugal, the southernmost tip of Spain and Gibraltar) and northern Africa (northernmost Morocco). Its common names are Portuguese sundew and dewy pine. It is unusual among carnivorous taxa in that it grows in dry basic rather than wet acidic soils, but the sites it inhabits are nonetheless nutrient deficient. No other carnivorous taxa grow in these habitats. It was originally described as a species of Drosera by Linnaeus in Species Plantarum (Linnaeus, 1753), and it does resemble the species of that genus, except for its reversely circinnate leaves (Fig. 4F; compare with Fig. 4E) that do not move when capturing prey. It forms an often stalked rosette of leaves covered with sticky, mucilage-secreting, long glands that trap prey (flypaper traps); plants reach approximately 40 cm in height. Once trapped, insects are dissolved by enzymes released by short glands on the leaves, and the released nutrients are absorbed by the plant. It is thus an active carnivore because it has all the cellular mechanisms to trap prey, digest them and absorb nutrients. Molecular phylogenetic studies have demonstrated that Drosophyllum is more closely related to Dioncophyllaceae (see above), Ancistrocladaceae (not carnivorous) and probably Nepenthaceae than it is to Droseraceae (all Caryophyllales; Fay et al., 1997; Cuénoud et al., 2002). It was known to Darwin, who drew sketches of the two types of glands on its leaves.

Nepenthaceae

Nepenthaceae include only one genus, Nepenthes L., and the most recent major treatment lists 120 species and five ‘incompletely diagnosed taxa’ (McPherson, 2009). The author of this work stated that

I have prepared this two volume work to provide a visually rich overview of the biology, ecology, diversity, distribution and conservation status of Nepenthes and Cephalotus. This work is not intended as a botanical or taxonomic monograph

and referred readers to a number of ‘appropriate printed sources’ for more detailed taxonomic treatments. Despite this disclaimer, this work (reviewed by Fay, 2009) represents the most complete work published in recent years, and the other works cited are regional treatments. The short account below is largely based on that of McPherson (2009).

Nepenthes is native to a large part of the Old World Palaeotropics (but not Africa), with the vast majority of the species occurring on the Sunda Shelf in southeastern Asia. Borneo, Sumatra, Sulawesi and the Philippines are home to notable radiations of species. Madagascar, the Seychelles, Sri Lanka, northern India, Papua New Guinea, northeastern Australia, New Caledonia and some isolated Pacific islands are home to small numbers of species. Nepenthes species generally grow in thin mats of leached organic matter, which can overlay alkaline, neutral or acid substrates.

Although known from Madagascar since the 17th century, Linnaeus (1753) based his description of Nepenthes on material from Sri Lanka, with a single species, N. distillatoria L. Common names are tropical or Asian pitcher plants and, for some species, monkey pots. McPherson (2009) listed a wide range of common names in other languages.

Seedlings of Nepenthes form rosettes and, in the majority of species, plants then form a scrambling or climbing stem. The traps of Nepenthes are pitchers (Fig. 8) formed from tendrils [similar in development to the grappling hooks in Triphyophyllum (Dioncophyllaceae; see above)]. All leaves can bear tendrils or pitchers and, when growing conditions are not ideal, the tendrils fail to develop into pitchers. The pitchers are borne on the end of tendrils of variable length (Fig. 9A) and are hollow vessels with a mouth into which prey can fall. The size of the pitchers varies between species from only a few centimetres in length to > 30 cm, and they can hold three or more litres (for more details about the morphology of Nepenthes, see McPherson, 2009).

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Figure 8. Nepenthes eymae Shigeo Kurata, cultivated at RBG Kew. Photograph taken by Maarten J. M. Christenhusz.

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Figure 9. A, Nepenthes sp., unfolding pitcher at the tip of a climbing tendril, cultivated at RBG Kew. B, Roridula gorgonias Planch. with a trapped bluebottle (Calliphora vomitoria L.), cultivated at RBG Kew. C, Sarracenia flava L., Carolina Beach State Park, North Carolina. D, Byblis liniflora Salisb., showing the glands, cultivated at RBG Kew. E, Byblis gigantea Lindl., habit, near Perth, Western Australia. F, Proboscidea louisianica Thell., Turku Botanical Garden, Finland. Photographs A–D and F taken by Maarten J. M. Christenhusz. Photograph E taken by Michael F. Fay.

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The pitchers are (at least in part) lined with secretory glands that produce the digestive liquid, and the upper part of the interior of the pitcher is generally smooth and waxy. The digestive process is thought to be partly because of the enzymes secreted in the liquid in the pitchers and partly because of micro-organisms in the liquid. In newly opened pitchers, the liquid can be highly acidic (pH 2.5; Heslop-Harrison, 1976). Nepenthes species are thus active carnivores because they trap prey, digest them and absorb nutrients. This active carnivory was already known in the 19th century. In his sole reference to Nepenthes, Darwin (1875) reported that:

Dr Hooker likewise found that, although the fluid within the pitchers of Nepenthes possesses extraordinary power of digestion, yet when removed from the pitchers before they have been excited and placed in a vessel, it has no such power, although it is already acid; and we can account for this fact only on the supposition that the proper ferment [enzyme] is not secreted until some exciting matter is absorbed.

Prey items are predominantly invertebrates, although occasional cases of the remains of amphibians and even small mammals being found in pitchers have been reported. Specialisation on particular types of prey has been reported (Moran et al., 2001), and some species appear to have forgone carnivory altogether. In addition to prey items, pitchers of Nepenthes can provide a home for various micro-organisms, invertebrates and even vertebrates, which may take advantage of the trapping of other animals by the plants (see McPherson, 2009 for extensive review of the fauna of Nepenthes pitchers).

Nyctaginaceae

Darwin (1875) reported that the stems and leaves of Mirabilis longiflora bear viscid hairs, and young plants ‘caught so many minute Diptera, Coleoptera, and larvae, that they were quite dusted with them’. On the basis of experiments with ammonium carbonate and infusions of meat he concluded, however, that the glands on these hairs had no power of absorption. His final words on the subject were:

We may further infer that the innumerable insects caught by this plant are of no more service to it than are those which adhere to the deciduous and sticky scales of the leaf-buds of the horse-chestnut [Aesculus hippocastanum L.].

Claims that the seeds of Pisonia grandis R.Br. trap and kill seabirds for the added nutritional benefit of germinating near a decomposing corpse were investigated by Burger (2005). His study was based on numerous reports of dead birds that apparently died after being entangled in the sticky infructescences of this coastal tree species. Burger found that any possible benefit to the germinating plants was outweighed by damage caused by scavenging crabs that were attracted to the decomposing birds. The killing of birds by P. grandis seeds, however morbid, seems more an adaptation to distribute seeds effectively to various oceanic islands than an actual adaptation for carnivory.

Plumbaginaceae

Like Passiflora foetida, species of leadwort, Plumbago L., have sticky, multi-celled, vascularized, glandular hairs in the inflorescence that capture many insects. Plumbago capensis Thunb. has been suggested to be carnivorous, but no evidence for protease and uptake was shown (Rachmilevitz & Joel, 1976). The glands are only produced in the inflorescence and are probably more of a defence mechanism against herbivory or a means to attach seeds to animals, aiding in their dispersal. If at all carnivorous, Plumbago shows a passive form of proto-carnivory.

Ericales

Ericaceae

Although Darwin (1875) mentioned the possibility that Erica tetralix could be carnivorous because of it having glandular hairs that secrete ‘viscid matter’ and occasionally catch minute insects, he showed that the glands have little or no power of absorption. We are not aware of any further publications providing evidence of carnivory in this species.

Primulaceae

In Primula sinensis Lour., the flower stems, leaves and petioles are clothed in longer and shorter hairs, and the longer hairs have an enlarged terminal cell, forming a gland which secretes a variable amount of thick, slightly viscid, not acid, brownish–yellow matter. Exposure to solutions and vapours of ammonium carbonate led Darwin (1875) to state that ‘in both cases there could hardly be a doubt that the salt had been absorbed chiefly or exclusively by the glands’. Weak infusions of meat, however, had no effect. Thus, although the glands appear to be capable of absorption of some material, there is no evidence for uptake of nutrients or digestion.

Roridulaceae

The single genus in this family Roridula consists of two species of South African subshrubs, R. dentata L. and R. gorgonias Planch. The leaves resemble those of Drosera in being covered in long glandular hairs and excrete a sticky sap that traps insects (Fig. 9B). Despite its morphological resemblance to sundews, they lack the specialized absorptive glands on the leaves that other carnivorous plants have, permitting the uptake of dissolved prey, suggesting that this is not a fully carnivorous species. However, two species of hemipteran, Pameridea roridulae and P. marthothii, are closely associated with Roridula, living their entire lives on these plants (Dolling & Palmer, 1991) without getting trapped. It was initially thought that these bugs intercepted the nutrients within trapped prey before it could be absorbed by the plant. Ellis & Midgley (1996) studied this relationship further and found that the faeces of Pameridea played an important role. This was the first account of a mutualist relationship between a carnivorous plant and an insect, in which Pameridia benefited from the trapped prey and Roridula benefited from the indirect digestion of nutrients. Anderson (2005) investigated the method of absorption in Roridula and found absorption through unusual cuticular gaps, which is most likely the way the nutrients from the Pameridia faeces are absorbed by Roridula.

The population density of Pameridia on plants of Roridula is negatively correlated with the density of the spider Synaema marlothi, a species that feeds on both trapped prey and Pameridia (Anderson & Midgley, 2002). Pameridia also sucks the sap from Roridula, and this herbivory reduces the benefit of the nutrients when the population of Pameridia becomes too large. Synaema marlothii is important in this mutualistic triangle to stabilize the populations of Pameridia and maintain the balance (Anderson & Midgley, 2007).

Sarraceniaceae

Sarraceniaceae include three genera distributed mostly in North America (Sarracenia L., with c. 11 species in eastern and northern North America and the monotypic Darlingtonia Torr. in the far west) with a disjunct distribution in northern South America (Heliamphora Benth., Fig. 10, with more than 15 species, and more being described as additional tabletop mountains, tepuis, are investigated). The plants of all three genera grow in nutrient-poor, acid bogs, and only the species of Sarracenia are full-fledged carnivores, exhibiting wax scales on the upper surfaces that increase the chances that an insect will lose its footing and fall into the pitcher, which produces digestive enzymes and absorbs the released nutrients. Heliamphora tatei Gleason, which is also unusual in the otherwise herbaceous family in being a shrub (up to 4 m tall), has also been found to have wax scales and produce digestive enzymes (Jaffe et al., 1992), so complete carnivores do exist in Sarrraceniaceae outside Sarracenia. Some species of Sarracenia also produce nectar around the rim of the opening (reputed to contain coniine, a narcotic, in S. flava L.; Fig. 9C, and nectar production appears to be more important in attracting prey than does the colour of pitchers (Bennett & Ellison, 2009; Schaefer & Ruxton, 2008)). Darlingtonia, like most species of Heliamphora, is a passive carnivore and depends on bacteria in the pitcher to digest prey, which it then absorbs.

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Figure 10. Heliamphora nutans Benth., cultivated at RBG Kew. Photograph taken by Maarten J. M. Christenhusz.

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In terms of phylogenetic relationships, Heliamphora and Sarracenia are sister taxa, with Darlingtonia sister to this pair (Bayer, Hufford & Soltis, 1996). This makes the biogeographical relationships more difficult to explain, most authors having hypothesized a South American origin for the family (Maguire, 1970; Juniper et al., 1989), and suggests that Darlingtonia has been an isolated taxon for a relatively long time. Some authors had also thought that Heliamphora was the most primitive morphologically, but this does not immediately mean that it should be sister to the other two genera. In any case, most features of Sarraceniaceae do not occur in any of the outgroup taxa, including Roridula, which most analyses have placed as sister to Sarraceniaceae (Albert et al., 1992; Chase et al., 1993) making polarization of morphological characters problematic.

[See Droseraceae, above, for discussion of lack of heliotropism in Sarracenia (Darwin, 1880).]

Gentianales

Gentianaceae

Saccifolium bandeirae Maguire & J.M.Pires, often treated as the sole member of Saccifoliaceae, has been shown to be a member of Gentianaceae by Struwe et al. (1998) and Thiv et al. (1999). This species has strange pouch-like leaves, leading to suggestions that it might be carnivorous, but with no convincing evidence having been presented. Struwe et al. (1998) discussed the possibility of it being a ‘carnivorous gentian’, concluding that its position in the asterids made this a possibility, but they then indicated that further research to demonstrate digestion and absorption would be necessary. The opening to the leaves is, however, downward facing, so it seems highly unlikely that the pouches could be effective trapping structures.

Lamiales

Byblidaceae

Long thought to comprise two species (Fig. 9D, E) endemic to Western Australia (although the first known collection was made in Queensland by Joseph Banks et al. during Captain Cook's first visit to Australia in 1770; McPherson, 2008), Byblidaceae now consist of seven species in the genus Byblis. Of these, two are perennial, occurring in Western Australia near Perth, and five are annuals distributed across tropical northern parts of Australia, with one species also occurring on New Guinea (McPherson, 2008).

The superficial resemblance of Byblis to Drosophyllum led many to treat them as related, particularly as the petals appear to be free. Close inspection, however, reveals that the petals of Byblis are basally fused (e.g. Lloyd, 1942). Phylogenetic studies have indicated placement in Lamiales, although with no strong support for interfamilial relationships within that order (e.g. Müller et al., 2004, 2006).

Darwin (1875), on the basis of a dried specimen of Byblis gigantea Lindl. (Fig. 9E) sent from Kew, noted two forms of glands on the leaves, ‘sessile ones arranged in rows, and others supported on moderately long pedicels’. The latter type were

far more simple in structure than the so-called tentacles of the preceding genera [of Droseraceae in the broad sense, i.e. Aldrovanda, Dionaea, Drosera, Drosophyllum], and they do not differ essentially from those borne by innumerable other plants… . As no instance is known of unicellular structures having any power of movement, Byblis, no doubt, catches insects solely by the aid of its viscid secretion. These probably sink down besmeared with the secretion and rest on the small sessile glands, which, if we may judge by the analogy of Drosophyllum, then pour fourth [sic] their secretion and afterwards absorb the digested matter.

Darwin appears to have been more or less convinced that B. gigantea was carnivorous, although he was unable to carry out his normal range of experiments to verify digestion and absorption because of the lack of living material at his disposal. Elsewhere in the book, he stated that:

There can hardly be a doubt that all the plants belonging to these six genera have the power of dissolving animal matter by the aid of their secretion, which contains an acid, together with a ferment almost identical in nature with pepsin; and that they afterwards absorb the matter thus digested. This is certainly the case with Drosera, Drosophyllum, and Dionaea; almost certainly with Aldrovanda and, from analogy, very probable with Roridula and Byblis.

Bruce (1905) demonstrated that the sessile glands (but not the stalked glands) had the power to digest egg albumen, although Lloyd (1942) was unable to demonstrate digestion of fibrin. Hartmeyer (1997, 1998) was not able to demonstrate enzyme production by sessile glands of B. liniflora, but more recent studies have shown that they produce enzymes in B. filifolia (Hartmeyer & Hartmeyer, 2005) and B. liniflora (Płachno et al., 2006; Fig. 9D).

Like Roridula, Byblis has been reported to have potentially mutualistic relationships with various arthropods and that at least some insects appear to be able to move over the surface of the plant without becoming entangled in the mucilage (Lloyd, 1942; McPherson, 2008). However, the dynamics of these relationships are not well understood.

Lentibulariaceae

Lentibulariaceae consist of three genera: Utricularia (bladderworts; Fig. 2) that is sister to Pinguicula (butterworts; Fig. 11), and Genlisea, which is sister to this pair of genera (Müller et al., 2006). Utricularia and Pinguicula have a more or less cosmopolitan distribution; Genlisea is only found in tropical America, Africa and Madagascar.

image

Figure 11. Pinguicula moranensis Kunth, cultivated at RBG Kew. Photograph taken by Maarten J. M. Christenhusz.

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The morphologically variable genus Utricularia consists of approximately 200 species and occurs nearly worldwide. It is especially diverse in the tropics, where species grow as aquatics, terrestrials, epiphytes and even vines. In many species, there are no clear distinctions between the roots, stems and leaves. Epiphytes (such as the Antillean U. alpina Jacq.) bear succulent tubers that contain water. In the North Temperate aquatic bladderworts, roots are absent, even in seedlings. The stems bear appendages that form bladder-like structures with trapdoors (Fig. 2). When a trapdoor trigger is touched by a small animal, the trap opens abruptly, and the animal is sucked into the bladder; the trapdoor then re-closes, and the bladder secretes enzymes and absorbs the products of digestion. Aquatic species form bladders on photosynthetic branches, and terrestrial species have their bladders on achlorophyllous runners. The traps can range in size from 0.2 mm to 1.2 cm (Taylor, 1989). Aquatic species possess bladders that are usually at the larger limit and can feed on more substantial prey, such as water fleas (Daphnia), nematodes, fish fry, mosquito larvae and even young tadpoles. Many species have chasmogamous and cleistogamous flowers in certain seasons and form buds to survive dry periods or winters. Adamec (2007) demonstrated low (approaching zero) partial pressures of oxygen in traps of Utricularia and Genlisea and hypothesised that the prey may, at least in part, die of anoxia. Sirováet al. (2009) showed that substantial proportions of newly fixed carbon are exuded into the trap fluid, possibly as a source of nutrients for the microbial community living there.

Utricularia humboldtii Schomb. is peculiar in that it is only found in the water tanks of Brocchinia spp. (Bromeliaceae) at high elevations. The Australian U. multifida R.Br. and U. tenella R.Br. were previously placed in the genus Polypompholix Lehm., based on a somewhat different shape of the trapping bladders. Molecular studies have, however, shown that this and another segregate genus, Biovularia Kamieński, are embedded in Utricularia (Jobson et al., 2003).

Pinguicula is named for its leaves that are greasy to the touch; pinguis meaning fat in Latin. The English common name butterwort results either from the juice being used on the udders of cows (either against magic or to treat chapping, etc.; Grigson, 1955) or the leaves being used to curdle milk and form a buttermilk-like fermented dairy product. Today, this is still consumed in the Nordic countries as filmjölk or tätmjölk (Swedish), tjukkmjølk (Norwegian) or viili (Finnish).

Pinguicula spp. are usually small, rosette-forming plants. The spurred flowers are produced singly on stalks from the centre of the leaf rosette and are held high above the leaves, possibly to prevent carnivory of potential pollinators. The leaves are usually bright green or tinged pink or red and are covered in specialized glands. There are two types of glands: stalked glands producing a slimy secretion that traps insects (Fig. 11) and sessile glands that produce enzymes. Insects in search for water are attracted to the droplets produced by the stalked glands. On contact with an insect, the stalked glands produce more mucus, entrapping the insect further while it struggles across the leaf. Some species bend their leaf edges somewhat, bringing additional glands into contact with the trapped insect. A leaf surface can only trap insects once, but new leaves are produced that cover the old ones when these are no longer functional.

There are c. 50–70 species occurring throughout the temperate Northern Hemisphere, extending south to the Himalayas, Atlas Mountains and southern South America. The genus is especially well developed in the Andes and Central America. An overview of the genus was provided by Legendre (2002).

Temperate species are well adapted to cold climates and form winter buds of densely packed non-carnivorous leaves. Roots are poorly developed in most species and wither in temperate species during the winter. Epiphytes like P. lignicola Barnhart form suction cups on their roots to anchor themselves to mossy branches.

Pinguicula produces a bactericide, preventing insects from rotting while being digested. The leaves were known to have a healing power and were used in European traditional medicine to heal sores and clean wounds. It took until the 19th century for this genus to be recognized as carnivorous, by Darwin (1875), who wrote:

We thus see that numerous insects and other objects are caught by the viscid leaves; but we have no right to infer from this fact that the habit is beneficial to the plant, any more than in the before given case of the Mirabilis, or of the horse-chestnut [Aesculus hippocastanum]. But it will presently be seen that dead insects and other nitrogenous bodies excite the glands to increased secretion; and that the secretion then becomes acid and has the power of digesting animal substances, such as albumen, fibrin, &c. Moreover, the dissolved nitrogenous matter is absorbed by the glands, as shown by their limpid contents being aggregated into slowly moving granular masses of protoplasm. The same results follow when insects are naturally captured, and as the plant lives in poor soil and has small roots, there can be no doubt that it profits by its power of digesting and absorbing matter from the prey which it habitually captures in such large numbers.

Conversely, the role of bacteria in the digestion process has been discussed extensively and Lloyd (1942) concluded:

Pinguicula is a carnivorous plant inasmuch as it catches small insects and digests them, at least in part, by means of its own ferments. The possible part played by bacteria is not excluded.

The small genus Genlisea counts c. 20 species in the tropics of the Americas, sub-Saharan Africa and Madagascar. They are small terrestrial or aquatic plants forming a rosette of leaves, and a few single flowers are held above the rosette that is similar in shape and colour to those of Pinguicula or Utricularia. The most peculiar feature of this plant is found underground. There, leaves form a pair of capillary tubes joined at the tip in a V-shape; there are spiral grooves down their lengths allowing soil-borne invertebrates and protozoa to enter. Inwardly pointing hairs prevent the prey from escaping; the only way they can swim is towards the apex of the tube, where they are digested (Płachno, Kozieradzka-Kiszkurno & Świątek, 2007).

It has been suggested that there is a water flow in these capillary tubes, but this does not seem to be the case, so an analogy with the digestive tract of an animal seems misplaced. For example, Heslop-Harrison (1976) wrote:

Genlisea, a genus of the same family [as Utricularia] has a digestive system almost simulating the intestine of an animal … A flow of soil fluid is maintained through the tube by the secretion of water through the walls of the utricle and tube. The diet seems mainly to be of protozoa, but small crustacea and larval forms of other groups are also induced to enter the tube. Undigested remains accumulate in the utricle. To simulate an animal digestive system it would be necessary only to replace the water flow by peristalsis and to provide the utricle with an anus!

Among angiosperms, Genlisea species have the smallest known genomes (Greilhuber et al., 2006). Those of Pinguicula and Utricularia are larger, but still also fall among the smallest angiosperm genomes.

Martyniaceae

Martyniaceae are closely related to Pedaliaceae (sesame, Sesamum indicum L., and relatives) and have been suggested to be carnivorous (Mameli, 1916). Martyniaceae occur in semi-dry to desert habitats from subtropical North America to tropical South America, where the peculiarly shaped fruits are distributed by large mammals, the hooves of which get caught on the hooks of the fruits. Martynia and Proboscidea are herbs with succulent stems, and the plants are covered in glandular hairs that produce a foul-smelling, sticky exudate. Even although the glandular hairs, especially of Proboscidea lutea (Lindl.) Stapf. (= Martynia lutea Lindl. = Ibicella lutea (Lindl.) Van Eselt.), catch many insects (Fig. 9F), no uptake of amino acids has been demonstrated (Rice, 1999; Wallace, McGhee & Biology Class, 1999). Mameli (1916) wrote:

Martynia lutea Lindl. is an insectivorous plant. It catches the insects by means of an abundant viscous substance, having an acid reaction, secreted by a large number of glandular hairs with which its above-ground parts are covered and by which it dissolves albuminous substances by a proteolytic enzyme, which has the character of trypsin.

Orobanchaceae

Various authors in the 19th century (see Groom, 1897) suggested that the glands and capitate hairs found in the lacunae in the scale leaves of some members of Orobanchaceae, including Lathraea, might be involved in nutrient absorption and entrapment and digestion of small organisms. Groom (1897) discounted this, demonstrating that these structures were instead involved in water excretion. Despite this, the suggestion that these plants are potentially carnivorous has persisted in the literature (e.g. Heslop-Harrison, 1976; Fitter, 1987; Jolivet, 1998). However, carnivory in Lathraea and its relatives appears to be mere speculation (see Fay, 2009, for further details).

Solanales

Solanaceae

Various members of the family have hairs that catch small insects. Darwin (1875) reported that Nicotiana tabacum is covered with innumerable hairs of unequal lengths, which catch many minute insects, but, in his experiments with infusions of meat and ammonium carbonate, he produced no convincing evidence of absorption. Zambelli (1929) stated that ‘Petunia violacea and Petunia nyctaginiflora are insectivorous plants’ on the basis of experiments demonstrating the presence of proteolytic enzymes, but was also unable to produce convincing evidence of absorption. Spomer (1999) demonstrated that leaves of Solanum tuberosum L. secrete proteinases, but Petunia hybrida Vilm. does not. Simons (1981) suggested that crop plants and their relatives with sticky hairs (notably wild tomatoes and potatoes; species of Solanum L.) are suitable study plants for investigation of ‘degrees of carnivory’.

Asterales

Stylidiaceae

The Australian trigger plants of the genus Stylidium Lour. are known to trap small insects such as gnats and midges using mucilage-secreting glandular hairs on their inflorescences and stems. Because trigger plants are remarkable for their active pollination mechanism, they had not been examined for carnivory. However, Stylidium uses the same mechanism to trap their pray as Drosera or Byblis, both genera that occur in similar habitats and often in the same places where Stylidium is found. Like Drosera, the glands produce proteases and can thus digest the insects caught (Darnowski et al., 2006). It has been shown that amino acids are absorbed by the surface of Stylidium, so some species of trigger plants appear to be fully carnivorous (Darnowski, Moberly & Płachno, 2007). They co-occur with species of Drosera in many places, so their habitat preferences fit the syndrome associated with carnivorous plants.

Dipsacales

Caprifoliaceae

The basal leaves of the common teasel, Dipsacus fullonum L., are united around the stem, forming a cup-shaped structure that fills with water after rain. All sorts of debris and insects get trapped in it, but no evidence of digestive enzymes or foliar nutrient absorption has been revealed. Christy (1923) did note that the fluid collected in the basin formed by the leaves has a lower surface tension which could be an adaptation to kill prey.

LOSS OF CARNIVORY IN CARNIVOROUS GENERA

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES

In some predominantly carnivorous genera such as Nepenthes and Utricularia a few detritivorous species are known. Some Malaysian species, such as Nepenthes ampullaria Jack and N. lowii Hook.f., and N. pervillei Blume from the Seychelles, catch few insects. Nepenthes ampullaria derives nutrition from fallen leaf litter, whereas the last two appear to obtain nutrients from the droppings of birds and tree shrews that feed on the nectaries in the pitchers and snail eggs laid on the rim of the pitchers (Corner, 1978; Clark et al., 2009).

Utricularia purpurea Walter may have partially lost its appetite for meat. Trapping rates of prey were significantly lower than in other species of Utricularia. The eastern purple bladderwort can still trap and digest prey in its traps, but does so sparingly. Instead algae, zooplankton and debris are present in the bladders, suggesting that U. purpurea favours mutualism instead of carnivory (Richards, 2001).

CONCLUDING REMARKS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES

We think that it is evident from the descriptions above that many plant species have the capacity to trap and kill insects and other animals and that some have refined the carnivorous syndrome to a high degree. Intermediates (the so-called ‘proto-carnivorous’ species) clearly do exist, and it is tempting to consider many of these to be good carnivores. Hartmeyer (1998), referring to species that do not produce their own digestive enzymes, stated:

The production of enzymes should not be a prerequisite for a plant to be considered carnivorous – a symbiosis with another digesting agent should be sufficient. In the past, symbioses were mistakenly considered strange exceptions, but now it is apparently a widespread syndrome with carnivorous plants. Indeed, with some plants it is an integrated part of the digestive system!

Other species exhibit even less specialized cases, but they too should not be discounted. The cost–benefit model for the evolution of carnivory indicates that there is a trade-off between the costs of carnivory and benefits to photosynthetic output (and ultimately to enhanced seed production), made possible by the additional nutrients acquired (Givnish et al., 1984; Benzing, 1987). The model also implicitly implies that, in some situations, carnivorous plants should have advantages over non-carnivorous plants in the same habitats. In his review of published data, Ellison (2006) showed that nitrogen, phosphorus and potassium often limit the growth of carnivorous plants, and their use of these elements is 20–50% that of non-carnivorous plants. All data indicated as well that under no circumstances do carnivores have a clear-cut advantage over non-carnivorous plants, and thus it appears that carnivory is in fact a choice of the lesser of two evils in difficult circumstances. Carnivores have poor competitive abilities because their lineages became adapted to living under conditions that were far from optimal in terms of nutrient availability, and carnivory represents a slight improvement over what would otherwise be the case. The habitats in which carnivores typically occur are those in which harsh abiotic conditions limit competition between species, and it is under such abiotic stress that carnivory represents an improved although certainly not optimal strategy. If the cost–benefit model cannot be used to explain why vegetable carnivory has been adopted by species with the full-blown syndrome, then it is difficult to see how it can be applied in situations where the species are these intermediate types described above. If mucilage-secreting hairs evolved in response to herbivory or to attach seeds to animals to aid in their dispersal, their co-option into providing increased nutrition necessitates minimal additional expense; it involves absorption of nutrients through roots, which happens as a matter of course. This minimal level of carnivory must surely be beneficial and confer some slight advantage. It is easy to imagine that this limited response is further constrained by the phylogenetic legacy inherited from the conditions under which their ancestors evolved and the limitations imposed by environmental stress in their current habitats. When confronted with two miserable options, the less miserable wins the day. Full-blown vegetable carnivory obviously threads a twisting trajectory through the maze of cost and benefit ‘peaks and valleys’ under highly stressful environmental conditions, and these are notoriously difficult to model. The ‘intermediate’ cases described above are no less difficult to understand, particularly when the basic requirements for being carnivorous, mucilage-secreting hairs, are ubiquitous, at least in the eudicots to which the majority of carnivores and so-called ‘proto-carnivorous’ (i.e. intermediate) taxa belong. If carnivory is far more common than previously held because of many species being subtly carnivorous, then the background comparisons of ‘carnivore vs. non-carnivore’ are also inappropriate because the latter category includes perhaps many species that are subtly carnivorous through symbioses with other organisms. The evolution of vegetable carnivores requires a great deal more study, both in terms of better documentation of the putatively intermediate cases as well as understanding of its evolutionary context, which may be constrained as much by phylogenetic history as by physiological responses. We may be surrounded by many more murderous plants than we think.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. OVERVIEW OF MURDEROUS PLANTS
  5. MONOCOTS
  6. EUDICOTS
  7. LOSS OF CARNIVORY IN CARNIVOROUS GENERA
  8. CONCLUDING REMARKS
  9. REFERENCES
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Footnotes
  • As in the movie, Finding Nemo, about a clownfish that lives in the tentacles of sea anemones without suffering from their stings.