Preface

  Contents

  Introduction

  1. Pitcher Plants
  2. Cobra Plant
  3. Sundews
  4. Venus Flytrap
  5. Butterworts
  6. Bladderworts

  References

  Home

                                            Introduction
Last Update: February 9, 2005

Prologue

The growth of plants is limited only by space, sunlight, water, carbon dioxide, and some inorganic nutrients. A deficiency in any of these basic requirements imposes a harsh environment for the plants. In an arid land, available water becomes the limiting factor.  In a dense forest, competition for sunlight is a life-or-death issue. The deficiency -- or environmental stress -- of any sort forces special adaptation to occur for plant dwellers in order to survive in those conditions.

In the wilderness of marshes and swamps exists a variety of life forms that have adapted to their peculiar environment through their never-ending struggle for survival. The exuberance of water in the wetland invites water-loving species from both animal and plant kingdoms. For the plant dwellers, this habitat is also typified by acid soil with a low content of mineral substances so vital for all green plants to sustain their existence. A lack of sufficient mineral in soil poses yet another difficult environment for the plant occupants within.  

This particular type of environmental stress has given rise to a habit quite eccentric in the normalcy of plant life as we know it. It is in such mineral-poor environment found in some regions of the globe that the plants that adopted carnivory can be found. These plants preferentially occupy the mineral-poor soil and thrive with a competitive edge over non-carnivorous plants with a more conventional lifestyle.

The geographical distribution of carnivorous plants encompasses the entire globe. Some species grow very widely throughout many continents, while others are confined to small restricted regions of the world. (World Map)

Kinds of Traps

This is a passive and the most primitive type of trap structure commonly referred to as a "pitfall". There is no movement in this trap. The leaves grow to form a pitcher in most species, though in some primitive ones multiple leaves together form a water-retaining pit at the rosette base. The pitcher retains some liquid at the bottom. Basically the prey falls into the slippery pitcher and dies. In some species, the active enzyme secretion is seen, but in many the digestion is heavily aided by externally introduced bacteria. Typically the pitchers are colorfully decorated and marked with ultra-violet patterns in some. Nectars are often offered as an attractant along with fragrance in some species. In terms of cost-effectiveness, the pitfall trap -- once constructed -- requires a minimal "operational" cost, since the trap does not involve any physical movement.

This trap type is found in five families, a total of eight genera of carnivorous plants: tropical pitcher plants in the Old World (Nepenthes in family Nepenthaceae), three genera of the New World pitcher plants (Sarracenia, Darlingtonia, and Heliamphora in family Sarraceniaceae), the western Australian pitcher plant (Cephalotus in family Cephalotaceae), and primitive monocotyledons (Brocchinia and Catopsis in family Bromeliaceae and Paepalanthus in family Eriocaulaceae).

Some carnivorous plants cover their leaves with finely distributed hairs tipped with a glue-like mucilage. This is called an "adhesive" or "flypaper" trap. In the sun, a sticky drop of mucilage glistens like a dewdrop. Insects are known to be attracted to shiny blobs. Many species in this group have also developed sensitivity to physical as well as chemical stimuli. Secretion of digestive juices are seen in many species. Tentacle bending as well as leaf folding is also seen in some species. This trap does not offer any "rewards" like the nectars in the pitcher trap,

The adhesive trap is found in seven families, a total of eight genera. The largest in number, by far, is sundews (Drosera of family Droseraceae) with about 150 species, followed by butterworts (Pinguicula of family Lentribulariaceae), containing over 70 species. Both sundews and butterworts have world-wide distribution.

The remaining genera of this group are Drosophyllum of family Drosophyllaceae, Byblis of family Byblidaceae, an African liana Triphyophyllum of family Dioncophyllaceae , Roridula of family Roridulaceae, and Ibicella of family Martyniaceae.

SUCTION TRAPS

Some aquatic and semi-aquatic species have developed a structure that can be termed a "suction" trap. Some 230 species belonging to Lentibulariaceae family use this type of trap in order to capture tiny water animals. Of these, the bladderworts (Utricularia, Biovularia, Polypompholyx), representing well over 200 species, develop numerous tiny sacs in the water and in the wet soil. Each sac, or bladder, is tightly sealed by the door that withstands the negative pressure inside built up by the constant pumping of water out of the bladder. When a prey, such as a mosquito larva or a water spider, breaks the delicate balance of the door latch by touching one of the levers attached to the door, the door seal is broken and the elasticity of the bladder causes it to pop to the normal, un-pressurized state. A resultant sudden inflow of water causes the water animal to be sucked into the bladder. The door shuts instantaneously behind the prey. All of these happen in an astonishing 1/30 of a second. The size of the bladder varies depending on the species, with 5mm being the largest end.

The trap is extremely efficient, as evidenced by many bladders being filled with multiple catches in an animal-rich environment. The sophistication and mechanical subtlety of this trap is without parallel in the plant kingdom. In spite of its apparent sensitivity, however, the trapping action itself is purely mechanical, and can be repeated many times. The bladderworts have world-wide distribution. 

In a related genus Genlisea, of family Lentibulariaceae, there are 20 or so semi-aquatic species occurring in South Africa and South America. These use a mild water flow in a spiral trap to capture aquatic prey.

Finally, we have come to the world-famous Venus' flytrap (Dionaea muscipula, of family Droseraceae) which employs a "steel trap" or "snap trap". In the marshy savannah of North Carolina, a Venus' flytrap beckons visiting meals with its red-tinted, nectar-covered lobes wide open -- that shut snap when a potential victim applies enough stimuli on the trigger hairs located on the inner surface of the trap lobes. The Venus' flytrap shows active secretion of digestive juices -- without relying at all on the external organisms for digestion. This trap has also developed a highly specialized sensory organ for trap closure/triggering. Along with the swift movement of trap leaves, the steel trap may very well have achieved the most advanced adaptation -- the triumph of plant carnivory. The distribution of the Venus' flytrap is highly limited to the coastal savannah of North and South Carolina in the United States.

This trap type is shared by only one other species in the Old World, commonly known as "water wheel plant" (Aldrovanda of family Droseraceae) -- an aquatic cousin of the Venus' flytrap. The water wheel plant occurs in Europe, Africa, Asia and Australia. Interestingly, the distribution of the water-wheel plant does not extend over into the New World, where the Venus' flytrap occurs.

Definition

Although the exact definition of carnivorous plants may differ depending on botanists, to be included in this exclusive society of plant carnivores, a plant must minimally exhibit the ability to capture the prey, digest it, and absorb its nutrients. Of these, the digestion is a process that might be helped by external microorganisms, like bacteria. In addition, many carnivorous plants provide some kind of attraction to allure the prey. Last but not least, the plant must derive survival benefit from carnivory. The general definition of the carnivorous plants is thus summarized as:

Carnivory

Attraction - 
For the purpose of attraction, traps use practically every element exploited by insect-pollinated flowers: color, ultra-violet patterns, fragrance, and nectars.

Movement ------ usually slow, Mimosa, 
Mucilage secretion
Sensory mechanism…
Digestion ---- secretion of enzymes

Is Carnivory Essential

It appears that there is no disagreement today that some plants are indeed carnivorous. Now the question becomes whether the carnivorous habit is beneficial to the plants, let alone essential for survival. In a typical habitat, we see many non-carnivorous plants growing side-by-side with carnivorous plants in the poor-soil environment. Obviously, some plants took different strategies (if not as drastic as eating insects) to enable them to survive in the mineral-poor environment. So, how essential is carnivory for carnivorous plants themselves? It is a matter of general observation that carnivorous plants show more vigorous and enhanced growth when fed with a rich insect diet, presumably producing more seeds. On the other hand, experience also shows, as confirmed by more controlled studies, that many or most of the carnivorous plants grow normally without being fed with any animal prey. Does this imply that carnivory is not essential for the survival of the species?

As we look at plants as part of the natural ecosystem, experiments demonstrating that a certain carnivorous plant can grow to maturity in a controlled insect-free environment tell us nothing about the intrinsic value of carnivory. The plant in a laboratory may receive enough light, may be protected from animal predators, may not face any competition from other plants. In nature, the growth rate, the plant size, and otherwise the general health of the plant, and certainly the amount of seed crop, all contribute to the increased probability for successful colonization of the species in the wild.

Cost/Benefit Consideration

If we accept that carnivory is beneficial to the plants and does increase their chance of survival, then we must beg the ultimate question: Why didn't many more members of the modern flowering plants adopt carnivory in the course of evolution, as an additional mode of nutrient acquisition, in this animal-rich planet? Of over 220,000 species of angiosperms extant on Earth today, there are only 590 or so known carnivorous plants. This is miniscule one quarter of one percent of the total flowering plants.

We also make note of the fact that a majority of carnivorous plants recognized today are tiny herbaceous plants. Some larger ones, like a recently recognized tropical liana from West Africa, Triphyophyllum peltatum (recognized as carnivorous in 1979) which can grow to a height of 30 feet, seems to abandon the carnivorous habit as they reach a certain size. This plant is also known to produce adhesive carnivorous leaves only before the rainy season when the insect life is presumably abundant.

Is plant carnivory such a rotten deal in light of the cost/benefit equation? "Could it be", Juniper et al. pose, "that carnivory exacts so high a price in terms of the diversion of biomass into the trap component, that only the richest of insect-containing sites, and then only at certain times of the year can effectively be exploited?"

I looked up an eucalyptus tree in my backyard that I planted several years ago. It was barely five feet then. It grew to well over 25 feet in height, with the trunk diameter reaching 10 inches. The eucalyptus is a leafy tree. In my mind's eye, I replaced all the foliage with sundew leaves covered with glistening tentacles. In an ideal condition, this tree would capture hundreds of mosquitoes for us every evening! Considering the total biomass of the tree, with the thick woody trunk, however, the amount of insects it can capture, in terms of a biomass ratio, is extremely small compared with, say, a tiny sundew plant that managed to have captured one mosquito. To put it in other words, a bigger stomach demands more substantial food. To make a meaningful nutritional contribution, this carnivorous tree would have to shoot for something larger than mosquitoes, say, a wild raccoon or two during the growing season. But to construct such a trapping device out of the leaf structure would be a prohibitively costly proposition.

"Despite the apparent advantage and the abundance of insect life, there are very few carnivorous plants. There are practically none below the angiosperms. There are no herbs, no shrubs or trees and there are large areas of the phylogenetic table where carnivory does not seem to have evolved at all." note Juniper et al. (1989).

Evolution did not bring about the plant carnivory beyond the point of what we have. That is to say, we do not have a situation where half the trees in our backyard are man-eating!

Price of Carnivory / Cost Analysis

COST ANALYSIS

Carnivory incurs costs, in terms of A) trap construction -- one-time expenditure due to diversion of biomass for trap structure, possibly at the expense of photosynthetic efficiency, and B) trap operation -- recurring cost during the life of the trap, such as the production of attractants, energy needed for trap-triggering, prey digestion, and so on. The amount of expenditure associated with carnivory differs depending on species, largely due to the type of trap used.

Looking at the four trap types we mentioned, some primitive pitcher traps do not seem to incur too much construction cost. Some pitcher plants may be sacrificing photosynthesis efficiency due to shape and color of the pitcher. And for all pitfalls in general, there is a minimal operational cost during the capture, mainly because of the lack of trap motion. Many pitcher plants do offer attractants/rewards in the form of nectar secretions during the operational life of the trap. Some pitchers secret digestive enzyme.

Pollinator/Prey Dilemma

It is generally considered, although with incomplete evidence, that all carnivorous plants are insect pollinated (Juniper, et al.1989). If true, this leads to an apparent paradox: The plants need to consume prey for additional nutrients, yet, at the same time, the plants need insects for successful pollination of their flowers.

HABITAT SEPARATION

In some aquatic/semi-aquatic species, this problem is averted by clear separation of prey-trapping and pollination spheres. In Utricularia, Genlisea, and Aldrovanda, the trap device that captures small aquatic animals lies in the water or in damp soil whereas their flowers protrude high in the air on a tall peduncle (stalk) that presumably get pollinated by flying insects hovering over the water.

For totally terrestrial carnivorous plants, prey and pollinators share the same habitat. In fact, some authors noted a remarkable resemblance between Darlingtonia flowers and its traps in terms of the arrangement and coloring of these organs. Presumably, both are designed to allure the same class of visitors? (Though, to this day, the pollination mechanism of Darlingtonia is not well understood.)

How do these terrestrial carnivorous plants manage to resolve this dilemma of prey/pollinator separation?

DUSTY SEEDS

"Both Drosera and Dionaea produce relatively small flowers. These flowers, which are white, violet or red, are pollinated by small insects that might also serve as prey" (Juniper, et al). This results in a seemingly paradoxical competition between the plants' flower and trap for the same insects (Juniper, et al). However, the plant's strategy to "produce numerous, dusty seeds per flower permits some seed dispersal, even when pollinator visits are rare" (Juniper, et al).

INSECT SEGREGATION

Drosophyllum, which, until recently, was considered a member of the family Droseraceae that contains Drosera and Dionaea, "produces larger flowers of a conspicuous yellow color which contrast strongly with the trap leaves"  (Juniper, et al). This prevents competition between its trap and flowers: The flower is pollinated by a far larger insect than the trap is capable of capturing. “This seeming lack of competition is consistent with the fact that each Drosophyllum flower forms a capsule containing small and languish seeds" (Juniper et al).

Also, in Byblis, Roridula and Ibicella, flowers are large and are clearly targeted for pollinators of the larger size relative to the typical size of the prey the glandular leaves are capable of trapping. For these plants the main prey is a small winged insect, such as a gnat, …… The prey/pollinator segregation based on the insect size is clearly at work in these species. This strategy may apply to some Pinguicula where intended pollinators seem to be larger than the normal size of the catch. Some African and Australian sundews also produce a relatively large flower seemingly intended for larger insects than the typical prey size commonly observed trapped.

SPATIAL SEPARATION

Some carnivorous plants use the "spatial" separation of trapping and pollinating zones. Western Australian pitcher plants, Cephalotus, produce an unusually tall flower stem to bear their white flowers. The majority of the prey for Cephalotus pitchers (typically 2-3 cm in size) that lie on the ground are crawling insects, notably ants. The flowers borne on a tall scape, that sometimes reaches 60 cm in height in the wild, are well isolated from the grassy trapping zone on the ground below. This tendency of the spatial separation by a tall flower stalk is seen in the Venus' flytrap as well as in many rosette sundews, if in lesser degree in some species. Some Pinguicula species, with their tall flower scapes, may be benefiting from the same strategy.

Catopsis berteroniana, unlike almost every other bromeliad, is known to raise its flower stalk up to 90 cm above the imbricate leaves (Juniper, et al).

TEMPORAL SEPARATION

One other way in which some group of carnivorous plants resolve this dilemma is the "temporal" separation. That is, pollination and trapping occur in sequence, separated in time. In many eastern North American pitcher plants, Sarracenia, though with some exceptions, the inflorescence occurs a month or so before the production of new pitcher leaves of the season. This means, during the anthesis, there are few functional pitchers to trap visitors to flower. Californian pitcher plants, Darlingtonia, follow suite. During the height of flowering that occurs in May-June in northern California and southwestern Oregon, there are no newly emerged pitcher leaves of the spring. Also, in Darlingtonia, given a typical pitcher height of 40-60 cm, a flower scape reaches 80 cm or more in nature, providing a spatial separation (albeit uncomfortably small) between prey-trapping and pollination, in case any functional pitchers remain from the previous season. Some butterworts that form winter hibernacula (winter buds) may be using the temporal separation in that at the time of flowering where the peducle emerges from the center of a tightly formed winter hibernaculum, there may not be any active glandular leaf formations.

It is noted that the Venus' flytrap temporarily suspends the production of new trap leaves during flowering. This may be simply due to the plant's energy being diverted to flower production. In cultivation, growers often cut off flower stems in order to avoid this energy loss, if a seed crop is not intended. Aldrovanda, referred to as a water wheel plant, and a sole, water-based relative of the Venus' flytrap, exhibits a similar tendency. The plant produces an 8-sided wheel-like structure at each node along a long stem as it grows floating near the water surface. A steel trap is formed at each wheel tip. During the flower season, the node that holds a flower stem does produce a wheel but, quite often, without the trap at any tip of the wheel. Since Aldrovanda has the habitat separation of water and air, the lack of trap during flowering must be due to energy diversion.

Evolution

As F. E. Lloyd  notes in his book The Carnivorous Plants (1942), the existence of carnivorous plants in both the choripetalae (with separate petal flowers) and the sympetalae (with united petal flowers) can be interpreted to indicate that the carnivorous habit arose among the higher plants at least twice -- and possibly more -- at distinct points in the phylogenetic tree.

A handful of fossil records of carnivorous plants -- only limited to fossil pollens and seeds --  tell us that many carnivorous families and genera were already well established from the beginning of the Tertiary period. Based on the generally accepted taxonomic relationship among various known carnivorous plants, it seems the habit of carnivory has arisen several times in the period between the Upper Cretaceous and the present in the phylogenetic tree of the angiosperms. (Juniper et al. 1989)

While the general notion of the evolution of carnivorous plants is summarized by these views, Croizat (Principia Botanica, 1961) offers a rather differing interpretation. In his analysis of the general plant morphology, Croizat -- to the surprise and unease of many --- focused his attention on (none other than) carnivorous plants, and made a meticulous and thorough analysis of the dispersal of carnivorous plants and their relatives.

"Step by step, Croizat establishes that Droseraceae, Nepenthaceae, Sarraceniaceae, and Lentribulariaceae and their relatives have, in fact, evolved stressing the same ancient, cardinal biogeographic nodes as angiosperms development itself.....(www.ento.psu.edu/home/ frost/research/ biogeography/ panbiogeography/ tuatara_articles/ PrincipiaBotanicaCarnivorousPlants)  

"The dispersal of carnivorous plants indicates that these taxa are not derived independently from other extant angiosperm families, as is commonly thought, but are instead the result of differentiation of a wide-spread ancestor, an ancestor as old as angiospermy itself.......  (www.ento.psu.edu/home/ frost/research/ biogeography/ panbiogeography/ tuatara_articles/ PrincipiaBotanicaCarnivorousPlants)

This fascinating subject of the evolutionary origin of plant carnivory lies heavily buried in the veil of geological times eons past.

A widely accepted criterion for plant classification for angiosperms is principally based on the reproductive organs of the plants, or flowers.  Traditional phylogeny for carnivorous plants primarily focuses on morphological analyses of floral as well as vegetative structures of the plants including their trap devices. As expected, it is difficult to come up with one single consensus among botanists. This, historically, has resulted in multiple phylogenetic trees depending on a school of thought one subscribes to. 

Recent advancement in DNA analyses have brought new tools in the field of phylogenetic analysis, largely eliminating subjective opinions of the individual researchers.

Traditionally, genera Drosera, Dionaea, Aldrovanda and Drosophyllum have been placed under the family Droseraceae.  The pollen morphology further indicated that Drosophyllum appeared to have separated from the rest early in the family.  A DNA sequences analysis has revealed, however, that Drosophyllum belongs to a different group than the other three genera of the family.  In fact, Drosophyllum shares the same linkage with Dioncophyllaceae (which contains genus Triphyophyllum). This strongly favors the placement of genus Drosophyllum in a separate family, Drosophyllaceae. (Rivadavia, et al. 2002) 

A generally accepted close relationship between Dionaea and Drosera has been supported by multiple phylogenetic analyses of DNA sequences with high statistical confidence. The phylogenetic analysis of chloroplast DNA sequences by Rivadavia, et al. 2002, did not offer any clue as to what trap type the common ancestor of these adhesive and snap trap structures might have possessed, or whether these two trap systems evolved independently from non-carnivorous plants.  

The analysis of Aldrovanda DNA sequences has revealed that Dionaea and Aldrovanda form a sister group in spite of their diverse ecological differences. This likely suggests a single evolutionary origin of their snap trap mechanisms, one terrestrial and the other aquatic. (Rivadavia, et al. 2002)

It has been revealed that Pinguicula and Utricularia form a sister group, confirming a traditionally recognized relationship between the two genera.   (Rivadavia, et al. 2002) 

It is speculated that some carnivorous plants we see today may have initially developed their trap mechanism as defense against insects. There are many non-carnivorous plants that produce a sticky substance much like sundews and their close allies with the adhesive trap. Ibicella (having not mentioned by Lloyd) appears to be a rather primitive member of the carnivorous plants exhibiting the adhesive type in light of this evolutionary transition.

There is some disagreement about whether certain plants are carnivorous. Rolidula, considered carnivorous by Darwin (1875), was not recognized as such by Llyod (1942) due to the structure of its tentacles. Brocchinia and Paepalanthus, both forming a primitive pitfall trap at the base of their rosette, are newly recognized carnivorous plants just recently, in 1984 and 1994, respectively. It is conceivable that more plants will be added to the growing list of carnivorous plants in years to come.

Some plants are speculated to be in the process of evolving into or out of carnivorous habit (Juniper et al.,1989).

Tropical pitcher plants (Nepenthes) generally produce two types of pitchers: lower and upper ones.  The lower pitchers are often more colorful and bulbous in shape, having wider wings, while the upper ones tend to be slender and plain in color. This variability seen in pitcher polymorphism extends to the wide variation of pitchers within an individual species. The diversity exhibited within a species is interpreted to be a sign of active, on-going evolution, leading to speciation.

Recently recognized as carnivorous (1979), a tropical liana from Ivory Coast of West Africa is known to exhibit carnivorous habit only on a part-time basis. In nature, a young plant of Triphyophyllum peltatum (family Dioncophyllaceae) produces glandular carnivorous leaves (similar to those of Drosophyllum) just before the rainy season. This seasonal carnivorous habit is said to be observed only during the juvenile stage before the plant enters the second phase of its life cycle. In this adult stage, the plant climbs rapidly into the canopy of tropical rain forest, attaining the height of 10 meters or more.  A characteristic hooked leaves are produced in this stage.  A recent report shows (Bringmann et al., 2002) that juvenile plants of Triphyophyllum peltatum grown in the greenhouse entered the adult phase -- without ever producing carnivorous leaves -- and flowered, successfully producing seeds.  Apparently, some, unknown, environmental signal triggers the plant in nature to produce carnivorous leaves. In the absence of this signal to go carnivore, the genetic information to construct a carnivorous device lays dormant in the sequence of DNA. 

Presumably, the greenhouse environment -- soil or otherwise -- was not severe enough to necessitate the production of the carnivorous organ for nutritional supplement. Bringmann et al. (2002) speculate that other Dioncophyllaceae species, which have never been observed to be carnivorous, might develop insect-trapping organs under some yet-unknown, specific, nutrient-deficient conditions.     

Could it be that many other plants on Earth share this genetic blueprint of how to become carnivorous, and, upon receiving a proper signal, are capable of transforming themselves to be carnivorous?

We are yet to fully understand the secret of DNA and its hidden protocols the billions of years of evolution has so meticulously created. For now, suffice it to say that plant carnivory is one of nature's unfinished attempts -- or pastime -- to push plants’ evolutionary adaptability to the limit.