February 9, 2005
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 --
stress -- of any sort forces special adaptation to occur for plant dwellers
in order to survive in those conditions.
During the profuse evolutionary history of modern flowering plants,
various kinds of environmental
stress have given rise to a staggering array of properties found in the richness of the plant kingdom of our planet today.
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
eccentric in the normalcy of plant life as we know it. It is in such mineral-poor
environments found in some regions of the globe
that the plants that have 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.
These plants demand not only the insect's service of pollinating
flowers, as many flowering plants do, but its flesh as well, as a
supplementary nutrition source to meet the survival demand of their
A relatively small group of these flowering plants have come to be known as
"insectivorous" or "carnivorous" plants. The menu of meals for these vegetable carnivores
includes a wide variety of insects as well as other small animals sharing the same
There are some 600 different species of carnivorous plants
recognized today, representing twelve (12) taxonomic families of floral
classification of angiosperms, or flowering plants. (Classification)
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
Kinds of Traps
Although the methods they devised in their attempt to capture the prey vary
among different species, the traps of all carnivorous plants are considered by
many to be modified leaves. The types of traps are summarized in
the next four categories:
PITFALLS / PITCHER 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 secretions 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. Nectar is 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
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. secretions 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 nectar 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 worldwide 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.
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 worldwide distribution.
In a related genus Genlisea, of family
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
SNAP TRAPS / STEEL TRAPS
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 savanna
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
secretions 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 savanna of North and South Carolina in the
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.
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.
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?
"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).
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
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.
berteroniana, unlike almost every other bromeliad, is known to raise its
flower stalk up to 90 cm above the imbricate leaves (Juniper, et al).
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.
As F. E. Lloyd notes in his book The Carnivorous Plants
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.
of fossil records of carnivorous plants -- only limited to fossil pollen 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
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/
"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 advances 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 Lloyd (1942) due to the structure of its
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
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
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.