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Pitcher plants are carnivorous plants whose prey-trapping mechanism
features a deep cavity filled with liquid known as a pitfall trap.
It has been widely assumed that the various sorts of pitfall trap
evolved from rolled leaves, with selection pressure favouring more
deeply cupped leaves over evolutionary time. However, some pitcher
plant genera (such as Nepenthes) are placed within clades consisting
mostly of flypaper traps: this indicates that this view may be too
simplistic, and some pitchers may have evolved from the common
ancestors of today's flypaper traps by loss of mucilage.
Whatever their evolutionary origins, foraging, flying or crawling
insects such as flies are attracted to the cavity formed by the
cupped leaf, often by visual lures such as anthocyanin pigments, and
nectar bribes. The sides of the pitcher are slippery and may be
grooved in such a way so as to ensure that the insects cannot climb
out. The small bodies of liquid contained within the pitcher traps
are called phytotelmata. They drown the insect, and the body of it
is gradually dissolved. This may occur by bacterial action (the
bacteria being washed into the pitcher by rainfall) or by enzymes
secreted by the plant itself. Furthermore, some pitcher plants
contain mutualistic insect larvae, which feed on trapped prey, and
whose excreta the plant absorbs.[1] Whatever the mechanism of
digestion, the prey items are converted into a solution of amino
acids, peptides, phosphates, ammonium and urea, from which the plant
obtains its mineral nutrition (particularly nitrogen and
phosphorus). Like all carnivorous plants, they grow in locations
where the soil is too poor in minerals and/or too acidic for most
plants to survive. |
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The families Nepenthaceae and Sarraceniaceae are the best-known and
largest groups of pitcher plants.
The Nepenthaceae contains a single genus, Nepenthes, containing
about 120 species and numerous hybrids and cultivars. In these Old
World pitcher plants, the pitchers are borne at the end of tendrils
that extend from the midrib of an otherwise unexceptional leaf. The
plants themselves are often climbers, accessing the canopy of their
habitats using the aforementioned tendrils, although others are
found on the ground in forest clearings, or as epiphytes on trees.
In contrast, the New World pitcher plants (Sarraceniaceae), which
comprise three genera, are ground-dwelling herbs whose pitchers
arise from a horizontal rhizome. In this family, the entire leaf
forms the pitcher, whereas in the Nepenthaceae, the pitcher arises
from the terminal portion of the leaf. The species of Heliamphora,
which are popularly known as marsh pitchers (or erroneously as sun
pitchers), have a simple rolled-leaf pitcher, at the tip of which is
a spoon-like structure that secretes nectar. They are restricted to
areas of high rainfall in South America. The North American genus
Sarracenia are the trumpet pitchers, which have a more complex trap
than Heliamphora, with an operculum, which prevents excess
accumulation of rainwater in most of the species. The single species
in the Californian genus Darlingtonia is popularly known as the
cobra plant, due to its possession of an inflated "lid" with elegant
false-exits, and a forked "tongue", which serves to ferry ants and
other prey to the entrance of the pitcher. The species in the genus
Sarracenia readily hybridise, making their classification a complex
matter.
There are two other families of pitcher plants, but both contain
just one or two carnivorous species.
The Cephalotaceae is a monotypic family with but one genus and
species, Cephalotus follicularis. This species has a small (2–5 cm)
pitcher similar in form to those of Nepenthes. It occurs in only one
location in southwestern Australia.
A few species of bromeliads (Bromeliaceae), such as Brocchinia
reducta and Catopsis berteroniana, are known or suspected to be
carnivorous. Bromeliads are monocots, and given that they all
naturally collect water where their leaves meet each other, and that
many collect detritus, it is not surprising that a few should have
been naturally selected to develop the habit into carnivory by the
addition of wax and downward-pointing hairs.
The Purple pitcher plant, Sarracenia purpurea, is the floral
emblem of the province of Newfoundland and Labrador, Canada. |
