Pitcher model
Utricularia bladder
Utricularia bladder TS
External gland/trichome
Carnivorous Plants
Above: left - a 3D computer model of a generalised pitcher of a pitcher plant (similar to Nepenthes). Right - a
drawing of a
Nepenthes pitcher. The pitchers are modified leaves and are beautiful structures and sometimes quite
large!

Pitcher Plants

Pitcher plants are extraordinary carnivorous plants. They have modified leaves that form special receptacles that
contain fluid. Insects and other small animals (and also plant debris) that fall into the pitcher are broken down and
absorbed. In particular, the nitrogen obtained in this way is a valuable nutrient as nitrogen often limits plant growth
and carnivorous plants often occur where nitrogen is in short supply. The pitchers can be quite large and dead rats
are occasionally found inside the very largest. Some pitchers secrete
enzymes and acid to digest their prey, whilst
others rely on bacterial decomposition or both. The enzymes that may be secreted include proteases (digest
proteins) and carbohydrases (digest carbohydrates like starch). Digestion is typically slow and to stop the animals
escaping various tactics are employed. Downward pointing hairs or spines inside the pitcher, combined with a
slippery, waxy surface make it hard for insects to crawl back out!

Darlingtonia, the California pitcher or cobra plant has particularly fascinating pitchers that look like snakes, with
their forked protruding appendage and curved over hoods. A spiral passage connects the digestion chamber to
the outside world and even if trapped insects can navigate this, the hood has a special trick to deceive them - it
contains
fenestrae or translucent windows which allow light through. The insect makes a line for these windows,
believing them to be exits, flies up, bumps into the hood and falls back down the spiral shoot into the trap! This
compensates for the fact that
Darlingtonia does not secrete digestive enzymes, but relies on the slower process of
bacterial decay.

Chance encounter might not be sufficient and pitchers have ways of luring their prey!
Sarracenia purpurea, a
pitcher from NE America has a green pitcher which is red at the top and has nectaries around the aperture and
releases a violet odour. The colour, promise of food and odour all serve to attract insects. Most of these insects
are actually not flies but largely crawling insects and
Sarracenia has a single ventral ridge the ala ventralis (ventral
wing) which is a causeway along which insects can crawl up the pitcher to the slick, waxy surface of the rim
(peristome) - and down they fall! Rows of downward pointing hairs, the slippery inside surface and the viscosity of
the fluid in the trap all make it hard for the insects to get back out. The nectaries secrete an alkaloid narcotic, in
addition to sugar, to anaesthetise the insect, again making it hard for the animal to avoid slipping in!
Nepenthes
has nectaries around the waxy rim and large downward-pointing teeth inside the tube and secretes digestive
enzymes.
Nepenthes is a tree-climbing vine.
Left a bladder of Utricularia. Utricularia is a
freshwater aquatic plant which lacks roots and
floats in the water. It gives off stalk-like stolons
which bear small bladders, which contain fluid
under reduced pressure, each no more than 5
mm long.

These bladders are sophisticated traps, primed
and ready to catch prey, which is typically small
crustaceans, like water fleas and copepods, and
insect larvae. These creatures try to hide
amongst the hairs (trichomes) that project from
the bladder (borne on wings (W) in the example
on the left) but these hairs are primed triggers
and when the prey brushes against them the
trap springs open! Water rushes into the
bladder, sweeping along the prey with it. The
trap then seals shut and enzymes digest the
unfortunate victim! R, rostrum; S, stalk; W, wing.
Below: a longitudinal section through one of the bladders. The trap is primed and a flap-like valve (V)
closes the entrance. This valve is wedged shut. Pulling on the trichomes (T) springs the trap, the
valve opens and water rushes in. Inside the bladder lining possesses small glandular hairs or internal
glands (IG), some of these have four cells at their tips (4 end-branches) and are called quadrifed
glands, whilst some have two terminal cells and are called bifid glands. The outside of the bladder
contains equidistantly spaced small spherical or nipple-like glands (external glands). The functioning
of these glands are described below.
Utricularia bladder in section
Left: transverse sections of Utricularia bladders.
Leftmost - when primed the bladders side-walls cave
in (they are concave) as the fluid inside the bladder is
at lower pressure than the water outside. Rightmost -
an activated trap - water has rushed in and stretched
the walls of the bladder which are now convex (curve
outwards).

Below: structure of a quadrifid or bifid gland, shown in
section. (Based on the model of Fineran).
The internal glands consist of an elevated basal cell, which is part of the epidermis (cell layer
covering the wall of the bladder). Above this is a pedestal cell and above this are 2 (bifid gland) or
4 (quadrifid gland) capital or terminal cells. Black and dark shading indicate water-impervious
regions of the cuticle and cell walls respectively. These ensure that water can only enter or leave
the gland through the terminal cells (which lack any substantial cuticle) and also block the
apoplastic pathway between the basal and pedestal cells.

Mechanism

These glands pump out water from the inside of the trap when it is being primed, lowering the
pressure in the trap and creating the suction when the trap springs open. This process is rapid
and so probably occurs through the apoplast (cell walls) as shown by the blue arrow. The
movement of this water is actively driven. Pumps in the cell-surface membrane of the pedestal cell
use cellular energy to pump water from the apoplast across the membrane into the cytoplasm of
the pedestal cell. The pedestal cell is a type of
transfer cell and has tubular wall-ingrowths which
increases the surface area of its cell-surface membrane to accommodate more pumps. These
pumps are membrane-spanning proteins that pump chloride ions. (Chloride ions are negatively
charged and their charges attract and drag positively charged sodium ions with them). This
increases the salt concentration inside the cytoplasm of the pedestal cell, lowering its water
potential. Water then follows by osmosis (drawn out by the salt) as it moves from higher to lower
water potential. This water is replaced by water from the apoplast and a stream of water movement
through the apoplast is set-up. Water then moves from the pedestal cell into the basal cell and
surrounding tissues through the symplast (cell cytoplasms) moving from cell to cell through pores
called
plasmodesmata.

The terminal cells possibly also secretes the enzymes to digest the prey when the trap is activated.
The products of digestion are absorbed by the terminal cells and are thought to follow the symplast
route (movement in the cytoplasm) moving from cell to cell through the plasmodesmata.

Once the trap has done its job, it is reset and can be used repeatedly. The water that is pumped
out every time the trap is reset has to be removed from the bladder and this is the function of the
external glands.
The external glands/trichomes consist of two cells - the pedestal cell and the terminal cell. Their
function is to excrete excess water taken up from inside the trap when it is reset. Water absorbed
by the external glands enters the tissues of the bladder wall and is transported from cell to cell
through the plasmodesmata until it reaches the external glands where it enters the pedestal cell
through plasmodesmata. The pedestal cell has tubular wall ingrowths (though not as extensive as
in the external glands). These ingrowths increase the surface area of the cell-surface membrane
follows them passively by osmosis. The cuticle of the terminal cell is highly porous (especially when
stretched by water entering the terminal cell) and the water passes out through these pores.
Utricularia (bladderwort)

Not all carnivorous plants use mechanisms as passive as those of pitcher plants. The next example we look at are
the aquatic bladderworts, or utricularias. These have extremely elaborate trap mechanisms!
Bibliography

  1. Glandular trichomes in Utricularia: a review of their structure and function. Fineran, 1985.
  2. Kinetics and Mechanism of Dionaea muscipula Trap Closing. Volkov et al., 2008. Plant Physiology
    146: 694-702.
  3. Leaf Closure in the Venus Flytrap: An Acid Growth Response. Williams and Bennett, 1982. Science
    218: 1120-1122.
  4. A quantitative study of tissue dynamics during closure in the traps of Venus's flytrap Dionaea
    muscipula Ellis. Fagerberg and Allain, 1991. Am. J. Bot. 78: 647-57.
Venus' Flytrap
Venus' Flytrap (Dionaea muscipula)

Similar to the bladderwort, the flytrap is a spring trap. Dionaea produces modified leaves as traps, with each trap
comprising a pair of valves. Each valve is fringed with hairs called cilia and has three trigger hairs, arranged in a
triangle, on its inside surface. Each plant produces 5-7 traps which are each 3-7 cm long.
Alluring glands inside the trap secrete a sugary mucilage to attract insects. When an insect touches one of the trigger
hairs electrical signals are generated. Initially these are local signals which decay rapidly over time and space. A
single brush does not activate the trap, otherwise many a false alarm such as a falling raindrop or debris could falsely
trigger the trap, but if any of the three hairs is touched again, then it generates a second electrical signal which is
added to the first. In this way, multiple touches that are close enough together in time will reach a threshold and the
trap will be activated. This process involves the transport of electrical signals throughout the plant neuroid/nervous
system (see the
sensitive plant).

When cells in the trap receive the signal, especially the hinge and central areas of the valves where most movement
occurs, then motor cells reversibly close the trap. The mechanism of closure is very different to the mechanism that
moves leaves to track the Sun - the pulvinus, or hinge joint at the base of a leaf stalk, has flexor and extensor cells
that swell and shrink reversibly as they take-up or lose water. In contrast, the movements that cause trap closure in
the flytrap are irreversible growth changes - flexor cells grow permanently larger. This growth appears to be triggered
by the active pumping of hydrogen ions out of the flexor cells and into the cell walls, lowering the pH of the apoplast.
This acidification of the cell walls activates wall-loosening enzymes, called
expansins, which break cross-links between
the cellulose microfibrils (microscopic fibres of cellulose) that make up the cell wall, allowing them to slide past
one-another and allowing the cell walls to stretch. As the cell walls looses, so the cells take in water and swell
irreversibly. Movement of water from the inner half to the outer half of the hinge provides the necessary water for the
flexor cells in the outer part of the hinge. This is a rapid example of the normal cellular growth process. Pumping of the
hydrogen ions is an energetic process and the cells use about 30% of their ATP (energy store) in this process.

Trap closure occurs in several stages:

1. Slow initial phase lasting 0.3s during which the trap only closes by 20%. During this phase, rapid growth of cells is
building up elastic energy which is stored inside the trap.
2. Rapid closure phase lasting 0.1s, during which time the trap closes by 60%. This is the phase that traps the
unfortunate insect before it has time to escape.
3. Second slow phase, during which the trap closes by the remaining 20% and lasting 0.3s.

Following closure is the appression phase, lasting about 30 minutes and during which the trap presses around its prey
and smoothers it in secreted mucilage. Following this is the sealing phase, during which the edges seal tightly to
prevent leakage of contents. this phase lasts for about one hour and is followed by enzyme secretion.

The cilia seal the edge of the trap after closure, but allow small prey to escape, since it is a waste of time the plant
digesting such small prey. If small prey escapes through the cilia, then the trap will soon open again (within 2 days).
Larger insects, however, cannot escape and the edges of the trap then more slowly form a tighter seal. Secreted
mucilage also entraps the insect and when the seal is formed, digestive enzymes are secreted and the prey liquefied
and absorbed. Digestion takes 4-14 days and the digestion glands give the inside of the trap its red colour (due to
anthocyanin pigment), which probably helps to attract insects. Extensor cells then grow larger and reopen the trap,
resetting it. Despite the fact that the growth changes are irreversible, successive growth cycles are possible and the
trap can be sprung and reset several times in its lifetime.

Semi-Carnivorous Plants

Many plants are not classed as fully-carnivorous but still 'eat' insects and make use of the nitrogen as a nitrogen
supplement. One such example of a semi-carnivorous plant is the
teasel.