Carnivorous Plants
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Utricularia Trap - Door Opening Mechanism

Door Opening  Observations of the trap operation utilizing high-speed videos have provided a new insight into the door opening mechanism. This replaces the long-held notion that the downward push of a trigger lever mechanically pulls the door edge out of the pavement depression, thus creating a small opening for water to enter. This precipitates more water flow, finally forcing the door to flap open completely. To make the idea more plausible, the trigger levers grow on the lower portion of the door just around what appears to be a secondary hinge. This makes it possible to unlatch the tight lock with a minimum of stress, the old theory speculates.

The new idea embraces "buckling" as the key mechanism for door opening. Buckling is a well-studied physical phenomenon and is thoroughly analyzed mathematically. Buckling is characterized by a sudden change of structural states under an increasing load — in bladderworts, a flip of the trap door curvature upon triggering, from convex to concave, as seen from the outside of the trap.

When the trap is set, the door is bulging outward. This surface curvature allows the door to withstand a strong outside pressure while a delicate structural equilibrium is maintained. If a trigger lever is touched, the surface area at the base of the lever is disturbed. This slightest perturbation of the door surface — under a near-critical pressure — causes the door to buckle. The buckling starts where the trigger lever grows and propagates swiftly to a larger area, reversing the curvature of the entire door surface. As the buckling reaches the free edge of the door, the change of the angle of the door edge toward the pavement depression virtually unlocks the door.

Giving in to the enormous pressure from outside, the door opens in a matter of 1/1000 second. The sudden inrush of water forces the door to be held in the open position. Water continues to flow into the trap, carrying the prey with it. The elastic energy of the trap walls now released and the trap fully inflated with no outside pressure, the door swiftly snaps back to the closed position, unbuckled.

Any disturbance of the trigger lever will cause buckling, however, the trigger lever may not be necessary for trap triggering. In fact, some traps do not have trigger levers, and some has only stalked mucilage glands in place of trigger levers (like U. purpurea). In reality, a more likely scenario would be that a prey animal bumps onto the door itself, which will surely create enough surface perturbation to cause buckling. For the majority of traps that do possess trigger levers, they might just be largely ornamental.

Perhaps the most critical structure for trap door buckling resides in the middle portion of the door where the wall becomes thinnest and most touch-sensitive — and appears to function as a secondary hinge. This is a vulnerable spot of the door and a slightest push by a potential prey will cause buckling (like your knee buckles if pushed from behind).

Not surprisingly, this is precisely where the trigger levers are located. The main purpose of the levers perhaps is to allure and direct a potential prey to this "sweet" spot of the door. The double-layered construction of the trap door — with the much thicker inner layer with wrinkles to allow for expansion — and the hinge mechanism in the middle of the door appear to hold the secret to “an astounding degree of mechanical delicacy depending on a fineness of structure scarcely equaled elsewhere in the plant kingdom.”

Besides pumping of water by the wall glands (which is physiological in nature), setting, tripping, and resetting of the trap are believed to be purely mechanical, unrelated to growth phenomena. Therefore, one bladder can repeat the triggering action without any biological growth limitation. One observer counted 24 times of trap resetting.  

(From "Carnivorous Plants in the Wilderness")



2017-10-03  Under construction


EVOLUTION:   Pinguicula -->> Genlisea -->> Utricularia

Flowers :  Pinguicula (salverform / snap-dragon) ->Genlisea (salverform / snap-dragon) -> basal Utricularia, sections Polypompholyx, Tridentaria (snap-dragon) ->Utricularia (snap-dragon)

Calyx lobes :  Pinguicula (x5) ->Genlisea (x5) -> basal Utricularia, sections Polypompholyx, Tridentaria (x4) ->Utricularia (x2)

True leaves :  Pinguicula (yes) ->Genlisea (yes) ->Polypompholyx, Tridentaria (yes) ->terrestrial Utricularia (yes) --> Utricularia (no)

Rosette habit : Pinguicula (rosette) --> Genlisea (rosette) --> Polypompholyx, Tridentaria (rosette) ->terrestrial Utricularia (yes/no) --> section Utricularia (free floating)

Root :  Pinguicula (yes) ->Genlisea (rhizophyll) ->Polypompholyx, Tridentaria (rhizoid) ->Utricularia (rhizoid)

Some speculations are that a rolled Pinguicula leaf going down into a moist substrate eventually led to ...

1.  Pinguicula -->> rolled leaf growing into moist substrate led to ...

2.  Genlisea --- cross section of Genlisea trap is very much reminiscent of Utric trap.... almost identical

3.  Primordial trap --- no functional door / maybe mild inward water flow  / lobster-pot trap / if door lightly closed, spontaneous triggering captures prey....

4.  Polypompholyx trap

5.  Terrestrial Utric trap --- generally very small door angle against the threshold.... animal can push itself into the trap --- no trigger lever?

6.  Aquatic Utric trap ----- more advanced door triggering mechanism --- possible physiological turgor control by sensory mechanism....

Evolution: Here are my current thinking and conjecture about Utricularia traps... ---> under construction 2017-April-02

Trigger hairs are not needed for trap triggering.... they are just ornaments, and very important ones at that... /aquatic/terrestrial/U.purpurea (illustr)

Cross section photo by Kiyoshi Shimizu (1966) - Utric winter bud --- primordial trap

- Theory of recapitulation ... winter bud shows a very primitive trap - prototype of terrestrial species of today
   primordial trap --> ((Genlisea-type trap)) --> Polypompholyx trap --> Terrestrial trap --> Aquatic trap

- Trap is epiascidiate ...like Sarracenia (bladder trap interior = adaxial surface)
         This is clear in Genlisea, and Utric is derived from Genlisea (i.e., Genlisea-like ancestor, evolutionarily speaking)

- Double-layer .... inevitable consequence since the trap is derived from a leaf (abaxial + adexial epidermis)

- The tip of the trap door corresponds to the leaf tip...

Terrestrial trap - similar to "primitive trap" structure (aquatic traps are more derived)

a)  If the door has a small gap ... a mild water flow like Genlisea (less derived than Utricularia). A water animal followed the water flow... or there might not have been any flow at all in a most primordial trap... just a static pouch.            

b)  If the door is lightly closed --- water animal can push the door and enter (lobster-pot) -OR- spontaneous triggering every so often!!!

- Water is expelled from near the threshold --- into a narrow corridor leading to the door (or along the length of the door). The area serves as a storage for a mouthful of water needed for the next trap firing.... The same water recycled - important consideration for traps not submersed in water...   .

Utricularia traps (aquatic) pop when pulled from water......... This is because the air is sucked into the previously set trap. This is a clear proof that the interior of the trap was lower than 1 atm (the  atmospheric pressure). In the water, the pressure becomes higher, and at the depth of 10 meter (34 feet) it becomes 2 atm. But bladderworts are floating near the water surface, so it is close to 1 atm (a tiny bit higher than 1 atm, but not lower). So the popping of the trap is not due to higher pressure of the air, but rather due to the disturbance to the trap door when the trap is lifted from the water....

Door Mechanism of Aquatic Trap

Upon triggering, the trap door opens completely in less than one millisecond (1/1000 sec). Probably another millisecond is needed to fully inflate the bladder. And then, with no more external pressure, the door swiftly returns to its closed position in a few milliseconds. So, overall, the entire suction operation of a bladderwort trap takes only 1/100 to 1/200 second to complete.

Buckling at the thinnest area of the door....  the secondary hinge

Illustrations ....  simulate the trap door with a sphere .... cut into half .... then half of that....  See from diff angles

The water pumping (out of the trap interior) is continuous. After triggering, it takes only 15-30 minutes to expel the water so that the trap is fully reset and ready again to fire.

After several hours of non-activity (no external stimulation), a trap seems to fire spontaneously, repeating this resetting process.

Copyright (c) 2017 Makoto Honda. All Rights Reserved.

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