Trap Closure Mechanism  
Many studies have been conducted to better understand the closing mechanism of the Venus flytrap. To this day many unanswered questions remain.

Trigger Hair Structure  
The trigger hair has sensory cells embedded at an indentation near its base, where bending strains are most pronounced when the hair is disturbed. The sensory cells contain a large amount of calcium (Ca+) and potassium (K+) ions.

Action Potential
Studies suggest strong correlation between calcium ions and the action potential. When a trigger hair is stimulated, the cytoplasmic concentration of Ca+ increases. These sensory cells generate an action potential upon stimulation.

Lobe Expansion
Exactly what happens in the trap at the time of triggering is not well understood. The entire lobe interior (mesophyll between the epidermis) seems to expand rapidly. Only the lower (abaxial) epidermis walls become "loose" (extensible) and get stretched. The lobe expansion occurs in the direction perpendicular to the midrib. This creates pressure for the lobes to warp inward.

Strain Buildup
When the trap is open, the lobe surface is slightly convex (as seen from above) - both in the direction perpendicular to the midrib and parallel to it. Because of this initial 3-dimensional curvature of the lobe surface, the pressure of the lobe expansion does not immediately produce a bending motion of the lobes, and the trap remains open. The strain from the expansion continues to build in the lobes. The lobes resist this buildup of the mechanical tension.

Sudden Flip
Then, suddenly, the lobes yield to the pressure and flip, precipitating a sudden change of curvature from convex to concave. This results in a swift closing motion of the two lobes. This flipping forces moderate expansion of the outer lobe surfaces (abaxial surface) in the direction parallel to the midrib. This stretching parallel to the midrib is not due to the lobe expansion (which occurs only in the direction perpendicular to the midrib).

Narrowing Phase
The lobe expansion, with only the lower epidermis extension, continues into the narrowing phase, exerting the pressure to close the two lobes more tightly. The pressure is so strong that if one lobe is removed from the trap, the remaining lobe curls more than 180 degrees. (See picture below.)

What Triggers Lobe Expansion and When
It is unclear what precipitates this lobe expansion. Is it a concentration of Ca+ ions itself (that causes an action potential) or the accumulation of some other chemical substance that serves as a memory until the threshold level is reached? A related question is when this build-up of the pressure starts: Right at the point of the first stimulation, or only after the second stimulation is received. The former suggests that the strain in the lobes itself serves as a memory that delays the closure until the lobes cannot endure additional strain anymore as a result of the second stimulation. This scenario is unlikely, because even a trap whose lobes are not fully convex waits for the second stimulus before inducing the motion.

Reopening
The reopening phase proceeds slowly, taking a day. Unlike the rapid closing phase, it is due to cell growth. The growth rate of an activated leaf is faster than an undisturbed trap and a reopened lobe has been measured to be several percent larger than the original size.

PHOTO:  Dionaea muscipula