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Carnivorous Plants in the Wilderness
by Makoto Honda

Carnivorous Plants

Snap Trap Evolution

2017-November-24  ------------------  Aldrovanda snap trap

Here is an excellent high speed video of the snap-trap operation of the waterwheel plant. I watched this video 100 times! I was trying to understand how this snap trap closes its lobes....

I was trying to determine which portion of the trap is changing its curvature...

This article by Joyeux & Poppinga claims that the snap trap mechanisms by the Venus flytrap (Dionaea) and the waterwheel plant (Aldrovanda) are entirely different --- and some articles quoting this article are even implying independent origins of these snap traps... I beg to disagree vehemently...

I looked at the above video many times  to see if I can tell if the snapping motion is caused by the deformation of the trap midrib (hinged motion) or by the warping of the larger area of the trap (near the midrib). Looking at the closed picture, and comparing it with the open trap picture, it is very difficult to see which portion of the trap lobes has caused the snapping. In the video, the left picture is a view from the tip of the trap, and the right picture is a lateral view of the same trap. In the left view, the camera is placed not straight along the line of the midrib, but a tiny bit lower, so that the bottom of the trap (and the midrib) is seen. Even so, the end point of the trap is obvious where the lobe opening ends. Let us call this Point A. Another point I focused on is at the midpoint of the end of the "motile" region (motor zone) of the lobe. Let us call this Point B. After some brain-storming, I decided to measure the distance between Point A and Point B - in order to determine if the snapping is due to the midrib hinging or the wall warping in the motor zone. Note that when I say the distance between A and B, I do not mean the actual distance between the two points , rather, what I am interested in is the projected distance, that is, projected on the left view. This is the idea: If the snap is due to the hinged rotation about Point A, there would be no difference in the A-B distance before and after the trap snap since there is no deformation of each lobe. However, if there is any lobe warping in the region, the arc A-B is stronger, and the distance A-B gets shorter after lobe closure.

Note also that, in the illustration below, I chose to show the frontal views of the trap (all views except one lateral view) as if the leaf blade is extending toward you (not from the tip of the trap, as in the video). You can see that by noting the bristles shown on the left of the trap.

                      Aldrovanda vesiculosa  snap trap closure                         

Well, based on my repeated trials of measuring the screen image of this video, I concluded that, indeed, the A-B distance got shorter (in the left view of the video) after trap snapping, and therefore, the Aldrovanda closure is driven by warping of the motor zone of the lobes. This is in concert with the traditionally-held view by many past investigators, including  ...

Snap Trap Evolved Only Once

... in the common ancestor of Aldrovanda and Dionaea.....I am not saying that these two snap trap mechanisms we see today in the extant species are identical. Of course, there are some differences. After all, both lineages have 30-40 million years of completely independent evolutionary paths for crying out loud. Not to forget also that one is terrestrial, the other is aquatic, both well fine-tuned in their respective, entirely different environments. What I am saying is that we do well seeking similarities --- rather than dissimilarities --- when we strive to unlock the mystery and wonder of these "most wonderful plants in the world."

Last update : 2017-Nov-05  ------------------  Observations of the trap

Molecular phylogenetics strongly points to a common origin of the two snap traps (Aldrovanda & Dionaea).

This most likely means that the snap trap mechanism evolved only once -- therefore, the basic mechanism for these snap traps must be similar... actually identical...
It is widely accepted that, in the case of Dionaea, the snapping of a trap leaf involves buckling. This is due to the initial convex curvature (doubly-curved) of the open trap lobes of a mature specimen. This "snap-through" buckling (or "flipping") does increase the speed of trap closure, a little. However, it has to be understood that the buckling, if it does happen, is not the main, driving force of the swift leaf closure, but rather, a result of it. The main cause of leaf closure is the pressure differential created between the opposite sides (upper & lower) of the trap lobes..... The same mechanism is responsible for the swift snap trap of Aldrovanda (no buckling here, though).

The molecular evidence further indicates that Aldrovanda and Dionaea form a clade that is sister to Drosera (sundews). This strongly suggests that the common ancestor of Aldrovanda & Dionaea came from an ancient sundew-like plant. This implies the mechanism responsible for the snap traps is most likely derived from a sundew-like plant --- its tentacle bending and  leaf folding.

The basic mechanism for leaf motion common throughout Drosera-Aldrovanda-Dionaea evolution is most likely to be a sudden (or relatively quick) drop of turgor pressure on one side of the structure in question, creating an imbalance of pressure on the structure to cause it to bend.... In this process, the other side (epidermis) might be forced to stretch a bit .... The recovery of the bending (or snapping for that matter) is achieved as a result of the side (epidermis) that lost turgor pressure restoring its lost pressure and then some to counter the stretch of the other side. This is accomplished by slow, normal, actual growth.


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