Carnivorous Plants
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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
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.