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Carnivorous Plants in the Wilderness
by Makoto Honda

Carnivorous Plants

Snap Trap Evolution

2017-November-08  ------------------  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 how this snap trap closes its lobes....

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

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.

                      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.....We are not saying that the snap trap mechanisms we see today in the extant species are identical - of course, there are some differences. One is terrestrial, the other is aquatic, both well fine-tuned in their respective environment. After all, both lineages have 30-40 million years of completely separate evolutionary paths. 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.

1.  Drosera (sundews) ---- Illustrations

  Sensory cells : epidermal cells at the tip of tentacle (covered by two layers of glandular cells)
 Tentacle bending (nastic / tropistic) - leaf folding
  Trap motion - 2 action potentials within 1 minute to cause a tentacle bending...
    - Sudden drop of turgor pressure on one side of the tentacle...


2.  Aldrovanda (waterwheel plant) ---- Illustrations

Sensory cells : trigger hair...
 motor region  - narrowing (free-side lobe / bristle-side lobe)
 Trap motion - 1 action potential to snap...
- Sudden drop of turgor pressure on the inner epidermal cells in the motor region...


3.  Dionaea (Venus flytrap) ---- Illustrations

Sensory cells : embedded in the indentation at the base of the trigger hair
Trap motion - 2 action potentials within 20 seconds to snap...
- Sudden expansion of mesophyll and loosening of the outer (abaxial) epidermal cell walls...


EVOLUTION : DROSERA - ALDROVANDA - DIONAEA ------------ Which came first..... Aldrovanda, of course!

Molecular phylogenetic reconstruction clearly points to the common origin of these snap traps....

Overwhelming molecular evidence supports common origin of these (extant) snap traps using different regions of DNA sequence (mat K, ...Chloroplast rbcL... etc)

Which lineage evolved first --- Aldrovanda or Dionaea?

Molecular evidence further suggests the common ancestor of these snap traps diverged from the ancient sticky-leaved sundew plant - the ancestor of the extant basal taxa such as D. regia  (see midrib)   

When we talk about which came first, Dioaea or Aldrovanda, we are not suggesting that today's Venus flytrap gave birth to today's waterwheel plant, or vice versa. We are discussing the possible ways in which the ancestors of these extant plants diverged tens of millions of years ago...  

Comparison of flower parts:   Petal / sepal / seed / pistil / stamen / pollen / stigma / placenta

Sepals / Petals
Drosera                 5 / 5 (4, or more than 5)
Aldrovanda          5 / 5 (4)
Dionaea                5 / 5

Drosera                 5
Aldrovanda          5
Dionaea              15

Drosera                 separate/undivided
Aldrovanda          separate/undivided
Dionaea                united

Drosera                 separate/undivided
Aldrovanda          separate/undivided
Dionaea                united

Drosera                  partial placentation    The ovary has five parietal placentas each bearing 8-13 ovules / pollen-to-ovule ratio 28.5...                          
Aldrovanda           partial placentation                                
Dionaea                 basal placentation      Superior ovary - single chamber


Drosera                multiple operculate pores                         
Aldrovanda         3 pores with an operculum united in tetrads with 4 grains    (Cross, 2012)                           

Basal placentation: placenta is at the base (bottom) of the ovary. Simple or compound carpel.
Apical placentation: placenta is at the apex (top) of the ovary. Simple or compound carpel.
Parietal placentation: placentas are in the ovary wall within a non-sectioned ovary. Compound carpel.
Axile placentation: ovary is sectioned by radial spokes with placentas in separate 
locules. Compound carpel.
Free or central placentation: placentas are in a central column within a non-sectioned ovary. Compound carpel.
Marginal placentation: only one elongated placenta on one side of the ovary, as ovules are attached at the fusion line of the carpel's margins . This is conspicuous in legumes. Simple carpel.

Venus flytrap is very tolerant of flooding condition - possibly implying an Aldrovanda-like, aquatic ancestor.

The structure of the mesophyll of the Venus flytrap leaves is more like that of aquatic plants....

THE THEORY OF RECAPITULATION - "Ontogeny Recapitulates Phylogeny"

1)  Lloyd (p.182) - In Venus flytrap, the trap tilts to the right - mainly in younger plants, just like the clear bend seen in the Aldrovanda trap (see Lloyd/Darwin) --- Regarding this trap posture, Lloyd commented on the distinct advantage for Aldrovanda but no benefit for Dionaea. However, Lloyd did not extend that to an evolutionary context.

2)  Lloyd (p.181) comments on a young Venus flytrap (2 mm trap) ----> "The number of parenchyma cells ranges between two to four courses with large interspaces, in this feature again resembling the mature leaves of Aldrovanda much more than do the thicker mature leaves of Dionaea."


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