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Carnivorous Plants in the Wilderness
by Makoto Honda

Carnivorous Plants

Snap Trap - Evolution

Last update : 2017-June-17  ------------------  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 evolved only once; therefore, the basic mechanism for these snap traps must be the same... 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

 Tentacle bending (nastic/tropistic) - leaf folding

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

 motor region  - narrowing (free side/lower side)

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


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

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

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)   

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

                          Stamen              Style                                 Ovule                                Pollen
Drosera                 5             separate/undivided        partial placentation        multiple operculate pores                         
Aldrovanda          5             separate/undivided        partial placentation                                
Dionaea              15             united                               basal placentation      

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

THE THEORY OF RECAPITULATION - "Ontogeny Recapitulates Phylogeny"

1)  Lloyd  - In Venus flytrap, the trap tilts to the right - particularly in younger plants, just like the clear bend in the Aldrovanda trap (see Lloyd/Darwin) --- a distinct advantage for Aldrovanda (no benefit for Dionaea).

2)  Lloyd comments on a young Venus flytrap (2 mm trap) ----> "The number of parenchyma cells between ranged from 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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