|Normal rest position.||Recovering from stimulation closure.||Three stamens moved to pistil.|
One well-known feature of the genus Berberis is the tactile sensitivity of its stamens (seismonasty). As insects visit the flower, seeking the nectar at the base of the filaments, contact with a filament causes it to spring toward the pistil. This is said to deposit pollen onto the insect, which in turn will carry the pollen to the stigmata of other flowers. While it is easy to trigger the stamen motion - almost any disturbance of the filament will do it - I have never personally witnessed the pollen transfer onto an insect or from an insect to a stigma. Once the insect stimulation has finished, the filaments slowly return to their normal position against the petals - this refractory period takes about 20 minutes.
The image below shows the stamens of B. trifoliolata in rest position and after tripping. Worthy of note:
A series of B. trifoliolata filament closure and eventual recovery:
|B. swaseyi — open,
|B. swaseyi — tripped||hybrid — tripped|
Petal, stamens & pistil
Bentley 1873, Fig. 860, p. 412
Stellung der nach dem Griffel hin bewegten
Position of the stamen having moved towards the pistil
Müller 1883, Fig. 40, p.124
Given the functional nature of filament sensitivity – pollination by insects and also perhaps prevention of self-pollination – modifications of floral structure (both androecium and gynoecium) could well be expected to trigger adjustments in the mechanics of the sensitive response to insects.
A recent observation by Neltje Blanchan suggests that self-pollination does not generally occur with B. vulgaris, given the relative length of its stamens:
So short are the stamens, it is improbable that a flower's pollen ever reaches its own stigma except through the occasional confused fumbling of a visitor. Usually he is so startled by the sudden shower of pollen that he flies away instantly.
The tradition of the taxonomic literature is to mention filament sensitivity only as a property of the genus, sometimes with detail that is clearly not valid for our Texas species. For example, De Candolle 1818 (pp. 4-5), provides the following description:
OBS. Stamina Berberidum plurimarum (B. vulgaris, canadensis, sinensis et verosimiliter omnium) acu secus filamentum irritata, subito supra pistillum se dejiciunt.
The stamens of most Berberis species (B. vulgaris, canadensis, sinensis and most likely all) when excited by a pin on the side of the filament immediately throw themselves down onto the pistil.
Filaments thick,...starting forward with a sudden jerk when touched with a point next the base on the inner side (thus projecting the pollen upon the stigma)and similarly for Bailey 1914 (v. 1, p. 487)"
The stamens are sensitive; when the filaments are touched with a pin, the fls. open, and the stamens fly forward upon the pistil.Lebuhn and Anderson's 1994 study of B. thunbergii demonstrates that "anthers that strike an object (e.g., a glass cover slip or floral visitor) deposit some of the sticky pollen that is held together by viscin threads" (p. 257), but, as with our species, the tripping action does not dump pollen on the stigma of the same flower.
As for where and how contact must be made to trigger filament tripping, I have found that a wide range of disturbance points within the corolla, anything that could be transferred to the filament, can bring about the sensitive response. The 'pin at the filament base' will indeed work, but is not necessary.
The following drawing from P. Fleurat-Lessard & B. Millet's recent article on Berberis stamen sensitivity indicates somewhat different morphology and reaction for B. canadensis than for our species: