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Earlier Study of Berberis Stamen Irritability

by Bob Harms  email-here

Discovery of Berberis Stamen Irritability

Although Berberis, especially B. vulgaris, was known to ancient botanists, the feature of stamen irritability remained undocumented until Linnaeus published the second edition of his Swedish Flora in 1755 – the 1745 edition makes no mention of this phenomenon. Linnaeus' description of B. vulgaris (p. 116) might seem to imply that the sudden movement of the filaments released pollen onto the stigma – thus leading to self-pollination of the flower:
When bees searching for honey touch the filaments, the anthers approach the stigma and release the pollen, which can be imitated by using a small pin. As related by Baal Hortulanus Monspessulanus.
Grieve's 1931 (A Modern Herbal v. 1, p. 82) credits Linnaeus with the observation and seems to have included details from later studies, perhaps Smith 1788 (see below):
Linnaeus observed that when bees in search of honey touch the filaments, they spring from the petal and strike the anther against the stigma, thereby exploding the pollen.
Duhamel du Monceau 1758 also noted the sensitivity of B. vulgaris:
si avec la pointe d'une aiguille on cause une légere irritation à la base des étamines de l'Épine-vinette, on les voit se contracter & se rapproche du pistile
if we cause a light irritation at the base of the stamens of B. vulgaris with the point of a pin, we see them contract and approach the pistil

Self-Pollination as the Unchallenged Perspective

In the second half of the 18th Century numerous studied explored the details of filament stimulation, but until late in the 19th Century the prevailing view was that insect-induced stamen tripping was a mechanism of self-pollination.

One of the first to study stamen irritability was J. G. Kölreuter, who presented the following in a 1772 paper (published in 1788, as reported by Göppert 1828, p. 236):
Die Staubfäden setzen sich ohne äussern Reiz niemals in Bewegung. Da nun die Staubbeutel zur Zeit Reife bei einer jedesmaligen Hinneigung zum Pistell, sich ihres Inhalts auf die Narbe entledigen [ set free ] und selbe dadurch befruchten, so würde dies niemals staff finden können, wenn nicht gewisse Insekten ... den im Grunde der Blume, durch die Honiggefässe abgesonderten Saft begierig aufsuchten, die Staubfäden dabei berührten und somit die Stelle eines künstlichen Reizes verträten.

The filaments are never set in motion without an external stimulus. Since the anthers when ripe release their contents onto the stigma in bending toward the pistil at that time and thereby fertilize the same, this would never have been able to occur unless certain insects had eagerly sought the nectar secreted by the honey-glands at the base of the flower, thus touched the filaments and consequently acted in the same manner as an artificial stimulus.

... so bewegen sie sich schnell zur Narbe, und ziehen selbst die Blumenblätter etwas nach, so dass die vorher geöffnete Blume nun halb geschlossen erscheint.

They [filaments] move quickly to the stigma and even draw the petals along somewhat, so that the previously opened flower now appears to be half closed.
[Note: The petals move as well as the stamens - as with the hybrid shown at the top of this page.]

A contemporary, James E. Smith 1788, added numerous noteworthy observations.
  1. With a very small stick I gently touched the inside of one of the filaments, which instantly sprung from the petal with considerable force, striking its anthera against the stigma. [158]
    [Note: Müller, see below, states that the anther does not actually strike the stigma.]
  2. ...the motion above described was owing to a high degree of irritability in the side of each filament next the germen [ovary], by which when touched, it contracts, that side becomes shorter than the other, and consequently the filament is bent toward the germen. [159]
    [Note: Contraction of the adaxial filament cells produces the tripping effect.]
  3. A very sudden and smart shock given to any part of a stamen would, however,sometimes have the same effect as touching the irritable part. [159]
    [Note: Direct contact with the filament base is not necessary.]
  4. From some flowers I carefully removed the germen, without touching the filaments, and then applied a bristle to one of them, which immediately contracted, and the stigma being out of the way, it was bent quite over to the opposite side of the flower. [160]
    [Note: The bending action is normally braked by contact with the pistil.]
  5. When the stamina stand in their original position, their anthers are effectively sheltered from rain by the concavity of the petals. ...till some insect coming to extract honey from the base of the flower thrusts itself between their filaments, and almost unavoidably touches them in the most irritable part : thus the impregnation of the germen is performed.... [160-161]
    [Note: Self-pollination results from the insect-induced filament motion.]
Perhaps the most influential of the 18th Century studies was Sprengel's 1793 Das entdeckte geheimniss der natur im Bau und in der befruchtung der blumen (Nature's secret in the structure and fertilization of flowers revealed). Sprengel first provided detailed analysis of the arrangement of Berberis flower components, especially the structure of the pistil and its position relative to the stamens, p. 205:
By the discovery of the actual stigma I was able to realize how meaningful was its special arrangement and that of the stamen and how the former must necessarily be pollinated by the latter as it lies on the pistil when touched by an insect.
[Note: Self-pollination is a consequence of stamen movement to the pistil - contra Müller below.]

The two pollen bags have still not opened. It is not long however, before they begin to open but not in the ordinary manner i.e., they split into two halves along their length, but in such a way that they detach themselves from the filaments from above downwards, turn around remain attached to the filament with their lower end and while the rest remains below its protected end ... consequently its side which contained the pollen is turned toward the pistil. Now, the filaments are exactly so long that when they lie on the pistil their pollen bag lies on the stigma. When therefore a filament on being touched by an insect lies on the pistil it presses the inner side of its sac which is full of pollen on the stigma....
[Note: Anther valves open, with the pollen-bearing side of the valve pivoting toward the pistil. For some strange reason Sprengel views the flower as hanging down vertically - thus in more standard descriptions his 'downwards' = 'upwards' and 'lower end' = 'upper end'.] ]

Göppert's 1828 article gives an excellent summary of the many experiments exploring the details of both natural and artificial mechanisms for stamen sensitivity in the late 18th and early 19th Centuries. These include:

And with few exceptions these studies were conducted in a broader context of plant sensitivity and insect/plant interactions, for which Berberis (almost exclusively B. vulgaris) served as only one example.

From my perspective, I find it rather strange that no-one seemed to wonder why a plant species would require insect intervention to achieve self-pollination.


Hybrid fully extended and in stages of recovery.

Cross-Pollination as the Dominant Perspective

Hermann Müller's 1873 detailed study of the B. vulgaris flower structure and insect/flower interaction made a compelling case that Berberis self-pollination was unusual, and uncertain. His interpretation quickly came to be the dominant view. [Passages below are from the 1883 English edition/translation, p. 92.]
Before the flower opens the anthers stand on a level with the stigma; they are closely applied to the stigma in the bud, but in the opening flower they are bent as far backwards as the surrounding petals allow. As soon as the petals begin to diverge, the anthers dehisce, the lid flying up and remaining attached only to the extreme end of the connective, which is here at it broadest. The anther-lid carries with it almost all the pollen, and after rising to its full height it turns that side which bears the pollen inwards towards the middle of the flower. If the stamens are now touched at the base and made to move inwards, the masses of pollen do not come on a level with the stigma, but stand above it; so that even those parts of the masses of pollen which do not touch the insect, excluding some little scattered particles, do not come in contact with the stigma: the insect's head or proboscis, however, in being drawn back and rubbed against the masses of pollen gets dusted with pollen just in those parts which will come in contact with the stigma of other flowers.

If insect visits altogether fail, then as the flowers wither, the masses of pollen come of themselves into contact with the stigma through the bending of the anthers. ... I cannot say
[1873, p. 126:
...ob diese späte Sichselbstbestäubung noch von Erfolg ist.
...whether this late self-pollination still succeeds.]

The last major work of the 19th Century was Knuth's 1898 Handbuch der Blütenbiologie ... (Handbook of Flower Biology based upon Hermann Müller's work 'The Fertilization of Flowers by Insects;' English translation in 1906-1909, used for the quotations below). Both Müller and Knuth are cited at length in Fedde's 1901 monograph on Mahonia (p. 46). Knuth first included a Mahonia species, B. aquifolia, stating that its floral arrangement was equivalent to that of B. vulgaris. He seems reluctant, however, to claim this as a genus feature (p. 55):
Many species possess sensitive stamens....
Beyond somewhat modifying Müller's skepticism concerning the possibility of fertilization via late self-pollination to imply that some flowers may be self-fertilized (p. 56)
does not always seem to be effective, for many flowers fail to set fruits
Knuth adds very little beyond further establishing Müller's overall cross-pollination view as correct.