Our methodological approach allows quantitative analysis of the forces involved in flower visits to different sage species and compares for the first time these results with the forces that different flower-visiting bee species are able to produce to gain access to a food source. In addition, the force measurements in Salvia flowers can be correlated with the functional morphology of the corolla tube and the structure of the staminal lever.
The first method used was to imitate the proboscis of a bee as it is inserted into a flower. This test procedure cannot be considered a perfect imitation of the actual process taking place during the visit of a bee because the functional mode of our testing device forces us to use a stiff metal rod with the same diameter as the bee's proboscis. A bee's proboscis is more flexible than a stiff rod, and a bee should be able to avoid adhering and becoming lodged within flower structures (at least part of the time) and piercing floral tissue during a normal flower visit. Because of the complex morphology of the flowers of some Salvia species, we were unable to avoid piercing the floral tissue during some of our measurements. But sticking and piercing events were easy to recognize after the force measurements when careful dissection revealed the resulting damage to the plant tissue. Because the exact position of the sensor in the flower is measured in the force-distance diagrams, we were able to trace the corresponding force peaks exactly in our data set and to exclude them from further biological interpretation. The high peak that followed the triggering of the staminal lever of S. sclarea is the consequence of such a sticking event. In addition to our morphological investigations, this interpretation is supported by the very high variability of this peak. In this species, the sterile base of the staminal lever arm is strongly sculptured (Figure 6.7B), and the sensor often sticks to these structures. In S. glutinosa, the lever arm is much less sculptured, and so the sensor typically passes smoothly.
The force-distance diagrams that resulted from the measurements are rich in information about internal flower morphology and the functional importance of the various flower parts during the activities of a visiting insect. Even very small structures that are found in a direct line from the flower entrance to the flower base where the nectar is present often caused a typical, detectable peak in force measurements when the sensor touched them. Peaks and structures are easy to correlate if one compares the position of the peak on the diagram with the structure's position in the flower.
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