Abstract
Conifer cones represent natural, woody compliant structures which move their scales as passive responses to changes in environmental humidity. Here we report on water-driven opening and closing motions in coalified conifer cones from the Eemian Interglacial (approx. 126,000–113,000 years BP) and from the Middle Miocene (approx. 16.5 to 11.5 million years BP). These cones represent by far the oldest documented evidence of plant parts showing full functionality of such passive hydraulically actuated motion. The functional resilience of these structures is far beyond the biological purpose of seed dispersal and protection and is because of a low level of mineralization of the fossils. Our analysis emphasizes the functional-morphological integrity of these biological compliant mechanisms which, in addition to their biological fascination, are potentially also role models for resilient and maintenance-free biomimetic applications (e.g., adaptive and autonomously moving structures including passive hydraulic actuators).
Highlights
Pinus sp. 1 (Eemian Interglacial, approx. 126,000–113,000 years BP, Tarantian, Late Pleistocene) was found in 1965 in the ‘Else’ mine on Lüneburg Heath, Germany, and was initially embedded in diatomaceous earth (‘Kieselgur’)
We tested if the coalified structures of the fossil Pinus and Keteleeria cones and scales were still capable of passive hydraulic motions as in their extant relatives
Our investigations demonstrated that the whole Pinus sp. 2 cone (Fig. 1c, Supplementary Video S1), as well as separated seed scales of Pinus sp. 1 and Keteleeria sp. (Fig. 1d) showed distinct bending movements after being transferred from a very dry environment into a container filled with tap water
Summary
Our investigations demonstrated that the whole Pinus sp. 2 cone (Fig. 1c, Supplementary Video S1), as well as separated seed scales of Pinus sp. 1 and Keteleeria sp. (Fig. 1d) showed distinct bending movements after being transferred from a very dry environment into a container filled with tap water. The X-ray absorption of most of the scale tissue was found to be in a similar range as for the recent pine cone scale specimens tested for comparison. 11), with only a few spots of strongly absorbing mineral material present, suggesting that the observed capability for hygroscopic movement most presumably has been conserved by the apparently dominant process of coalification. This is the first report of passive-hydraulic actuation in plant organs that are millions of years old, revealing the exceeding resilience of such mechanical systems. Functionality, raising the bar to a new level for current research on the biomimetic transfer of plant movement principles into movable technical systems[12,13,14,15,16,17]
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