Sclerenchymatic tissue in Banksia follicles – The effect of moisture on dimensional and mechanical propertiesPart of:
The seed capsules (follicles) of some Banksia species, a plant genus native to Australia, do not release their seeds immediately after maturation but remain closed for up to two decades before they open upon the heat of a bushfire. Their seeds are released subsequently into an environment favorable for germination. For seed protection and final release follicles must stay intact and functional – properties desirable to incorporate in various materials. However, follicle properties are only partly understood. Our investigations aim to determine structural, thermal, hygroscopic and mechanical properties of selected follicle tissues by WAXS, experiments on hygroscopicity and tensile tests. The data is essential for a fundamental understanding of both initial opening and seed release which occurs upon several wetting and drying cycles. The experiments revealed differences between pericarp sections. Within the endocarp cellulose fibrils are oriented rather along the longitudinal follicle axis, in the mesocarp their orientation is shifted by approx. 70°. Experiments on the hygroscopic behavior of longitudinal pericarp sections show increasing swelling from the follicle inside to the outside, even within the mesocarp. The tensile Young’s modulus of both dry and wet tested samples revealed large differences between endo- and mesocarp and also between the moisture-states. These values largely exceed those of other lignocellulosic materials. We hypothesize that a moisture-induced softening particularly of the endocarp promotes the complete opening of the capsule for seed release after a bushfire. The presented data describe large effects of humidity on tissue properties. It might be groundwork for bioinspired materials with regard to composite structures and durable functionality originating from Banksia’s natural reproduction mechanism.