Seed protection strategies of the brainy Elaeocarpus ganitrus endocarp: Gradient motif yields fracture tolerance

Abstract Be it animals or plants, most of the organism's offspring come into existence after their embryos develop inside a protective shell. In plants, these hard protective shells are called endocarps. They serve the function of nourishing and protecting the seeds from external mechanical damage. Through evolution, endocarps of plants have developed various structural strategies to protect the enclosed seeds from external threats, and these strategies can vary according to the habitat or lifestyle of a particular plant. One such intriguing hard plant shell is the endocarp of the Elaeocarpus ganitrus fruit. It mostly grows in South Asia's mountainous forests, and its endocarps are known in the local communities as unbreakable and everlasting prayer beads. We report an in-depth investigation on microstructure, tomography, and mechanical properties to cast light on its performance and the underlying structure-property relation. The 3D structural quantifications by micro-CT demonstrate that the endocarp has gradient microarchitecture. In addition, the endocarp also exhibits gradient hardness and stiffness. The toughening mechanisms arising from the layered cellular structure enable the endocarps to withstand higher loads up to 5000 N before they fracture. Our findings provide experimental evidence of outstanding fracture tolerance and seed protection strategies developed by Elaeocarpus ganitrus endocarp that encourage the design of synthetic fracture tolerant structures. Statement of significance Endocarps are low-density plant shells that exhibit remarkable fracture resistance and energy absorption when they encounter impact by falling from high trees and prolonged compression and abrasion by the predators. Such outstanding mechanical performance originates through structural design strategies developed to protect their seeds. Here we demonstrate previously undiscovered structural features and mechanical properties of Elaeocarpus ganitrus endocarp. We scrutinize the microstructure using high-resolution x-ray tomography scans and the 3D structural quantifications reveal a gradient microstructure which is in agreement with the gradient hardness and stiffness. The multiscale hierarchical structures combined with the gradient motif yield impressive fracture tolerance in Elaeocarpus ganitrus endocarp. These findings advance the knowledge of the structure-property relation in hard plant shells, and the procured structural design strategies can be utilized to design fracture-resistant structures.