Acrocomia is a genus of palms which is native to the Neotropics , ranging from Mexico in the north, through Central America and the Caribbean , and through South America south to Argentina. Acrocomia is a genus of spiny , pinnate -leaved palms which range from large trees to small palms with short, subterranean stems. The species bears branched inflorescences which are located among the leaves. The unisexual flowers ; female flowers are born near the base of the inflorescence, while male flowers are borne towards the tips. Fruit are large, single-seeded, and vary in colour from yellow, to orange, to brown.
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A Nature Research Journal. Fruit and nut shells can exhibit high hardness and toughness. In the peninsula of Yucatan, Mexico, the fruit of the Cocoyol palm tree Acrocomia mexicana is well known to be very difficult to break. Here we report the mechanical properties, microstructure and hardness of this material. Our findings reveal a complex hierarchical structure showing that the Cocoyol shell is a functionally graded material with distinctive layers along the radial directions.
These findings demonstrate that structure-property relationships make this material hard and tough. The mechanical results and the microstructure presented herein encourage designing new types of bioinspired superior synthetic materials.
Natural materials can exhibit outstanding mechanical properties that are often superior to those of their constituents 1. These remarkable properties found in biological materials from both plants 2 , 3 and animals 4 are the result of evolutionary developments and architecture optimization leading to high-performance lightweight materials, made of relatively weak and mundane constituents 5 , 6 , with complex hierarchical structure and topography 5.
Classic examples of natural materials with exceptional performance of mechanical properties made of limited constituents include seashells, wood, bamboo, bone and teeth 7 , 8 , 9 , In recent years, the study of biological materials to understand their mechanical properties and microstructure is growing at a fast rate 11 with the aim to develop sources of inspiration for the design of superior synthetic materials 12 , 13 , 14 , Bioinspired material design brings the possibility of developing new multifunctional materials with diverse technological applications 1 , 16 , However, a deep understanding towards structure-property relationships at different length scales, to unveil the underlying governing mechanisms and design principles of biological materials, is needed 16 , Plants are an excellent source of inspiration for biomimetic materials 19 , 20 because they can exhibit desirable mechanical properties such as high bending stiffness and toughness 21 , 22 , 23 , impact resistance 24 , and hierarchical multifunctional structures Recent studies have been focusing on the mechanical performance and microstructure of the hard shell of nuts 26 and fruit seeds 27 , which are known to protect the seed against predators 27 , for the development of impact-resistant and energy-absorbing materials Some studies have been performed to measure the force required to break nut and fruit shells in order to understand fracture behaviour 29 , 30 , 31 , 32 ; however, fracture resistance may be influenced by the size and shape of the shell 33 and thus more studies are needed to understand the intrinsic mechanical properties of the material.
More complete studies of the endocarp of Macadamia nuts to understand the high toughness and hardness observed in the shell of this biomaterial have been performed It was found that the hierarchical cellular structure is responsible for its good mechanical performance 34 , 35 , Nanoindentation of the seed coats of tropical plants have shown high hardness and a complex hierarchical structure in these materials The Cocoyol or Coyol Acrocomia mexicana Karw.
In recent years, the fruit of this palm as Acrocomia aculeata has been extensively studied because of the great potential for the production of solid biofuel 39 , 40 due to its high content of cellulose and lignin The hardness of Cocoyol endocarp as Acrocomia aculeata has been mentioned only anecdotally in the scientific literature 42 , 43 , 44 and in personal narrative books 45 , Stephens in during his visit to the Maya ruins of Uxmal 48 , 49 , in which the Cocoyol hardness is mentioned.
The Governor thought that the Dwarf would die before it was his turn in the challenge; however, the Dwarf survived the challenge and the Governor died when the Cocoyol fruits were smashed in his head. The spectators hailed the Dwarf as the new Governor of Uxmal.
While some parts of the legend are very questionable, the questions that remain out of curiosity are: Is the Cocoyol fruit very hard indeed? In this work, this is addressed with an extensive study of the microstructure and mechanical properties of the Cocoyol endocarp to unveil structure-property relationships that improve our understanding of the hard fruit shell.
We start by analysing the microstructure in the radial direction using optical microscopy. The cross section of the fruit shows its constituent parts. The cross section of the endocarp shows that its microstructure consists of sclerenchyma cells.
Specimens used for microscopy studies and mechanical characterizations obtained from the endocarp are shown in Fig. The average density of cylindrical samples is 1. Cocoyol fruit and endocarp specimens. Cross sections of the fruit endocarp were polished and then investigated using optical microscope at two different magnifications, as detailed in the Methods section.
Typical optical images of cross sections in both equatorial plane Fig. The microstructure consisting of sclerenchyma cells does not vary systematically with the radial position; however, two main layers of cells outer layer and inner layer in Fig. The transition between these two layers is gradual; however, some distinctive features can be identified for each layer.
In the outer layer, which is near the external edge, the cells have polygonal shape Fig. Some of the spheroid cells in the inner layer are elongated cells perpendicular to the scanned area, as shown by SEM and micro-CT images in Fig. Both spheroids and elongated cells appear to be grouped in bundles, which is more obvious in the area closer to the inner edge, where it seems that the bundles of elongated cells are better aligned with the ring inner edge and the length of these cells is larger than those cells in the top part of the inner layer.
Optical micrographs of Cocoyol fruit endocarp. The brighter contrast indicates distribution of cells and darker regions indicate cell lumina and space between cells.
From the 2D micrographs, the measured diameter of the cells is between microns and they have lengths of cell diameters. It can be seen in Fig. Also, groups of spheroid cells are observed, which may be elongated cells perpendicular to the cross section Fig.
A similar microstructure with a similar size range has been reported for Macadamia shell 34 , 35 , 36 ; however, that type of shell contains more hollow space, i. One cuboid per location obtained from a single nut was used. The representative density of the selected sample near the inner edge location 1 , in the middle location 2 , and near the outer edge location 3 was 1. Here, we report one measured density per location. The three cuboids were obtained from the same slice of a single ring specimen, as shown in Fig.
This apparent gradual increase of density from the inside to the outside surface of the endocarp, with a gradient of density of 0.
It is noted that the measured difference of density between cuboids is within the range of experimental error due to the resolutions of the analytical balance and the corresponding volume measurement, and further studies are needed to correlate the density gradient to the mechanical property gradient; however, this density gradient trend is consistent with the stiffness gradient and cell packing discussed later.
As mentioned before, it seems that several cells are isodiametric or spheroids Fig. It is noted that although micro-CT resolution is insufficient to quantitate certain cell features such as primary and secondary wall thicknesses, it is sufficient to qualitatively distinguish the 3D cell shape and orientation. If we focus on the corners of the scanned volume, it is revealed that the cellular structure is indeed a complex entanglement of bundles of elongated cells Fig.
For instance, a cell in the front plane looks like a polyhedral cell while the same cell in the side plane looks like an elongated cell. Specimens have a height to diameter ratio of height 3. Typical uniaxial compression true stress-true strain curves. Different energy-absorbing mechanisms can be identified from this image: A. Cell tearing which occurs when the crack runs through a cell or a cell bundle; B.
Middle lamella breakage which occurs when the primary cell wall remains fairly intact. This resembles somehow delamination between cells; C. Primary cell wall breakage, which occurs when primary wall is severely damaged. This failure mechanism C is interesting because it seems that the middle lamella remains fairly intact, indicating that the intercellular bonding is very strong.
It can also be observed in Fig. It is also noted that because of the difficulty to differentiate between primary wall and the outer layers of the secondary wall, failure mechanism C may include in some cases the breakage of some external layers of the secondary wall. From Fig. SEM images of crack surfaces. Identified failure mechanisms include A. Cell tearing; B. Middle lamella breakage; C. Primary cell wall breakage; D. Pull-out of elongated cells.
The mechanical properties obtained from compression tests are shown in Table 1. Toughness work of fracture is defined as the area under the stress-strain curve up to failure. The fracture surface is coplanar but rough. Some of the failure mechanisms identified are similar to those observed in compression, i. Pull-out D of elongated cells is also observed. In areas where cells are mainly polyhedral near the outer edge Fig.
In areas where elongated cells are grouped into bundles near the inner edge and in the middle section, Fig. It can be seen that the pull-out D of elongated cells Fig. It seems that the damage mechanism is closely related to the cells orientation. The combinations of these mechanisms lead to extended crack paths and thus an increase in toughness is developed. Negligible influence was observed on the measured mechanical properties for different maximum load levels ranging from 10 mN up to mN.
The measured reduced elastic modulus E r and hardness H at different positions are shown for both meridional and equatorial orientations in Fig. It is clear from Fig. This indicates that the structure is fairly homogenous along the circumference of the ring samples.
It can also be seen that both the reduced elastic modulus and the hardness increase slightly towards the outer edge of the Cocoyol ring specimen. This increase is more obvious for the hardness value. The gradients of E r and H were determined from the slope of the linear fits to the data. The gradients of the E r were The gradients of the H were Nanoindentation and Vickers hardness results.
It can be seen that hardness values are in the range of 0. Moreover, a slight increase of hardness from inner edge to outer edge can be observed for the Vickers hardness as well for both meridional and equatorial directions.
Identifying Commonly Cultivated Palms
Acrocomia aculeata is a species of palm native to tropical regions of the Americas , from southern Mexico and the Caribbean south to Paraguay and northern Argentina. Petioles of the leaves are also covered with spines. The flowers are small, produced on a large branched inflorescence 1. The fruit is a yellowish-green drupe 2. The inside of the seed, also called endosperm , is a dry white filling that has a vaguely sweet taste like coconut when eaten. These nuts, which are so hard as to be difficult to break with a heavy hammer, are crushed to a pulp by the powerful beak of this macaw.