Figure 1.
Fracture forces related to the mean shell thickness for different seed, nut and drupe shells.
The sketches show the loading direction for each species. The bars are mean values, the black lines denote the standard deviations. The cross-head speed was 5 mm/min.
Table 1.
Fracture forces of different seed coats, nut shells and hard inner drupe shells (endocarp, “fruit stone”); all specimens were tested in compression in the as-received state without additional drying with a cross-head speed of 5 mm/min.
Figure 2.
Hierarchical microstructure as observed for Macadamia integrifolia follicles and seeds: The overview in a) shows the different hierarchical levels (H0–H8) and the structural elements they contain.
The coloured boxes link our structural levels with the corresponding classification scheme generally used in biology (organ, tissue, cells, sub-cellular structures). The images in b) give examples for structural units/features of each hierarchical level.
Figure 3.
Photographs of an entire Macadamia integrifolia seed, showing the hilum, the micropyle and the outer suture (a, b).
Light-brown speckles form an individual pattern on the surface of each shell (b). Photographs of seed coats cut normal (c) and parallel (d) to the outer suture show a locally varying thickness of the shell. The “normal” section (c) exhibits a nearly constant thickness, while the shell thickness of the “parallel” section (d) varies. The white structures within the coat material are the vascular bundles. Reconstructed 3D images from CT scans show the density, orientation and branching of the vascular bundles running from the hilum to the micropyle within the seed coat (e, f).
Figure 4.
Schematic representation of how the relative area fractions of certain cell types and of the vascular bundles were analysed.
Several high resolution micrographs (a) were combined (b). The different cell types and the vascular bundles were marked and colour-coded (c). The area fractions were then determined by means of a quantitative image analysis software (d).
Table 2.
Quantitative data (min–max value; mean ± standard deviation) describing the structural features of the Macadamia seed coat at the different hierarchical levels (HL) (fig. 2).
Figure 5.
Sandwich structure of the Macadamia seed coat:
a) schematic illustration While layers L2 to L5.1 are visible at the resolution of the SEM micrograph of a fracture surface shown in (b), layers L1 and L6 may only be discerned at higher magnifications (fig. 6).
Figure 6.
SEM micrographs of the outer- and innermost layers of the Macadamia integrifolia seed coat: a) & b): epidermis, c) & d) inner testa layer.
The one cell-layer thick epidermis is composed of pancake-like cells, which form a smooth surface with many pores on the outside. The inner contour of the epidermis cells follows the shape of the sclereid cells. The inner testa layer is a thin homogeneous layer, which is connected to the cream-coloured or dark brown inner layers. The surface of the inner testa layer shows no pores (d).
Figure 7.
Light (LM) and scanning electron (SEM) micrographs of corresponding sections of the outer surface of the same Macadamia seed coat: a) & b) with natural wax layer, c) & d) after dewaxing.
In both states many pores are seen on the shell’s surface. They appear as dark circular objects in the light micrographs, and are even better visible in the SEM micrographs. The arrows and the numbers denote corresponding pores in the LM and SEM micrographs. The polyhedral surface structure visible in the natural state is less well visible after dewaxing.
Figure 8.
SEM micrographs of the different sclerenchymatous layers.
a) Outer sclereid layer (L2), which is composed of a dense arrangement of polyhedral sclereids; b) the sclerenchymatous fibre layer (L3), which consists of fibrous cells, so-called sclerenchymatous fibres; c) in some regions of the shell, another relatively thin “inner” sclereid layer (L4) was observed, which contains ellipsoidic, kidney- or dumbbell-shaped sclereids; d) the sclerenchymatous fibres are arranged in compact bundles, which are entwined with each other.
Figure 9.
SEM micrographs of Macadamia seed coat fracture surfaces broken normal (a) and parallel (b) to the outer suture.
The “normal” fracture surface (a) is rougher with many sclerenchymatous fibres protruding at different angles. The “parallel” fracture surface (b) is smoother because sclerenchymatous fibres are mainly orientated parallel to the fracture surface. The diagrams in c) show the area fractions of different cell shapes within the sclerenchymatous tissue for sections cut normal or parallel to the outer suture.
Figure 10.
Microstructure of the inner layers and the interfaces between them and to the adjacent layers: Light micrographs of polished sections show the sclerenchymatous and the cream-coloured (L5.1; a, b) and dark brown (L5.2; a, d) inner layers and the interfaces between the different layers.
The cream-coloured layer (a, b) is composed of polyhedral cells with thin cell walls. The SEM micrograph in c) of a fracture surface shows the fine and fibrous microstructure of the cells in the cream-coloured layer. The light micrograph in d) shows the dark brown layer, which is composed of slap-shaped cells with thickened cell walls.
Figure 11.
SEM micrographs of cells in the outer sclereid layer show that they have an isodiametric shape near the outer surface (a) and a more and more ellipsoidal shape with increasing distance from the shell’s outer surface (b). Light micrographs of polished sections show the structural composition of sclereid cells (c) and of sclerenchymatous fibre cells (d), which have a similar microstructure. Both types of cells have thickened and lignified cell walls and a less dense inclusion within their lumen.
Figure 12.
SEM micrographs of vascular bundles within the sclerenchymatous fibre layer:
the vascular bundles are surrounded by sclerenchymatous fibres (a). Each bundle consists of many tube-like cells, so-called spiral vessels and tracheids (b).
Figure 13.
SEM micrographs showing a deflected and branched crack path on different length-scales:
a) fracture surface of an entire Macadamia seed coat after loading in compression; crack deflection took place in three dimensions, as the topography of the fracture surface and the secondary cracks visible on it show. A magnified view of the crack part near the inner surface shows that the crack was deflected at the interface between the inner sclereid layer (L4) and the sclerenchymatous fibre layer (L3). c) The secondary crack stops at the interface between the sclerenchymatous fibre layer (L3) and the outer sclereid layer (L2). d) In the main fracture surface, the crack mostly follows the interfaces between the cells; in this area, however, the secondary crack fractured some sclerenchymatous fibres vertical to their long axis.