We have identified the wood species used for the wooden kris sheaths based on the anatomical characteristics of samples of the sheaths by SRX-ray μCT. All of the samples were identified to be hardwood species (e.g., Dysoxylum spp., T. indica, and K. hospita) (Table 1). This is not surprising, because only a few species of softwood belong to the families Araucariaceae, Pinaceae, and Podocarpaceae grown in Southeast Asia and the Pacific [12]. The wood species used for making sheaths 1, 2, and 5 contained septate fiber, which is commonly observed in the genus Dysoxylum, family Meliaceae. Several species in the Meliaceae family, including some in the genera Entandrophragma and Guarea, have nearly identical anatomical features as Dysoxylum. However, based on their geographical origin, the samples are most likely from the genus Dysoxylum. Furthermore, Dysoxylum acutangulum has traditionally been utilized as kris sheath material [17]. Septate fiber is a type of fiber containing thin transverse primary vessels with a thin transverse primary vessel wall [10, 18]. Another characteristic of this genus is the appearance of banded axial parenchyma [12]. In sheaths 3 and 6, one of prominent characteristics of T. indica was the appearance of gum or other deposits in the vessels. In addition, crystals were present in the chambered axial parenchyma. The existence of tile cells for sheaths 4 and 7 is one of the key characteristics of K. hospita. Tile cells occur in only 1% of hardwoods [19], and they are an important characteristic in distinguishing the Malvales members. The type of tile cells of this species is Durio type. The tile cells in sheaths 4 and 7 had the same height as the procumbent cells [10].
In this study, part of the sample observed was approximately 1 mm in both diameter and length. With a small amount of sample, information about the wood porosity and growth-ring morphology that is important for wood identification may not be obtained [5]. However, by utilizing SRX-ray μCT, we could obtain features such as perforation plate, morphology of pits, septate fibers, tile cells etc. that can take time and effort to obtain using conventional sectioning method. When using the sectioning method, some iterations are required to get appropriate sections to observe such features. However, these iterations will be difficult to apply due to limited sample availability. For SRX-ray μCT, infinite repeated sectioning is possible. Therefore, compared to traditional sectioning, SRX-ray μCT is better to have more successful wood identification with minimum sample size, while keeping the sample is still intact.
A kris sheath is typically made of selected materials. A high-quality kris sheath is usually made of fine, esthetic, rare, and expensive wood [13, 15, 17, 20]. Several wood species are often used for kris sheaths, such as Santalum album, K. hospita, Dysoxylum acutangulum, Wrightia javanica, Melia azedarach, Murraya paniculata, Ficus septica, Dalbergia latifolia, Mesua ferrea, Tectona grandis, Cassia siamea, Pterocarpus indicus, T. indica, and Cassia laevigata [13,14,15,16,17]. Sheaths 4 and 7 were K. hospita with dark-brown stains on their surfaces (Fig. 1e and h). The appearance of a dark-brown stain was one of the considerations for selecting K. hospita [17]. Sheaths 3 and 6, which were made of T. indica, also had dark-brown stains on their surfaces. Although K. hospita species can be identified through the appearance of a dark-brown stain, to avoid misidentification, observation of the wood anatomy is more accurate to distinguish the wood species. Meanwhile, Dysoxylum acutangulum, also known as trembalo (local name in Indonesia), is preferable, because this species has a beautiful grain.
In general, xylem tissue and the surrounding air provide sufficiently fine contrast when observed by SRX-ray µCT. Because mineral inclusions have higher density than xylem tissue, they can be identified in xylem cells [21]. By performing simple segmentation, we reconstructed the mineral inclusions with the prismatic crystal shape in volumetric space. The crystal may have some functions, such as protection against insects, detoxification of toxic substances, tissue mechanical support, as well as light gathering and reflectance [21,22,23,24,25,26]. As previously mentioned, we observed two different patterns of the crystal distribution. First, on T. indica, the crystals in longitudinal alignment in the axial parenchyma cells around the vessels as if they were pile foundation on a building structure. Second, radial short and long series of crystals were observed on K. hospita as if they were beams of load bearing wall on a building structure. Similar observation was conducted on bark structure and suggested that the presence of abundant prismatic crystals contributed as mechanical reinforcement to prevent compression fracture [27]. The spatial distribution of the crystals may influence the mechanical function to strengthen the structure of the vessel for water conduction in many species with axial parenchyma arrangements that encircle the vessel entirely (vasicentric, aliform, and others), mainly belonging to the family Fabaceae, including T. indica. The radial alignment of the crystals in K. hospita may be useful in strengthening the structure of the ray cells.