4.1.2. Silica–Amorphous Hydrated Silica

Zancajo et al. [34] and Corrales-Ureña et al. [35] identified the vibration of the Si-O-Si bonds at 1090 and 1020 cm−1, in amorphous silica bodies (phytoliths) from *Sorghum* and *Ananas comosus*. In *Echinocactus texensis*, amorphous silica bodies were found in the EH spectrum region, with the vibration of the Si-O-Si bond at 1014 cm−<sup>1</sup> (Figure 2D). These silica bodies were distinctive in the anatomical sections of the hypodermis (Figure 3E,F). Anatomically, silica bodies were described and characterized from the visualization of stem sections [20,21,36]. In their chemical composition, just one previous study analyzed with Raman spectroscopy the presence of amorphous silica bodies in three *Opuntia* species and *Stenocereus thurberi* (Engelm.) Buxb. [36] and identified differences in their structural composition, mainly in the SiH bonds. Therefore, with Raman spectroscopy, the structure of the phytoliths could be analyzed in future studies. We consider that these amorphous silica bodies are the product of controlled biomineralization [36,37] and should be considered as a taxonomic character.

The EDS analysis supports the presence of Si in the hypodermis of *Echinocactus texensis* with the highest concentration, while the concentrations of Ca and Mg were lower. Although the precipitation phases of hypodermic biominerals were not studied in *Echinocactus texensis*, Si and Mg may be the first phase of biomineral precipitation along with some form of calcium carbonate or amorphous calcium oxalate (both have carbon, oxygen, and calcium), as in *Ficus microcarpa* L.f and *Morus alba* L. cystoliths. In both Moraceae, Si and Mg precipitate in the first phase of biomineral development, and amorphous calcium carbonate is deposited later [38]. Biominerals of *Echinocactus texensis* precipitate in cells with a small lumen (20 μm on average) compared to *Ficus* idioblasts that exceed 100 μm [38]. Density was not quantified in *Echinocactus texensis* but they are quite abundant in hypodermis strata. For these reasons, we consider that the silica bodies differ from the cystoliths and the calcium carbonate does not precipitate.
