*3.1. Analysis by FTIR*

Figure 2 shows the spectra of the four tissues analyzed from the five species. In all spectra, the characteristic calcium oxalate crystals peaks were present (Table S1) in all species and in the four tissues. Furthermore, in all the tissues of the five species, a weak peak at 915 cm−<sup>1</sup> and a wide peak ranging from 3000 to 3600 cm−<sup>1</sup> due to the OH stretching were present and reflected the occurrence of dihydrate calcium oxalate (CaC2O4·2H2O). This dihydrate calcium oxalate was detected in *Astrophytum asterias*, *Echinocactus texensis* and *Mammillaria sphaerica* with the peak 517 cm−<sup>1</sup> (Figure 2A,E,I), although there is noise on the spectra. The peaks between 1014 and 1048 cm−<sup>1</sup> in some tissues of the five species studied were assigned to opal and silicates (Figure 2B,D,F,H,J), whereas some peaks in the pith of *Ariocarpus retusus* subsp. *trigonus* were assigned to calcium carbonate (Figure 2C,D). The peaks at 555 and 560 cm−<sup>1</sup> could represent some contaminants or unknown biomineral residue (Figure 2A–J). The occurrence of primary cell wall debris was assigned to hemicellulose residues in

the pith (4) of *Ariocarpus retusus* subsp. *trigonus* (Figure 2D) and to cellulose residues in the pith (4) and epidermis-hypodermis (1) of *Astrophytum asterias* (Figure 2A,B), *Ariocarpus retusus* subsp. *trigonus* (Figure 2C,D) and *Mammillaria sphaerica* (Figure 2I,J).

**Figure 2.** Spectra of biominerals in each tissue studied. The numbers below each spectra (line) indicate the wavenumbers of the peaks. A, C, E, G, I 4000–400 cm−<sup>1</sup> spectra. B, D, F, H, J 4000–650 cm−<sup>1</sup> spectra. (**A**,**B**) *Astrophytum asterias*. (**C**,**D**) *Ariocarpus retusus* subsp. *trigonus*. (**E**,**F**) *Echinocactus texensis*. (**G**,**H**) *Mammillaria melanocentra* subsp. *rubrograndis*. (**I**,**J**) *Mammillaria sphaerica*. Tissues: 1(black line) = Epidermis-hypodermis, 2(red) = Cortex, 3(purple) = Vascular cylinder, 4(green) = Pith, W = weddellite (calcium oxalate dihydrate), O = opal (amorphous silica), C = Calcium carbonate, S = silicates.
