*3.2. Anatomy with Light and Polarized Microscopy*

The five species studied have the typical stem anatomy for the Cacteae. There is an epidermis with straight or convex outer cell wall and a hypodermis of one to five strata of parenchyma or collenchyma (Figure 3A,E and Figure 4A). Both cortex and pith have exclusively parenchyma cells (Figure 4A) and in the vascular cylinder, nonfibrous wood with wide medullary unlignified rays (Figure 4F–H). The studied species do not form visible biominerals in the epidermis (Figure 3). Two of the five species studied here had biominerals in the hypodermal cells. *Astrophytum asterias* presented prismatic biominerals (Figure 3A,C) that were birefringent under polarized light (Figure 3B,D) and *Echinocactus texensis* had amorphous silica bodies that lacked birefringence under polarized light (Figure 3E,F). Biominerals in the hypodermal cells of *Ariocarpus retusus* subsp. *trigonus* and both *Mammillaria* species were not detected in the anatomical sections.

**Figure 3.** Biominerals in the hypodermis of stem in Cacteae, transverse sections (TS) and isolated (IS). (A, C, D, E, F) Bright field. (B, D) Polarized light. (**A**,**B**) *Astrophytum asterias* (TT846), TS. (**C**,**D**) *Astrophytum asterias* (TT1020), IS. (**E**,**F**) *Echinocactus texensis* (TT859), TS. Bar is 50 μm in A, B, E; 20 μm in C, D, F. e = epidermis, co = cortex, h = hypodermis, white arrows = amorphous silica bodies.

In the cortex (Figure 4A,B,D), vascular cylinder (Figure 4F–H) and pith (Figure 4I,J), all species had spherulites or prisms, which were birefringent with polarized light (Figure 4C,E,G,L,N,P). In the vascular tissue, the biominerals were deposited in the ray cells (Figure 4F,H).

**Figure 4.** Biominerals in the stem tissues of transverse sections (TS) and isolated (IS) using bright field (b) or polarized light (p). (**A**) *Ariocarpus retusus* subsp. *trigonus* (TT1005), TSbAR, epidermis, hypodermis (orange arrowhead) and cortical tissue. (**B**,**C**) *Astrophytum asterias* (TT846), TSbpMR, spherulites in cortical cells. (**D**,**E**) *Astrophytum asterias* (TT1020), IS, spherulites of cortical tissue. (**F**,**G**) *Echinocactus texensis* (TT859), TSbpBR, spherulites in vascular cylinder. (**H**) *Mammillaria sphaerica* (TT1051), TSbBR, vascular cylinder. (**I**) *Ariocarpus retusus* subsp. *trigonus* (TT879), TSbAR, pith. (**J**) *Echinocactus texensis* (TT859), TSbBR, spherulites (black arrows) in pith. (**K**,**L**) *Ariocarpus retusus* subsp. *trigonus* (TT1005), ISbp, spherulites of cortical tissue. (**M**,**N**) *Mammillaria melanocentra* subsp. *rubrograndis* (TT1050), ISbp, spherulites of pith. (**O**,**P**) *Mammillaria sphaerica* (TT1051), ISbp, spherulites of cortical tissue. Bar is 300 μm in A, B, C, F–J; 100 μm in M, N; 50 μm in D, E, K, L; 20 μm in O, P. AR = apical region; MR = medium region; BR = basal region; X = tracheary cells; \* = rays.
