*2.3. Field Emission Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray Spectrometry (EDS) Analysis*

For FE-SEM observations, samples from pith to epidermis of stems (<1 cm) per species were placed between to coverslips and dried in an oven (56 ◦C) overnight. The dried samples were fixed to aluminum specimen holders with double-sided tape and coated with gold in a Hitachi-S-2460N sputter coater. The coated samples were then observed using a Field Emission Scanning Electron Microscope (FE-SEM; JEOL-JSM7800, JEOL Ltd., Akishima, Tokyo, Japan) (20 Kv) coupled to Energy Dispersive X-ray Spectrometry (EDS; Oxford X-Maxn<sup>2</sup> 50 mm2, Oxford Instruments, Tubney Woods, Abingdon, UK) at the Physics Institute, UNAM. It was calibrated with copper and standards were: C (Cvit), O (SiO2), Mg (MgO), Al (Al2O3), Si (SiO2), K (KBr) and Ca (Wollastonite). The relative concentration of the element is given in percentages of weight.
