Boraginales are typically characterized by a dense cover of stiff, mineralized trichomes, likely providing protection against herbivores. These trichomes have long been known to be mineralized with silica.
Mustafa et al. investigate detailed biomineralization patterns in representative species from nine families of the order Boraginales by using cryo-SEM and EDX element analysis. Three zones (tip, shaft, and base) of the trichomes showed distinct mineralization patterns: (i) tips contained either silica or calcium phosphate; (ii) shafts usually contained calcium carbonate, or calcium phosphate in few cases; (iii) bases often had additional silica deposits. A comparison with the phylogeny of Boraginales reveals a striking phylogenetic signal in the details of biomineralization patterns.
Mustafa, A., Ensikat, H.-J., & Weigend, M. (2018). Mineralized trichomes in Boraginales: complex microscale heterogeneity and simple phylogenetic patterns. Annals of Botany, 121(4), 741–751. https://doi.org/10.1093/aob/mcx191