Distribution of cystoliths in the leaves of Acanthaceae and its effect on leaf surface anatomy

Cystoliths are large outgrowths of cell wall material and calcium carbonate with a silicon-containing stalk found in the leaves, stems and roots of only a handful of plant families. Each cystolith is contained within a cell called a lithocyst. In leaves, lithocysts may be found in the mesophyll or the epidermis. A study by Koch et al. (2009) reported unique, indented features on the surface of superamphiphilic Ruellia devosiana (Acanthaceae) leaves which the authors named ‘channel cells’. We report herein that such ‘channel cells’ in the Acanthaceae are actually lithocysts containing fully formed cystoliths in which only a portion of the lithocyst is exposed at the epidermis, forming a leaf epidermal impression. Intact leaves and isolated cystoliths from 28 Acanthaceae species (five in the non-cystolith clade and 23 in the cystolith clade) were examined using light and scanning electron microscopy and X-ray microanalysis. All 23 members of the cystolith clade examined contained cystoliths within lithocysts, but not all showed leaf epidermal impressions. In four species, the lithocysts were in the leaf mesophyll, did not contact the leaf surface, and did not participate in leaf epidermal impression formation. The remaining 19 species had lithocysts in the epidermis and possessed leaf epidermal impressions of differing sizes, depths and morphologies.