Stomata length is a reliable characteristic for distinguishing infraspecies and ploidy levels of Opuntia mesacantha (Cactaceae)

Opuntia mesacantha includes two subspecies, O. mesacantha subsp. mesacantha (tetraploid) and O. mesacantha subsp. lata (diploid), that are difficult to distinguish in the field. We show that (1) stomata length is effective for distinguishing the two subspecies, and (2) this can be visually assessed, reliably and non-destructively, using a microscope with a reticle (ocular data). We compare our ocular results with digital imaging and chromosome counts from mitosis in root tips to confirm that our approach is effective for these taxa.