Comment:
recommendation of W. T. Stearn, Botanical Latin ed. 4: 261. 1992 followed in retaining original spelling: "The initial ρ of a Greek word transliterated as rh, e.g. rhiza (ριζα, root, rhizome), should have an additional r added to it when preceded by a vowel, e.g. Glycyrrhiza (γλυκυρριζα, liquorice, from γλυκος, sweet, ριζα, root), leptorrhiza (λεπτορριζα, with a thin root, from λεπτος, small, slender), but as many reputable authors, Linnaeus among them, have omitted this additional r, it is best regarded as optional and an author's original spelling, as in Lemna polyrhiza, should be accepted."