monomorphic: species of which only one sex (female) is known to exist.
If you can find a dimorphic Oxalis it will be a new point, for all known species are trimorphic or monomorphic.
monomorphic mon·o·mor·phic (mŏn'ō-môr'fĭk) or mon·o·mor·phous (-fəs)adj.
Having only one form, as one crystal form.
Having one or the same genotype, form, or structure through a series of developmental changes.