Comparative inflorescence and floral ontogeny in the genus Mentha (Mentheae: Nepetoideae: Lamiaceae): variable sequences of organ appearance and random petal aestivation

The genus Mentha is a taxonomically complex genus, characterized by large morphological variations. Only a few, frequently overlapping, characters are of value in taxonomy. Comparative floral developmental studies provide an opportunity for better understanding the systematic relationships among different species. The inflorescence and floral ontogeny of three Mentha L. species (M. piperita L., M. pulegium L. and M. suaveolens Ehrh.) were investigated using epi-illumination light microscopy. All three species studied have thyrses with the same developmental pathway. The lack of higher order bracts and the monochasial branching of the higher order inflorescence apices were found as special features of inflorescence ontogeny. Sepals appear unidirectionally from the adaxial side in all except for M. pulegium which shows a modified unidirectional sequence. Variable sequences of petal and stamen appearance from unidirectional to reversed unidirectional sequence are present in all. Significant ontogenetic features include (1) appearance of the corolla as a rim before petal lobes become visible and (2) instability in petal aestivation. Morphological features including densely hairy calyx, five-lobed corolla tube, smaller adaxial stamens and hairy ovary with included style distinguish M. pulegium from the other species. On the basis of our results floral ontogenetic features could be considered important characters for delimiting or diagnosing different sections in the genus Mentha. Variable sequences of petal lobe appearance and instability in petal aestivation were found as unusual developmental characters.