In basal monocots (e.g. Acoraceae, Alismatales s. APG 1998) it is difficult to unravel the ancestral features if floral morphology by comparison with their outgroups (magnoliids) and more derived clades of monocots. Two general types occur:1) flowers with distinct sepals and petals, often two or several whorls of stamens and carpels, and intercarpellary nectaries (e.g. Alismataceae, Butomaceae, Hydrocharitaceae, Tofieldiaceae);2) flowers with an undifferentiated perianth consisting of tepals, one or two whorls of carpels, which are often centrally fused and lack intercarpellary nectaries (e.g. Acoraceae, Araceae, Aponogetonaceae, Juncaginaceae, Potamogetonaceae, Scheuchzeriaceae).Type 1 is most similar to flowers in magnoliids such as Annonaceae, Aristolochiaceae, and monocots such as Dioscoreaceae and Liliaceae, whereas type 2 is similar to magnoliids such as Saururaceae and Chloranthaceae. In current phylogenetic analyses Acoraceae are sister to all other monocots, and Aponogetonaceae are basal within Alismatales. Therefore, the second type may represent a suite of morphologically plesiomorphic character states. However, because these basal clades stand on long branches, the actual features may as well have undergone extensive anagenesis, and therefore represent autapomorphies.To elucidate floral evolution in these basal monocots, a better understanding of perianth organ identity, differentiation and development is critical. Some of the genes responsible for the differentiation of the perianth are transcription factors of the MADS-box gene family, known as B-class genes: orthologs of APETALA3 and PISTILLATA in Arabidopsis, and DEFICIENS and GLOBULOSA in Antirrhinum. Expression of B-class genes in perianth organs of basal monocots exhibiting a morphologically undifferentiated perianth would be consistent with a petaloid developmental program for these organs, even though the perianth may appear bracteolar or sepalous in later stages of development. B-class genes in the genus Triglochin (Juncaginaceae) are characterized and the application of reverse transcription in situ PCR (RT-ISPCR) is discussed.