AbstractFontinalis squamosaandFontinalis antipyreticaexplants were cultured with 0, 10−4, and 10−3M ACC, an ethylene precursor. Synlptoms of ethylene effects included truncated apices, loss of growth capability in the apical bud, colour changes, leaf undulations, inhibition of rhizoid production, and crumpled branches and leaves. The two species responded differently to treatment, and a mode of control of morphological expression relating to currently used taxonomic characters is implicated.