Abstract: The organization patterns of 18S-25S rRNA gene (45S rDNA) in the meristematic cells of ten plants belonging to five families were comparatively analyzed using fluorescence in situ hybridization (FISH).The 45S rDNA probe produced two types of signals in the interphase nuclei of all tested species: strongly fluorescing perinucleolar knobs and weakly fluorescing intranucleolar spots,indicating the existence of similar organization patterns of rDNA chromatin at interphase in higher plants.The spots associated with or emanating from the knobs were observed in a portion of the interphase nuclei in each tested species.It was obvious that the more spots produced,the smaller the knobs became,and the number of spots was positively correlated with the activity of the cell.These findings indicated that the spots resulted from the decondensation of knobs,the decondensation of rDNA heterochromatin was the cytogenetic ma-nifestation of active rRNA gene transcription,and the number of spots represented the degree of transcription activity of rRNA gene at interphase.Our results supported the speculation that these spots were the structural units in the organization of active ribosomal genes and the sites that rRNA synthesis took place.Our results also revealed some differences in the organization of rDNA chromatin at interphase among species.The maximal number of spots in the nucleolus differed considerably among the tested species.In prometaphase cells of each tested species,the rDNA sites displayed incompact signal blocks and many spots by FISH with rDNA probe,indicating an active rDNA transcription during prometaphase.