The interphase nucleolus in Allium porrum meristematic cells is characterized by the presence of 1-4 dense fibrillar zones of rather complex organization. Each such zone appears to consist essentially of a convoluted, evacuolated, filamentous structure approximately 1.5 µm in diameter. At the ultrastructural level, these structures exhibit an intricate array of lacunar spaces each of which is surrounded by a dense coating. These lacunae are filled with a loose fibrillar material and the largest ones sometimes also show a dense central core. In appropriate preparations, certain of the peripherally located lacunae are found to be continuous with segments of chromosomes. High-resolution radioautography reveals, moreover, that DNA is present within both the dense and lighter portions of the nucleolar loops. These observations add further support to the hypothesis that the convoluted filamentous structures in question correspond to loops of chromosomal origin and are thus related to the nucleolar organizer.

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