A Particulate Component Found in Nucleoli of Allium cepa and Vicia faba

During an electron microscopic investigation of mitosis in the meristematic cells of Allium cepa and Vida faba, a particulate component of distinctive character has been observed in many nudeoli. The most striking feature of these granules in thin sections of osmium-fixed material (1) is their great electron density which makes them stand out dearly from the rest of the nudeolar material (Fig. 1). The majority of such particles are donghnut-like in appearance but, within one field, wide variations in shape are found. They may be round, oval, or rod-like in profile, possibly as a result of the angle of sectioning. In diameter they vary from approTimately 70 A to 200 A, with the majority at about 140 A (Figs. 2 and 3). Wide variations have also been observed in the occurrence of the particles. They may or may not be present in nudeoli which otherwise look identical and which, moreover, belong to nuclei showing similar distribution of chromatin material. They are not found in the less dense areas of nudeoli, and are quite unevenly distributed throughout the rest of the nudeolar material. They occur in pairs and small dusters of three or more. Whereas the above general description applies to nudeoli both from AlUum cepa and Vicia faba material, a number of significant differences have been observed in the distribution of the dense partides in these two plants, especially during certain stages of the mitotic cyde. During interphase (AlUum cepa), nudeoli usually show a few light spaces of various sizes within which no particles are found. There also often occurs a large central area which either appears empty or contains material resembling the chromatin masses in density and fine structural (Fig. 1). In such nudeoli, the particles are always localized in the outer dense material only. In Vicia faba two different types of interphase nudeoli have been found. The most common type