NUCLEOLONEMA AND PARS AMORPHA IN ROOT MERISTEMATIC CELLS OF VICIA FABA

Abstract

In osmium-fixed, Epon-embedded root meristematic cells of Vicia faba stained according to the Feulgen – methylene blue procedure, the interphase nucleolus is seen to consist of a metachromatic cabbage-green colored material in which are embedded numerous unstained vacuole-like structures of varying size. By using an orange Wratten filter in the illumination system, it is observed that the metachromatically stained nucleolar material is, in fact, made up of two components, each segregated into zones distinguishable by differences in staining intensity. The first more lightly stained component is found in the central and bordering regions of the nucleolus as well as in a number of areas of varying width extending radially in between them. The second more intensely stained component is observed in areas of varying length and width usually occupying the intermediate regions of the nucleolar mass. Under electron microscopy, the first component is seen to consist of densely packed fibrils and granules some 150 Å in diameter; in places, those constituents are assembled into seemingly coarse, thread-like structures approximately 0.1 μ in diameter. The second component, on the other hand, is made up mainly, if not exclusively, of tightly packed fibrils 60 to 100 Å in diameter. Judging from their topographical distribution and ultrastructural characteristics, it would seem as if the two nucleolar components just described correspond respectively to what is usually referred to as nucleolonema and pars amorpha in animal nucleoli. In addition to nucleolonema and pars amorpha, two ultrastructurally and topographically distinct types of vacuole are recognizable within the interphase nucleolus of Vicia faba. The vacuoles of the first type are usually large or medium-sized, contain loosely and uniformly scattered fibrils and granules approximately 150 Å in diameter, and are observed exclusively within the nucleolonematic zones of the nucleolus. The vacuoles of the second type are small, contain only loosely dispersed fibrils, 60 to 100 Å in diameter, and are confined to the pars amorpha of the nucleolus.