Ultrastructural aspects of nucleolar fibrillar centres in meristematic cells ofAllium cepa

This paper deals with the fine structure of the fibrillar centres of the nucleolus inAllium cepacells in ultrathin, sections of in vivo fixed roots. The ultrastructural observations have allowed us to consider each nucleolar fibrillar centre as an active zone in the nucleolar chromatin loop, and to propose a possible model for the organization of the different components of the nucleolus within it.