Some Cytological Aspects of the Differentiation of Metaxylem in the Root ofAllium Cepa

Abstract

SUMMARYA cytological study was accomplished on the immature metaxylem of Allium cepa roots in the histological region between 500 and 3000 μ from the tip. Labelling experiments with 3H-thymidine, 3H-uridine and 3H-lysine were also carried out.The results show that in metaxylem maturation four morphologically detectable stages can be distinguished: 1) Cells close to the region of the apical initials undergo a process of chromosome endoreduplication.2) Cells of the histological region between 800 and 1200 μ from the root tip show amplification of the genes of the nucleolar organizing region.3) Cells of the histological region between 1200 and 2000 μ from the root tip show extrusion of the nucleolus from the nucleus (about 30% of cells show the phenomenon). Nucleoli, either extruded or not extruded, incorporate 3H-thymidine in their peripheral area in concomitance with chromatin incorporation. On the contrary, the inner part of the nucleolus is active in DNA synthesis when chromatin is almost inactive.4) After nucleolar extrusion, nuclei are observed either: i) in the despiralized chromatin condition and without any nucleolus, or ii) with chromatin accumulation around the nucleolus. Both types of nuclei show the presence of micronucleoli, containing both DNA and RNA.The results of 3H-uridine and 3H-lysine incorporation are also reported.