LOCALIZATION OF CELL DIVISION ACTIVITY IN THE PRIMARY THICKENING MERISTEM IN ALLIUM CEPA L

Stems 1, 2, 3 months old of Allium cepa L. were labelled with tritiated thymidine, fixed in FAA, sectioned, stained with the Feulgen reaction, and prepared for autoradiography. The serial transverse sections were outlined with a camera lucida, recording labelled nuclei as dots. These drawings were used for 3-dimensional reconstructions of the locations of labelled nuclei. Near the top of the stem, labelled nuclei occur in a broad band, whereas they occur in narrower bands at successively lower levels in the stem, and finally labelled nuclei disappear. The locations of the labelled nuclei correspond to the location of the primary thickening meristem (PTM) in the stem of onion as determined by previous histological and histochemical observations. Microspectrophotometry was used to measure the relative amounts of DNA in Feulgen-stained nuclei of the PTM in serial transverse sections of 1- and 2-month-old onion stems. A bimodal distribution was obtained which can be explained by changes in DNA levels during the cell cycle. No evidence of polyploid nuclei was observed. One can conclude, therefore, that the PTM is the site of cell division activity during the primary stem thickening process in onion.