RESUMEN
Seedlings of Crepis capillaris were irradiated after pulse-labelling with tritiated thymidine ([3-H]TdR), and both chromosomal aberrations and presence of silver grains were recorded in the same metaphase cells at various intervals throughout the whole mitotic cycle. The following results were obtained: (a) irradiated roots were homogeneous with respect to the number of aberrations, and heterogenous with respect to labelling index (LI); (b) time--effect curves for labelled (L) and unlabelled (U) cells showed no significant difference from one another; (c) no significant quantitative difference of aberration spectra produced in S and G2 stages was found. These results support the view that the major factor which determines both quantitative and qualitative variation in the production of chromosomal aberrations by radiation is the time lapse between irradiation and fixation rather than relation of the time of irradiation to the time of DNA synthesis. In addition, it was found that labelling with [3-H]TdR modifies the effect of radiation on chromosomes.