During their first hours of germination plant seeds are often used to study the effects of physical factors, mutagens, herbicides, and othcr chemical compounds with growth-regulating activity. Some changes in seed sensitivity to these effects, depending on the stage of germination, have been observed in many investigations. Thus, the exposure of Crepis capillaris seeds to )I-radiation at 4 h of germination resulted in 37-39% chromosomal aberrations, whereas similar exposure at 5 and 10 h (before the beginning of the S-phase in root meristematic cells) resulted in only 7 and 16% aberrations, respectively . Similar changes have been found in experiments with 5-fluoro-2-deoxyuridine , ethylenimine , and thiophosphamide . The seeds of Brassica nigra were sensitive to fungicide action only after 6 h of germination , and maximal far red inhibition of tomato seed germination occurred after exposure at 8 h . The reasons for these seed sensitivity modifications are still not clear.