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Summary

The cytological changes occurring in dormant embryo radicle meristem cells of Fraxinus excelsior during the period of chilling before germination are described. The only events observed to be dependent on low temperature occurred in the nucleolus, which increased in volume and complexity of structure. A large, low temperature-dependent increase in total cellular RNA occurred during chilling, and autoradiography showed preferential incorporation of RNA precursor into the nucleolus during this treatment. Other changes observed included the formation of smooth ER, the development of plastid inclusion bodies, and the formation of cytolysomes. As these changes also took place in control embryos maintained at 22° C, they were probably not directly concerned in the relief of dormancy by chilling. Certain deleterious changes are discussed in relation to the stress which may be experienced by the embryo during dormancy.