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It has been shown that when Arabidopsis seeds are incubated with different heteropycnic labelled DNA during the first 4 days of germinating, large fragments of exogenous DNA are taken up by the seedling plant tissues (cotyledons and rootlet) and remain polymerized and double-stranded during the entire growth period. Only a small percentage of the population of exogenous DNA molecules appears to be destroyed and reutilized for DNA synthesis de novo. Most of the exogenous DNA remains free or becomes integrated with the Arabidopsis DNA as double-stranded molecules, not separable by alkali denaturation, but easily separable by ultrasonication.Analysis of Werent tissues and organs of growing plants reveals that a large amount of foreign DNA remains in the cotyledons during the vegetative growth phase, but is redistributed a t the flowering stage (when cotyledons etiolate). At this time, the foreign DNA leaves the cotyledons and migrates towards the flowering buds where it accumulates as a highly molecular and double-stranded material. I n the E", progeny, radioactive molkcules are found which do not correspond to the endogenous material and which have a density depending on that of the foreign DNA used.Arabidopsis thaliana (L.) Heyhn, is a facultative long-day autogamous crucifer (2 n = 10 chromosomes) which has a short life cycle (about 35 days under our experimental conditions), and can be mass cultured in a relatively small area. After the incubation, the seeds were washed and grown aseptically in closed glass vessels on perlite soaked with a mineral medium [5], at 24 "C, under continuous Gro-Lux (Silvana) fluorescent illumination (10000 lux).Plants were harvested a t different growth stages (Fig.1). The organs were excised, frozen and kept a t -30 "C in 20 ml ethanol for several days, which

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