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

The root patterns of Rumex palustris (Sm.), Rumex acetosa (L.) and Plantago major (L.) ssp. major, three species occurring in the river forelands, were studied in experimentally waterlogged or drained compacted soils and compared with specimens growing in drained loosely packed-soils as a control. A modified method for endoscopy in root boxes was developed. The species studied showed different patterns of root development as a result of soil waterlogging or compaction. R. palustris was the least sensitive to waterlogged soils, as shown by the formation of new, morphologically distinctive roots; R. acetosa was the most sensitive and P. major had an intermediate response. With respect to soil compaction P. major was the least affected species, followed by R. acetosa and R. palustris, respectively. The fractional root porosity of these species was studied by using a flow-through system to create hypoxia, a small soil-pore diameter or a combination of both. Hypoxia resulted in a higher root porosity. In both Rumex species small soil-pores inhibited this increase. Contrasting results were found for the porosity of Plantago roots. Results are discussed in relation to the distribution of these species in the field.

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