A negative correlation between genome size and AT/GC-ratio in seed plants has been claimed. This assumption was not confirmed by the flow cytometric analysis of 54 plant species belonging to 17 families. Instead of that, partly significant differences between the families with respect to their range of AT/GC-ratios were found but without a phylogenetic coherence above the family level. The flow cytometric analysis of the exact AT/GC-ratio in plants is hampered by the influence of the non-randomness of the DNA base sequence on binding of base specific fluorochromes. Nevertheless, the comparison with HPLC results reveals, that flow cytometric results do not reflect the exact AT/GC-ratio but the dimensions of AT/GC-ratios between species. Because of the non-stoechiometric binding of base specific dyes to DNA, instead of absolute AT-fractions, a dye factor was calculated from base specific fluorescence and Propidium iodide fluorescence of two species. According to literature, endopolyploidy in seed plants is related to genome size, family and organ. The flow cytometric analysis of 54 seed plant species belonging to 16 families reveals that endopolyploidy is closely related to family affiliation, less closely to life history and organ and weakly to genome size. However, the four tested factors are not independent from each other and a phylogenetic coherence above the family level was not discernible. Therefore, other causal relations (with genome size and life history) may just have been covered by the factor family affiliation. Nutritive demands and habitats of 15 Central European species revealed that endopolyploidy is particularly exhibited by species of disturbed habitats with high nutritive demands. Since endopolyploidization probably accelerates growth of plants, endopolyploid species grow in habitats requiring and supporting fast development.