The existence of sympatric interspecific hybrids of Sideritis serrata Cav. ex Lag. and S. bou r­ gaeana Boiss. Reut, has been shown by HPLC analysis of the flavonoid glycosides accumulated in the cell sap, and by the methylated flavonoid aglycones exudated by the plants and deposited onto the plant surfaces. The hybrids tend to produce in a single plant the compounds characteristic of the parental taxa. This "complementation" effect in the hybrids permitted relatively easy detec­tion of F1

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... ds. Generally, there was a close correlation between the biochemical results and a morphological analysis which used the Anderson's index, although a specimen which morphologi­cally was a hybrid, showed a flavonoid pattern as S. bourgaeana. The external flavonoids analysis has revealed that the activity of sideritoflavone O -methyltransferases is much more important in S. serrata than in S. bourgaeana, while the hybrids show intermediate values. M oreover, S. bour­gaeana accumulates sideritoflavone 3′-m ethylether whereas S. serrata accum ulates the 4′-methylether. The hybrids produce both com pounds in similar amounts. In this study, external flavonoids have proved more useful than vacuolar flavonoid glycosides in the biochemical documentation of hybridization. This is probably due to the fact that methylated flavonoid aglycones are physiologi­cally more stable as they are externally located and therefore preserved from the action of enzymes.