Five years ago, in 1986, only a limited number of glycosidically bound volatiles were known and little could be reported on the distribution of those compounds in the plant kingdom. Since then an explosion of research activities in this field has led to the publication of many interesting results concerning structurally diverse terpenoid and non-terpenoid glycosides, their occurrence, and their role in different plant species. Up to now, about 200 different aglycone structures have been found in about 150 species belonging to 35 different families including ferns, gymnosperms, dicotyledons, and monocotyledons.All the diverse results concerning the role of these compounds in the plants should not lead us into ascribing to them an exclusive role, such as being involved in essential oil metabolism, as early workers have postulated. Although considering new electron microscopical observations of a transformation of carbohydrate-rich substances into lipophilic droplets at the tonoplast of Menha x piperitu secretory cells, the range of possible roles should not be underestimated. New experimental work should not only provide new analytical data but should also attempt to throw more light on the role of the glycosides.

The essential oil from fresh aerial parts of Thymus x citriodorus, probably a hybrid between T. vulgaris L. and T. pulegioides L., was analysed by GC and GC-MS. Geraniol was found to be the main compound (more than &I%), which kept its high level from July to October. Geranyl acetate (1.0%), geranyl butyrate (0.8%), nerol (2.8%), and citronellol (0.3%) were also detected. The lemon-scented compounds geranial and neral amounted to 8.2% and 5.5% respectively. The content of thymol (0.5%) was remarkable.In fresh plant material the glycosidically bound volatiles amounted to 0.003-0.013%. After enzymatic hydrolysis, geraniol was found to be the main aglycone (45%), bound to glucose in the glycosidic fraction. Other important aglycones were a farnesol isomer (7.3%), nerol (3.5%), a-cadinol (0.4%) and citronellol (0.4%). Thymol only amounted to 0.5% in the aglycone fraction. Other aglycones, such as the aliphatic alcohols present, and benzyl alcohol, 2-phenylethanol, eugenol, and the terpenes terpinen-4-01 and linalol, can more or less be considered as ubiquitous in aglycone fractions. The correspondingly high level of geraniol in the essential oil and in the aglycone fraction shows a correlation between the free and bound volatiles in the plant.