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The two forms of Douglas fir, Pseudotsuga menziesii var, menziesii and var. glauca, differ quite considerably with regard to the monoterpenes of their leaf oils. Several chemical races, differing quantitatively in certain monoterpenes, appear to exist in each variety and the leaf oil composition seems to be particularly useful in classifying intermediate forms. INTRODUCTION Chemosystematic studies based on terpene analysis have advanced from the qualitative approach, as used by Erdtman' in his pioneer work on the heartwood extractives of conifers, and a semi-quantitative one by Mirov2 with the turpentines of most pine species, to a quantitative method with the development of gas—liquid chromatography (GLC)3. Several groups of workers3'2 have shown that quantitative analysis can lead to useful systematic correlations at the species and sub-species levels. Qualitative differences appear to be significant only in higher taxa. Since one is dealing with volatile compounds precautions in sampling, transportation and storage of the plant material as well as in steam-distillation, recovery and GLC analysis must be taken to ensure reproducible results. The relative amounts of individual terpenes in an oil can be determined with a reproducibility of 0.1 per cent, but this increases to 0.5 to 1 per cent when different columns and instruments are used. Losses owing to volatility, autoxidation and polymerization can increase the error further. To this one must add the inherent biological variability within a given plant sample. Hence, an error of 2 to 5 per cent in the individual values must be accepted, but this is usually well within the variation of one plant or population of plants to another and hence is entirely satisfactory for most quantitative chemosystematic studies. In our chemosystematic studies of North American conifers we have concentrated on the volatile oils of the leaves (or foliage) for several reasons. These oils contain a wide spectrum of monoand sesqui-terpenes and in some cedar species also diterpenes'3. In contrast, the volatile oils of the wood, blister resins or bark2' 9. 10, 14 are composed mainly of monoterpene t Part II of the series 'Chemosystematic Studies in the genus Pseudotsuga.'

Geographical variation in the terpene composition of the leaf oil of Douglas fir