Scent as Differentia for the Hedeoma drummondii Complex
by Bob Harms  email-here

In 1973 R.S. Irving and R.P. Adams expanded on a major finding in Irving's 1968 dissertation, the correlation between the scent–producing volatile terpenoid components and clearly recognizable freshly collected taxa (recognizing at that time H. reverchonii var. reverchonii and H. reverchonii var. serpyllifolia as distinct from H. drummondii). The basic results were (listing only the major, scent–determining components):

Percentage of monoterpenes for each taxon1:
Monoterpene HEDR HERE HESEassociated scent
Pulegone 50.0 --- ---peppermint
Isomenthone 40.0 --- ---mint
β-Pinene Tr2 Tr2 30.0camphor
Bornel --- 3.5 26.0camphor
Camphene Tr2 1.0 17.0camphor
Geranial --- 31.7 Tr2lemon
Citronellal --- 22.0 Tr2citrus
Neral --- 21.0 Tr2lemon
14-letter abbreviations may be used for the 3 taxa on this web site.
2Tr =< 1%.

Irving & Adams further note (192):

Although there is quantitative variation found both within and between populations, the overall quantitative and qualitative profile remains invariable.
The scent correlation, which has become the basis for recent keys only after Irving's work (and his 1980 key), seems to remain unknown in spite of the difficulties of identification solely based on morphological traits. At times it is not ignored, but isn't used for want of a reliable sense of smell when collections are made. Even labels for early Irving collections failed to note a scent.

I find the work of Irving & Adams on monoterpenes compelling, and it corresponds to my own field experience with these taxa.
If it is correct:

Caveat: Known hybridization within this group can change the monoterpene relationships in unpredictable ways, and can thus lead to incorrect identifications.

Collections in TEX/LL with scents noted for each taxon. Scent/taxon mismatches in yellow.
total scent
mint camphor lemon no specific
scent type3
Irving collections
total scent noted
H. drummondii 190 16 6 6 2 2 29 4
H. serpyllifolia 102 10 0 4 3 3 20 6
H. reverchonii 151 17 2 1 11 3 20 9
3Includes: aromatic, very aromatic, not aromatic, odor seems variable, strongly scented,
strong but agreeable scent, smell peculiar.

In effect only 8 percent of the 443 holdings in TEX/LL have a specific scent noted; and a third of these appear to have the wrong scent for the determined taxon. Less than a third of Irving's 69 collections are accompanied by a scent notation.

Most scent mismatches are for H. drummondii. In my opinion there may be several reasons for this:

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