by Bob Harms 

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):

Irving & Adams further note (192):

Although there is quantitative variation found both within and between populations, the overall quantitative and qualitative profile remains invariable.

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:

• There are many collections in various herbaria that have been incorrectly determined.
• To the extent that such collections have in turn been used as models to determine other collections in which scent information is not available, the damage has been compounded.

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:

• Two 'common names' for H. drummondii include "limoncillo" and "lemon verbena"
  [found on a herbarium label for a Hedeoma with a lemon scent].

• The common names in turn may have led to the association of lemon scent with H. drummondii in floral literature — which in turn has led to incorrect identifications; e.g., "all parts of this plant smell like lemon when crushed!" (but the images given, especially the broad leaves & spreading upper calyx teeth, indicate H. reverchonii (variety can't be determined from the photos).

• And vice versa a plant correctly identified as H. drummondii may be accompanied by a statement of lemon scent based not on the field observation, but simply to share the assumed knowledge of this plant.

• If scent is ignored, confusion with H. reverchonii var serpyllifolia is quite likely if only calyx & corolla length are considered. These are roughly the same in these two taxa (note Irving's 1980 key & the recent BRIT treatment). Consequently plants with camphor scent would be considered to be H. drummondii. Note that flower and leaf color, both in the BRIT key, do not adequately distinguish these three taxa.