From 7 Oct 2007

How to distinguish Eryngium heterophyllum and Eryngium lemmoni
Why I don’t like to use the Kearney & Peebles Flora of Arizona

Here is how Kearney & Peebles distinguish these two species:
“3. Plants from a cylindric taproot; lower cauline leaves pinnatifid to bipinnatisect; inflorescence paniculately branched, the heads comate; bracts linear-lanceolate to lanceolate, entire or with 1 or 2 pairs of lateral spines near the middle, commonly yellowish above . . . . . . . 3. E. heterophyllum
3. Plants from a fascicle of fibrous or fleshy roots; lower cauline leaves spinose-serrate; inflorescence successively trifurcate, the heads not comate; bracts broadly lanceolate to oblanceolate, spinose-serrate with 2 or 3 pairs of teeth, silvery-white above . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. E. Lemmoni

And here are my observations on the various characters:
“Plants from a cylindric taproot” vs. “plants from a fascicle of fibrous or fleshy roots”. This appears to be accurate and, although not necessarily a useful field character, this can be a useful distinction with good herbarium specimens.
“Lower cauline leaves pinnatifid to bipinnatisect” vs. “lower cauline leaves spinose-serrate”. This is also essentially accurate, although it could be better worded, for instance by including a more quantifiable distinction rather than descriptive terms that can be somewhat subjective (how deep must the divisions be before the leaf is pinnatifid?).
“Inflorescence paniculately branched” vs. “inflorescence successively trifurcate”. This is simply inaccurate. Inflorescences of the two species are quite similar. In both, as we move up the plant we have first several alternately arranged primary inflorescence branches, then a whorl of ca. 3-7 primary branches. Each primary branch of the inflorescence is determinate, and may either terminate in 2-3 heads arising from a single node, or the lateral head(s) may be replaced by secondary branches terminating in groups of 2-3 heads. It seems to be more common for E. heterophyllum to have the terminal groups with only 2 heads, and E. lemmoni to have groups of 3 heads. However, this is by no means a uniformly applicable identifying characteristic, and neither species has an inflorescence that is accurately characterized as “paniculately branched” or as “successively trifurcate”, although the primary branches of the inflorescences of either species may (or may not) be “successively trifurcate”.
“The heads comate” vs. “the heads not comate”. “Comate” is, first, a needlessly obscure term. I do not recall having heard it before, in any context, and although it sounds much like the more commonly used “comose” the meaning is quite different. In any case, a comate head is one in which the bracts of the head are greatly enlarged at the apex of the head and form a leafy projection beyond the flowers. Pineapples are comate. The heads of E. lemmoni are indeed not comate and most heads on most specimens of E. heterophyllum are indeed comate. But some heads on many specimens, and all heads on rare specimens of E. heterophyllum are not comate, or at best indistinctly so. So this is a one-directional character; plants with comate heads must be E. heterophyllum, but plants with non-comate heads could be either species.
“Bracts linear-lanceolate to lanceolate, entire or with 1 or 2 pairs of lateral spines near the middle” vs. “bracts broadly lanceolate to oblanceolate, spinose-serrate with 2 or 3 pairs of teeth”. This is accurate, although unfortunately there is overlap in the descriptions.
Bracts “commonly yellowish above” vs. “silvery-white above”. I cannot tell if this character is inaccurate, or simply variable and of limited utility. I have only seen E. heterophyllum in the field at two locations (Rucker Canyon in the Chiricahua Mts. and Clanton Draw in the Peloncillo Mts.), but both times the bracts were silvery-white above. No difference in bract coloration is apparent from the herbarium specimens I looked at earlier today, but colors are often unreliable in dried material. Presumably any specimens that did clearly have yellowish bracts could be readily identified but, as with non-comate heads, specimens with silvery-white bracts (which appear to be the overwhelming majority) could be either species.

Although this is the most annoying example I have encountered recently (since this key has resulted in my misidentifying E. heterophyllum as E. lemmoni not once but twice), it is unfortunately not an isolated example. Most keys in the Kearney & Peebles flora are well written and eminently usable. However, a significant minority are not, and while these keys will still usually yield correct identifications if used carefully while comparing specimens of all of the relevant taxa, they often make me feel rather confused and can easily lead to misidentifications if used incautiously.