This is the only complete flora of Colorado.  All others are abbreviated and therefore the results of your keying in these other books should be checked against Weber.

Consult the on-line Catalog of the Colorado Flora

Arnica cordifolia, arco9

Each grouping (1a/1b, 2a/2b, 3a/3b) is  called a couplet.  Note that the distinguishing characteristics in each couplet are physical features that we can observe:

Opposite vs. alternate leaves
Shape of phyllaries
Shape of flower head
Size of leaves
Kind of root

  Terms to understand:
Ligulate, pappus, involucre, plumose, alternate/opposite, phyllaries, turbinate, rhizome, receptacle: (think of what is left of the Dandelion after the silvery puff of seeds is gone).

Arnica cordifolia, arco9

Fragrances are useful in identifying plants.  Arnicas often have a lemon fragrance.

Terms to understand:
Ray/disk, cauline, obtuse/acute, subplumose, barbellate, rosette, lanceolate, petiole, basal rosette, cordate, achene, apomictic, and the many terms that describe hairiness and hairs (trichomes): pubescent, puberulent, pustulose, tomentose, stellate, strigate, sericeous, hirsute, villous, pilose, glandular, glaucous. 

Apomictic (without mixing): producing seeds from unfertilized ovules.

These terms don't have to be memorized; refer to Weber's excellent glossary and be assured that the meaning will
sink in with centuries of use.