I sampled spine length in the keystone Carnegiea gigantea (Engelm.) Britton & Rose in two northern Sonoran Desert populations where this species is ultimately limited by cold temperatures. I compared spine length (1) between two sites, (2) on north versus south facing sides of plants, (3) by level of shading, (4) by proximity to other potential conspecific competitors, and (5) by presence near sources of surface runoff from which all their water is obtained. Spine length increases, providing thermal protection and reducing photosynthesis, where temperature is more extreme (winter and summer) and where conditions are more xeric. The difference in spine length by direction on plant was much smaller at the more extreme site. Clumped plants that necessarily had increased competition for water but also greater shading were split; at the more extreme site, greater spine shading occurred where water competition was fiercest, while at the less extreme site, greater shading occurred in association with sunlight rather than water.

Aphyllon epigalium Colwell & A.C. Schneid. is described as a new species from Oregon and California. This taxon is distinguishable from the other members of Aphyllon Mitch. sect. Aphyllon in Western North America (i.e., A. fasciculatum [Nutt.] Torr. & A. Gray, A. purpureum [A. Heller] Holub, and A. uniflorum [L.] Torr. & A. Gray) in its host preference for Galium L., by having 2–4 yellow flowers per stem, and pedicels longer than the stem. Two subspecies are described: Aphyllon epigalium subsp. epigalium, which occurs in the Cascade and Klamath ranges in Oregon and California, and the Coast Ranges and Sierra Nevada in California; and A. epigalium subsp. notocalifornicum Colwell & A.C. Schneid., currently known from few sites in montane southern California. The new subspecies differ from one another in flower size, corolla lobe shape, host preference, geographic range, and nuclear and plastid genetic markers. An updated key to California and Oregon Aphyllon sect. Aphyllon is presented.