Published in Systematic Botany, Volume 14, Issue 4, October 1, 1989, pages 583-588.
Porophyllum pygmaeum is a new species from coarse calcareous soils of the Desert National Wildlife Range in Clark Co., Nevada. It has subterete leaves that contain a continuous double layer of palisade mesophyll surrounding a central area of larger, achlorophyllous, polyhedral parenchyma cells and veins. The hollow foliar oil glands lie just below the abaxial epidermis and are deeply invaginated within the parenchyma layers. Porophyllum pygmaeum is a tetraploid (n = 24) perennial herb that apparently is most closely related to P. greggii a hexaploid herbaceous species of western Texas with much longer and narrower leaves. In its fleshy subterete leaves P. tridentatum resembles the subshrubby P. tridentatum of Baja California from which it differs by its chromosome number (n = 15 in P. tridentatum) by its dwarf herbaceous habit, by leaves that are always entire and that bear several pairs of submarginal glands, and by much longer achenes. The new species apparently is not closely related to P. gracile the only other species of that occurs in southern Nevada and adjacent regions of Arizona and California.
The definitive version is available at http://www.jstor.org/stable/2419003.