A Comparison of Aztekium and Strombocactus (Cactaceae)

Aztekium and Strombocactus, monotypic genera of Mexican cacti, have been considered to be congeneric. Studies in the field and laboratory dealt with habitat, distribution, flowers, fruits, outer cell layers of the stem, spines, pollen, and seeds. Although the genera are similar, particularly with regard to habitat and seeds, there are numerous distinguishing features. These two genera are compared to each other and to eight other genera of Cactinae in a table of 30 characters. Aztekium and Strombocactus should continue provisionally to be recognized as distinct genera. Aztekium and Strombocactus, two monotypic genera of Mexican cacti, are similar in general appearance and occur in remarkably similar habitats. Buxbaum (1950) studied their gross morphology and stated that although "Strom- bocactus disciformis has much smaller seeds than Aztekium, the very similarity of the morpholog- ical types of their seeds, as well as their vege- tative organs, suggests that Aztekium is a true Strombocactus and only has subgeneric value." The verrucose seeds of both genera possess what Buxbaum (1950) called a strophiole, a corky, spongy mass of material covering the hilum.