Evolutionary Trends in Dion (Zamiaceae)

Distribution, ecology, and morphology of the species of Dion are presented. The genus Dion is distributed in Mexico and in Honduras, where it is generally scattered and restricted to rocky, steep habitats. On the basis mainly of the morphology of the seeds, of the seedlings, and of the female cones, three natural groups were recognized. The first group, which is the most distinct morphologically, is made up of D. mejiae endemic to Honduras. The second group is made up of D. spinulosum and D. rzedowskii, whose distribution is restricted to a very small area in Mexico. They appear to be the only Dion spp. to be adapted to moist and shaded habitats. The third group is made up of the remaining spp. of Dion, all Mexican in distribution. They appear to be well adapted to open and dry habitats generally. Stems, female cones, and seeds show the trends to decreasing size from south to north in this group. A key of the genus is given.