Stem anatomy of <i>Turbinicarpus s.l</i>. (Cacteae, Cactaceae) and its contribution to systematics

The stem anatomy of Turbinicarpus s.l. was studied with the aims of finding characters to support the three clades (Rapicactus, Kadenicarpus, and Turbinicarpus) in which Turbinicarpus s.l. was recovered in the most recent phylogeny of the Cacteae tribe. Thirty-five taxa were prepared, and their tissues were compared. Substantial variation was found in the epidermal surface. The hypodermis has concentric druses (Rapicactus clade) or prismatic crystals (Kadenicarpus and Turbinicarpus clades) in the cell lumina. There are abundant collateral cortical bundles, but they are amphicribal in a few taxa, and xylary fibers occur in the Kadenicarpus clade. All members of Turbinicarpus s.l. have phloem without sclerenchyma and nonfibrous wood, except for T. subterraneus, which has wood with few fibers. The periderm has an epidermal origin, and the phellem may have thin-walled cells or alternating thin- to thick-walled layers. Our results support the three clades. The Kadenicarpus clade comprises the species with xylary fibers in cortical bundles, but it shares prismatic crystals in the hypodermis, thin-walled phellem cells and partially dilated rays with the Turbinicarpus clade. The members of the Rapicactus clade have concentric druses in the hypodermis. The anatomical features proved to be valuable to support the recognition of monophyletic clades.