LAUSR

Premise of research. The extant genera of cycads (order Cycadales) can be readily distinguished by the anatomy of their leaflets. In particular, the genus Dioon possesses a unique cellulosic fiber type in the leaflet mesophyll, not found in any other genus. We examine living material of all 15 species of Dioon to confirm preliminary observations and provide details of wall structure in comparison with fibers in the leaf axis. Methodology. Unembedded sample material was sectioned on a sliding microtome transversely and longitudinally, and standard histochemical tests were utilized to identify cell types. SEM was utilized to analyze fiber anatomy. Maceration of samples provided cell types for comparison. Pivotal results. All species of Dioon have the same configuration of cell types in the leaflets, the most conspicuous element being elongated fibers with multilamellate cellulosic cell walls. Such fibers do not appear in the leaf axis. This fiber type is seemingly unique among all cycad genera. Conclusions. Dioon leaflets possess a fiber type of distinctive wall structure, unique among the extant cycads. This finding clarifies an earlier report in which such fibers were described as “gelatinous fibers.”