Arabidopsis leaf hairs (trichomes) are polyploid epidermal cells with a predictable branching pattern. More than 15 genes have been identified that are involved in the regulation of branching. The cloning of theZWICHEL,ANGUSTIFOLIAandSTICHELgenes points to two mechanistic aspects of branch formation: (i) a role of the microtubule cytoskeleton; and (ii) a link to the regulation of cell divisions. The latter aspect is supported by the recent identification of anArabidopsismutant with multicellular trichomes, thesiamesemutant, suggesting thatArabidopsistrichomes are evolutionarily derived from multicellular forms. We speculate that the spatial information for branch formation is derived from mechanisms employed in cell divisions.