Seed coat anatomy, karyomorphology, and relationships ofSimmondsia (Simmondsiaceae)

The development of the seed coat ofSimmondsia, whose relationships are extremely problematic, is documented, and its structure is compared to those of putatively related families Euphorbiaceae and Buxaceae. InSimmondsia, the young seed coat is composed of (1) the palisadal exotesta, (2) the thick aerenchymatous mesotesta which is further differentiated into the outer and the inner tissue of mesotesta, and the undifferentiated (3) endotesta and (4) tegmen. At maturity, only the palisadal exotesta composed of thick-walled and prismatic cells, as well as the outer tissue of mesotesta composed of elongate, thick-walled cells, are persistent, while all the remainder is crushed. These distinctive structural features of the exo- and mesotesta inSimmondsia are not found in Euphorbiaceae, but prevalent in Buxaceae. Evidence from seed coat anatomy and other sources supports the view thatSimmondsia has close affinities with Buxaceae, and be placed as a distinct family along with Buxaceae in Buxales.Simmondsia has 2n=52 as a tetraploid ofx=13, and probably represents a taxon adapted to desert areas by polyploidization in the order.