Vessels are demonstrated in metaxylem of both roots and rhizomes of both species of Acorus (Acoraceae) by scanning electron microscopy (SEM). The end walls of vessel elements are characterized by perforations that retain porose pit membranes; the pores sometimes coalesce into larger openings. Lateral walls lack pores in primary walls as detectable by our SEM. Striations in pit membranes, as in Nymphaeaceae, were observed. Although vessel elements, as defined by porose pit membranes in end walls, occur in both roots and rhizomes, the long scalariform nature of end walls of vessel elements and the porose nature of the membranes are interpreted as primitive. Thus, the nature of vessels in Acorns is compatible with the recent idea that Acorus (Acoraceae) is the sister group to the remainder of the monocotyledons. Cheadle's (1942) view that presence of vessels only in roots is primitive in monocotyledons was based on light microscopy. The addition of SEM data has revealed new vessel distributions. The pores in pit membranes in end walls of vessels of Acorus are larger than pores in pit membranes in end walls of tracheids in vessel-less dicotyledons. The difference between vessel elements and tracheids in groups such as Acorus may be not the absence of pit membranes in perforation plates but the size of pores in the membranes of the perforation plates. The conductive capabilities and air bubble transmission capacities of pores of various sizes in pit membranes need physiological analysis in view of SEM findings on porosity in pit membranes of vessel perforation plates.
