SEM studies on vessels in ferns. 17. Psilotaceae

Perforation plates are reported in aerial and subaerial axes of Psilotum nu dum and in aerial axes of Tmesipteris obliqua. In Psilotum, both perforatio ns lacking pit membranes and perforations with pit membrane remnants were o bserved-Perforation plates in Psilotum may consist wholly of one type or th e other. In Tmespteris, perforations have threadlike pit membranes or consi st of porose pit membranes. Wide perforations alternating with narrow pits, a conformation observed in various ferns, were observed in Psilotum (subae rial axes). In Psilotum, perforations are more common in metaxylem than in protoxylem; perforations in protoxylem consist of primary wall areas contai ning small circular porosities ar relatively large circular to oval perfora tions. There are no modifications in the secondary wall framework of protox ylem or metaxylem in Psilotum or Tmesipteris that would permit one to disti nguish presence of perforations or perforation plates with light microscopy , and scanning electron microscopy (SEM) is required for demonstration of p orose walls or perforations. The tracheary elements of the Psilotaceae stud ied have no features nor also observed in other ferns with SEM.