SEM studies on the vessels of heterophyllous species of Selaginella

Macerations of rhizomes of five species of Selaginella were studied with sc anning electron microscopy (SEM). Although only tracheids were observed in two species of Selaginella (pit membranes thin or weblike but intact), vess el elements were seen in the remainder. Pit dimorphism resulting in intermi ttent perforation plates featuring wide perforations alternating with narro w pits occurs in S. lepidophylla. Highly porose pit membranes were observed in three species. Although the number of species examined is small, the ap parent absence of vessels in some species and the absence of vessels with s imple perforation plates are evidence that the heterophyllous species of Se laginella have xylem less xeromorphic than that of the homophyllous species ; S. lepidophylla has more adaptations for potentially rapid conductive rat es than other heterophyllous species. Structural features of vessels and tr acheids of Selaginella as seen with SEM have also been reported in ferns.