Origins and nature of vessels in monocotyledons. 9. Sansevieria

Sections of fixed material of four collections of three species of Sansevieria were studied by means of scanning electron microscopy (SEM) to initiate study of distribution of vessel elements and the ultrastructural nature of tracheary elements in the genus. Roots of Sansevieria have scalariform perforation plates in vessels; only tracheids are present in stems (= rhizomes) and leaves. This distribution is similar to that of genera of Asparagales claimed in recent molecular phylogenies to be close to Sansevieria: Dracaena, Ophiopogon, and Ruscus. Also similar in tracheary element types is Orchidaceae, now considered a member of Asparagales. Ultrastructural study of Sansevieria tracheary elements reveals intermediacy between vessel elements and tracheids because porose pit membranes extend across entire end walls of tracheids, and portions of scalariform perforation plates of vessel elements. This correlates with longevity of roots in Sansevieria, and probable moderate transpiration rates in leaves. In Sansevieria, as in other monocotyledons, vessel type occurrence, organographic distribution, and tracheary element ultrastructure relate primarily to ecology, although molecular-based trees, in fact, help understand the patterns of shift in ecological adaptation with attendant change in tracheary element structure.