Origin and nature of vessels in Monocotyledons. 7. Philydraceae and Haemodoraceae

SEM studies of macerated stems and roots showed that long scalariform perforation plates are present in roots and probably also stems of Philydrum (Philydraceae). In Haemodoraceae, clearly recognizable vessel elements are present in roots; perforation plates range from scalariform to simple. In stems of Haemodoraceae, however, less clearly recognizable vessel elements are present: the presence of thread-like pit membrane remnants is one criterion that argues for vessel presence. Such tracheary elements can be considered transitional between tracheids and vessel elements. End walls of transitional tracheary elements may have narrower bars between perforations, less prominent borders, and pit membrane remnants rather than non-porose pit membranes. However, only one or two of these three character expressions may be present, suggesting that clear definitions of vessel elements and tracheids may not be possible. Haemodoraceae show a marked difference, or disjunction, in morphology between vessel elements in roots and the vessel elements transitional to tracheids of stems. The physiological and evolutionary correlations of this morphological disjunction may be related to seasonality of water availability.