Specialization in Secondary Xylem of Dicotyledons. I. Origin of Vessel

Abstract

1. Tracheids are characterized by great average length, small cross-sectional diameter, angularity of outline, evenly thickened walls, thin walls, and the absence of a distinct end wall. 2. Vessel segments which retain these primitive characteristics nearly always have scalariform perforations; the scalariform perforation is therefore primitive. 3. There is a high correlation between the diffuse arrangement of vessels and the scalariform condition of the end wall. 4. Vessel segments with scalariform perforations are characterized by scalariform lateral pitting; therefore scalariform lateral pitting is primitive, and this leads to the natural inference that the tracheid type from which the vessel was derived was also scalariform. 5. A high correlation was found between the scalariform condition of the lateral walls of vessel segments and the presence of bordered pits in the fibrous elements. 6. There was some evidence to indicate that a sequence from the protoxylem to the secondary xylem would reflect the origin of the vessel. 7. Many primitive woods show, in this positional sequence, all transitions from scalariform tracheids to scalariform vessel segments. 8. The evidence would indicate that vessel segments with scalariform pitting on both the end and side walls are more primitive than vessel segments with scalariform pitting on the end walls and opposite to alternate pitting on the side walls. 9. The wood of the vessel-less angiosperms Trochodendron, Tetracentron, and Drimys is unquestionably very primitive. The primitive tracheids of these genera resemble, to a considerable degree, the scalariform tracheids characteristic of the primary wood of primitive angiosperms.