Specialization in Secondary Xylem of Dicotyledons. II. Evolution of End Wall of Vessel Segment

The scalariform perforation is primitive and the porous perforation is specialized. The phylogenetic order of development is: scalariform, scalariform-porous, oblique porous with vestiges of the scalariform condition, and transverse porous. 2. The primitive fully bordered aperture of a scalariform perforation gradually loses its border as the perforation becomes specialized. 3. The primitive scalariform perforation has many bars, which are generally lost in slow stages in correlation with the evolutionary development of the perforation. 4. Specialization of the scalariform perforation frequently results in a widening of the apertures of the perforation. 5. The inclination of the end wall changes from the highly inclined position to the transverse position as the scalariform perforation develops into the porous perforation. 6. The lateral pitting specializes more rapidly than the specialization of the perforation.