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

Abstract

Specialization of wood vessels results in a decrease in their length, and a change from scalariform to simple perforations of the end walls. There are but two significant types of porous perforations, those which are bordered and those which are non-bordered. The bordered scalariform type is primitive and its transition to either simple porous or bordered porous is slow. The number of bars in the scalariform vessel decreases with specialization. The porous condition arises from sudden loss of bars. As specialization progresses the end wall changes from highly inclined to transverse position. Width of the apertures is increased either by the loss of borders or actual reduction in the number of bars. Primitive scalariform perforations are commonly oblique and specialized porous ones nearly always transverse. Lateral pitting is specialized more rapidly than the perforation.