Specialization in Secondary Xylem of Dicotyledons. III. Specialization of Lateral Wall of Vessel Segment

Scalariform intervascular pitting is primitive in the organization of the vessel segments of the dicotyledons. 2. Specialization of the scalariform pit produces transitional and opposite pitting. The re-arrangement of opposite pits gives rise to the highly specialized alternate arrangement of intervascular pits. 3. Vessel-parenchyma pitting may be scalariform, transitional, opposite, or alternate. The evolutionary development is the same as in the development of intervascular pitting. 4. Vessel-parenchyma pits may be fully bordered, half-bordered, or non-bordered. The fully bordered pit appears to be primitive and to give rise during specialization to the half-bordered and the non-bordered type. 5. The introduction of tertiary spirals in secondary vessel segments is an evidence of specialization. Spirals do not occur in the most primitive dicotyledonous woods.