The Three-Dimensional Structure of Vascular Tissues in Stegnosperma (Phytolaccaceae)

The three-dimensional structure of anomalous secondary growth in Stegnosperma is described. The initiation of anomalous increments of vascular tissue is related to the branching pattern in stems and roots of young plants. In more mature stems the relationship between anomalous layers and plant architecture is obscured by the complexity of the system. Tangential connections between vascular bundles in the same anomalous increment and radial connections between successive layers are frequent. These anastomoses do not occur simultaneously in the xylem and phloem, resulting in a complex netlike structure similar to that described for Avicennia, an unrelated genus with anomalous secondary thickening by successive cambia. Anomalous secondary growth in the closely related Amaranthaceae, Chenopodiaceae, and Nyctaginaceae may represent an advanced form of the more slowly developing pattern in Stegnosperma.