The primary vascular system of Callixylon

Abstract

The primary vascular system of Callixylon (main stem axes of Archaeopteris) is composed of a discontinous cylinder of longitudinal bundles from some of which traces diverge radially in a regular, helical pattern. Tangential divisions and fusions of primary xylem strands occur in approximately equal frequency and, thus, the number of bundles in the system remains constant in the specimen studied. Following divergence, traces increase in size and become ribbed distally. The available evidence suggests that all of the traces observed in this study are branch traces. Branch bases, embedded in secondary xylem, have the anatomical characteristics of penultimate axes of lateral branch systems of Archaeopteris. It is suggested that some regions of main axes of Archaeopteris might have borne lateral branch systems only, lacking entirely simple leaves. Lateral branch systems, therefore, might have been more similar to compound leaves than previously supposed.