Root Phloem of Fossil Tree-Sized Arthrophytes

Distribution and histology of the primary phloem is described from specimens representing a variety of branch orders of the petrified calamitalean root genus Astromyelon In all but the smallest roots the primary phloem occurs as a series of strands alternating in position with those of the primary xylem. Vertically elongate cells with horizontal end walls present in this tissue compare favorably with midinternodal stem and root sieve elements of the extant genus Equisetum. Large roots at advanced stages of development have a narrow zone of secondary phloem outside of the secondary xylem. This tissue exhibits both a ray system of parenchyma and an axial system of vertically elongate cells present in longitudinal cell files These latter cells are regarded as sieve elements produced by fusiform initials of the vascular cambium. The present study is the first to document adequately the presence and derivative tissues of a functionally bifacial vascular cambium in the Calamitales.