XYLEM STRUCTURE IN BOTRYCHIUM DISSECTUM SPRENGEL AND ITS RELEVANCE TO THE TAXONOMIC POSITION OF THE OPHIOGLOSSACEAE

The radially seriate xylem of Botrychium dissectum Sprengel resembles secondary xylem, particularly that of gymnosperms, in many important details. It is derived from a layer of cells which strongly resembles a vascular cambium. Presumptive cambial initials are fusiform, and derivatives are radially seriate. The walls of the initials and derivatives have a beaded appearance when viewed in tangential section. The number of xylem elements increases in seasonal increments. Circular-bordered pit pairs occur where tracheids abut other tracheids, and specialized cross-field pit pairs occur where they abut the radially-aligned parenchyma or rays. Cambial activity in Botrychium differs from that found in seed plants and progymnosperms in not producing secondary phloem. Tracheids are less similar to those known in progymnosperms than previously assumed, and some supposed similarities may be less significant than previously assumed. The significance of these dissimilarities is unclear. The recognition that the bulk of the xylem is secondary and that protoxylem strands are arranged as sympodia suggests that Botrychium may be eustelic rather than siphonostelic.