New information on Bostonia perplexa--an unusual member of the calamopityaceae from North America

Abstract

A calamopityacean axis exhibiting multiple segments of primary xylem surrounded by secondary vascular tissue is analyzed here for its morphological and systematic significance. The plant is fundamentally protostelic with a deeply three-ribbed column of primary xylem. Each rib consists of a semi-discrete bundle of tracheids at the tip, intermittently connected to the stelar center by an extensive primary xylem parenchyma. The appearance of separate vascular segments at some levels is associated with departure of paired leaf traces. Between levels of trace departure, the three-ribbed protostele is reconstituted with primary xylem ribs following a helical course through the stem and supplying a regular Fibonacci phyllotaxis. Attached petiole bases are broadly of the Kalymma-type but exhibit a distinctly three-ribbed medial petiole bundle. The new specimen is assigned to Bostonia perplexa requiring an expanded concept of the taxon. A restricted cladistic analysis of stelar architecture and nodal anatomy within the Calamopityaceae produces two phylogenetic hypotheses. One is preferred on morphological grounds but necessitates viewing at least some protostelic calamopityaceans as exhibiting a derived condition within the group. The redefinition of Bostonia proposed here, a protostelic calamopityacean with medullosan-like vascular segments, reinforces long-standing views of a calamopityacean origin of Medullosales. Cladistic interpretation of stelar form and petiole anatomy in members of these groups, however, reveals character incompatibility hinging upon whether Quaestora (a protostelic medullosan) or Bostonia (a calamopityacean with medullosan-like dissected vascular system) should be preferred as a candidate for medullosan ancestor.