Vascular Cambium and Wood Development in Carboniferous Plants. IV. Seed Plants

The activity of the vascular cambium was investigated in stems of Cordaites (Mesoxylon sp. and Cordaixylon sp.) and Medullosa noei, primitive gymnosperms from the Upper Carboniferous of North America. Developmental characteristics of the meristem are inferred from quantitative and qualitative changes in the anatomy of the secondary xylem determined from serial tangential sections. Tracheids in cordaite xylem increase in size through the inner-middle wood and stabilize in the middle-outer wood, where the elements average ca. 5.0 mm long and 35 μm wide. In Medullosa wood, the tracheids are more than 20 mm long and 120 μm wide; mean cell size is relatively constant radially across the secondary xylem zone. Within individual tracheid files of both cordaite and medullosan wood, a centrifugal pattern of progressive cell enlargement is present, punctuated by abrupt reductions in cell length. This pattern is indicative of anticlinal division of fusiform initials. The orientation of the anticlinal partition was apparently pseudotransverse, as in extant plants with nonstoried cambia. New fusiform initials were formed at a significantly lower frequency in the cambium of Medullosa than in the cordaites. This may reflect the unusual "polystelic" organization of the stem and/or the effects of selective pressure acting at the level of the cambium for wood with elongate tracheids.