Vascular Cambium and Wood Development in Carboniferous Plants. II. Sphenophyllum plurifoliatum Williamson and Scott (Sphenophyllales)

Wood development and cambial activity in the Carboniferous articulate stem Sphenophyllum plurifoliatum Williamson and Scott were studied by analyzing mature interfascicular secondary xylem. The wood parenchyma is interpreted as being a true vascular-ray system. Rays are mostly biseriate and appear as groups of cells in the interstices between tracheids. Ray cells are longitudinally elongate, and some exhibit radial processes that extend between tracheids. The radial arrangement of the ray cells was apparently disrupted by tracheid enlargement during wood development. A distinct pattern of tracheid enlargement from inner to outer wood is apparent, suggesting that the fusiform initials increased in size during secondary growth. There is no evidence for multiplicative division of fusiform initials, and the increase in cambial circumference seems to have been accommodated primarily by fusiform initial enlargement. The absence of a mechanism to augment the number of fusiform initials in Sphenophyllum implies limited lateral growth, thus providing additional evidence that the vascular cambium was determinate. The results of this investigation, in conjunction with those of earlier work on Carboniferous lycopods, suggest that the vascular cambium in woody pteridophytes was fundamentally different from that of modern seed plants.