Botrychium dissectum Sprengel rhizomes were examined at monthly intervals from February 1993 through December 1994. Sampies taken ranged from those with an inactive cambium and only mature tracheids to those having an active cambium and immature tracheids. The vascular cambium became activated in the early fall prior to maturation of the leaf and fertile spike complex. Intertracheid pit membranes with tori were present in all sampies, although the morphology of the torus varied. The presence of tori was first observed in a tracheid that had just initiated its secondary wall formation. As the pit membrane matured, matrix material was hydrolyzed first from the margo area, then from the torus, and eventually the pit membrane was represented only by a very thin network of microfibrils. In addition, studies confirmed that tracheids bordering parenchyma cells developed a torus thickening, aIthough no thickening of the parenchyma cell wall occurred. Torus ontogeny in B. dissectum combined features previously described for angiosperms and gymnosperms.