SPECIAL PAPER: MERISTEMS AND EVOLUTION: DEVELOPMENTAL CORRESPONDENCE AMONG THE RHIZOMORPHS OF THE LYCOPSIDS

This paper deals with a comparison of the rooted axes (rhizomorphs) of Isoetes with those of Stigmaria, Pleuromeia and other fossil lycopsids from the standpoint of developmental correspondence. The objective is to present a mechanistic approach to the problem of evaluating possible interrelationships among the genera. The results complement those from other comtemporary approaches to the same problem. These main points are advocated: 1) The rhizomorphs of Isoetes and Stylites correspond. Because neither of these contains an apical meristem, neither corresponds to Stigmaria with respect to organization of the axis or root production. 2) Rooted portions of Isoetes and Pleuromeia correspond, although external lobing in Pleuromeia parallels lobing of the stele, whereas in Isoetes external lobing occurs between the lobes of the stele. In both cases, lobing of the stele is associated with root production. 3) Paurodendron rhizomorphs appear to correspond to Stigmaria developmentally with regard to the presence of an apical meristem and the arrangement of roots, but Paurodendron rhizomorphs are shorter. If several lines of root production were added to the vascular cambium of the kind of rhizomorph seen in Paurodendron, a plant part like the rooted portion of Pleuromeia could result. Further extrapolations of the Paurodendron form of growth are explored for their bearing on comparison of rhizomorphs among the lycopsids.