Vascular Morphology of the Ophioglossaceae.

Abstract

This study tested the suggestion of a possible evolutionary relationship between the Ophioglossaceae and the progymnosperms by comparing various aspects of the vascular morphology of the Ophioglossaceae with those known for the progymnosperms. This involved four major parts: the study of stelar morphology in the family, the study of tracheid development and lateral wall ornamentation, the study of the ontogeny of cladogenic roots and of the root-stem vascular connection, and the study of two leptosporangiate ferns to form a basis of comparison with the Ophioglossaceae. The patterns of provascular bundles and protoxylem str and s in the family were found to resemble those found in the progymnosperm-seed plant line of evolution. The circular-bordered pitted tracheids and the secondary growth in the genus Botrychium were found to differ from those found in the progymnosperm-seed plant line. It was noted that circular-bordered pits occur in diverse taxa, extant and extinct, and may be a shared primitive character state or may reflect parallel evolution. Furthermore, the unifacial cambial activity in the Ophioglossaceae differs from the bifacial cambial activity in plants of the progymnosperm-seed plant line. The Ophioglossaceae were found to have cladogenic root ontogeny and root-stem vascular connections of types not previously described. The similarity of provascular and protoxylem patterns notwithst and ing, the differences in the morphology of the tracheids and in the nature of cambial activity led to the conclusion that there is insufficient basis for concluding that the Ophioglossaceae and the progymnosperms are closely related. The leptosporangiate ferns, Osmunda cinnamomea and Woodwardia virginica, were found to have patterns of provascular bundles and protoxylem str and s which differ from those found in the Ophioglossaceae. The meaning of these differences was unclear. Further work on the vascular morphology of leptosporangiate ferns was indicated.